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Human facial expressions as adaptations: Evolutionary questions in facial expression research

November 2001
Yearbook of Physical Anthropology 44(S33)

DOI:10.1002/ajpa.20001.abs

Authors:

Karen L Schmidt

University of Pittsburgh

Jeffrey F Cohn

University of Pittsburgh

The importance of the face in social interaction and social intelligence is widely recognized in anthropology. Yet the adaptive functions of human facial expression remain largely unknown. An evolutionary model of human facial expression as behavioral adaptation can be constructed, given the current knowledge of the phenotypic variation, ecological contexts, and fitness consequences of facial behavior. Studies of facial expression are available, but results are not typically framed in an evolutionary perspective. This review identifies the relevant physical phenomena of facial expression and integrates the study of this behavior with the anthropological study of communication and sociality in general. Anthropological issues with relevance to the evolutionary study of facial expression include: facial expressions as coordinated, stereotyped behavioral phenotypes, the unique contexts and functions of different facial expressions, the relationship of facial expression to speech, the value of facial expressions as signals, and the relationship of facial expression to social intelligence in humans and in nonhuman primates. Human smiling is used as an example of adaptation, and testable hypotheses concerning the human smile, as well as other expressions, are proposed. Yrbk Phys Anthropol 44:3-24, 2001.

Basic facial expression phenotypes. 1, disgust; 2, fear; 3, joy; 4, surprise; 5, sadness; 6, anger. Posed images from Kanade et al. (2000).

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Eyebrow flash of greeting (Eibl-Eibesfeldt, 1989).

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Muscles of facial expression. 1, frontalis; 2, orbicularis oculi; 3, zygomaticus major; 4, risorius; 5, platysma; 6, depressor anguli oris. Original drawing from Huber (1931). (Reprinted by permission of The Johns Hopkins University Press.)

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Non-Duchenne and Duchenne smiles. a: Non-Duchenne smile. b: Duchenne smile. Images from Kanade et al. (2000).

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Automated method for quantifying facial movement. a: Posed smile with outlined features tracked automatically (Tian et al., 2001). b: Movement (radius length relative to mouth center) of right and left lip corners calculated from positional change in tracked features over first 400 msec of expression (Schmidt and Cohn, 2001).

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Figures - uploaded by Jeffrey F Cohn

Content may be subject to copyright.

Content uploaded by Jeffrey F Cohn

Content may be subject to copyright.

Human Facial Expressions as Adaptations:

Evolutionary Questions in Facial Expression Research

KAREN L. SCHMIDT

AND JEFFREY F. COHN

Departments of Psychology and Anthropology, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

Departments of Psychology and Psychiatry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

KEY WORDS nonverbal communication; social intelligence; signaling systems

ABSTRACT The importance of the face in social inter-

action and social intelligence is widely recognized in an-

thropology. Yet the adaptive functions of human facial

expression remain largely unknown. An evolutionary

model of human facial expression as behavioral adapta-

tion can be constructed, given the current knowledge of

the phenotypic variation, ecological contexts, and ﬁtness

consequences of facial behavior. Studies of facial expres-

sion are available, but results are not typically framed in

an evolutionary perspective. This review identiﬁes the

relevant physical phenomena of facial expression and in-

tegrates the study of this behavior with the anthropolog-

ical study of communication and sociality in general. An-

thropological issues with relevance to the evolutionary

study of facial expression include: facial expressions as

coordinated, stereotyped behavioral phenotypes, the unique

contexts and functions of different facial expressions, the

relationship of facial expression to speech, the value of facial

expressions as signals, and the relationship of facial expres-

sion to social intelligence in humans and in nonhuman pri-

mates. Human smiling is used as an example of adaptation,

and testable hypotheses concerning the human smile, as

well as other expressions, are proposed. Yrbk Phys An-

TABLE OF CONTENTS

Introduction ................................................................................................................................................................. 4

Facial expressions as adaptations .......................................................................................................................... 4

Behavioral Phenotype Sets ......................................................................................................................................... 4

Anatomical variation and facial expression .......................................................................................................... 5

Variation in neural control of facial muscles ........................................................................................................ 6

Variation within facial expression phenotype sets ............................................................................................... 7

Individual differences in observable facial behavior ............................................................................................ 8

Methods in facial expression research ................................................................................................................... 9

Phenotypic variation in facial perception .............................................................................................................. 9

Diversity among facial expression phenotypes ................................................................................................... 10

Ecological Contexts and Fitness Consequences ..................................................................................................... 11

Facial expressions as social signals ..................................................................................................................... 11

Socioecological contexts of human facial expression .......................................................................................... 11

Infant/caregiver interaction .................................................................................................................................. 12

Long-term cooperative social interaction ............................................................................................................. 13

Positive ﬁtness consequences ............................................................................................................................... 15

Facial expressions during speech ......................................................................................................................... 15

Courtship and facial expression ........................................................................................................................... 16

Strangers, competitors, and conﬂicts of interest ................................................................................................. 16

Signal properties of facial displays ...................................................................................................................... 17

The functions of facial expression ........................................................................................................................ 18

Phylogenetic Perspectives on Facial Expression .................................................................................................... 18

Homology in facial expressions ............................................................................................................................ 18

Functions of facial expressions in nonhuman primates ..................................................................................... 20

Conclusions ................................................................................................................................................................ 20

Acknowledgments ...................................................................................................................................................... 20

Literature Cited ........................................................................................................................................................ 21

YEARBOOK OF PHYSICAL ANTHROPOLOGY 44:3–24 (2001)

DOI 10.1002/ajpa.20001

INTRODUCTION

Facial expressions as adaptations

One of the central questions in human evolution is

the origin of human sociality and ultimately, human

culture. In the search for the origin of social intelli-

gence in humans, much attention is focused on the

evolution of the brain and consciousness. Many as-

pects of human cognition and behavior are best ex-

plained with reference to millions of years of evolu-

tion in a social context (Byrne, 1995; Cosmides et al.,

1992; Humphrey, 1976). Human brainpower can

thus be explained, in part, by increasing social de-

mands over the course of human prehistory (Dun-

bar, 1998). Social intelligence, however, is not re-

ﬂected only in the brain, but in every adaptation

that allows successful interaction in social groups.

New advances in studying the biology of social be-

havior have not fully explored that most visibly so-

cial part of the human body, the face. The face is a

visible signal of others’social intentions and moti-

vations, and facial expression continues to be a crit-

ical variable in social interaction.

Although social intelligence is an increasingly rich

source of hypotheses of cognitive and behavioral ad-

aptations, the anthropological study of facial expres-

sion remains focused on essentially nonadaptive

questions. Current anthropological views of facial

expression tend to focus on the contrasts between

universal and culture-speciﬁc explanations of facial

expressions. Facial expression is either interpreted

as a human universal, with basic expressions repre-

sented in all known human populations (Brown,

1990), or it is conceptualized as the natural out-

growth of cultural differences, with little overlap in

expression from population to population (Bird-

whistell, 1975). Physical anthropologists, with im-

portant exceptions (Blurton Jones, 1972; Fessler,

1999; and see Chevalier-Skolnikoff, 1973; Goodall,

1986; Hauser, 1996; Preuschoft and van Hooff, 1995

for comparison with nonhuman primates), have gen-

erally avoided the study of human facial expressions

and nonverbal communication, leaving the interpre-

tation of facial expression largely to psychology and

to other branches of anthropology (Birdwhistell,

1970; LaBarre, 1947). The current state of research

in facial expression, combined with the current in-

terest in social intelligence as a driving force in

human evolution, calls for the re-emergence of the

study of facial adaptation in physical anthropology.

Establishing human facial expressions as biologi-

cal adaptations requires a rigorous review of our

current knowledge and ultimately the formation and

testing of evolutionarily based hypotheses. The def-

inition by Reeve and Sherman (1993) of adaptation

is a guideline for developing evolutionary hypothe-

ses, and allows the exploration of behavioral adap-

tations that have remained relatively unknown in

physical anthropology. They deﬁne an adaptation as

“a phenotypic variant that results in the highest

ﬁtness among a speciﬁed set of variants in a given

environment.”This deﬁnition is particularly suited

to adaptive hypotheses of human behavior, because

its requirements can be met with observation of

current phenomena, and reference to phylogenetic

factors is not required (Reeve and Sherman, 1993).

What is required, however, is evidence of phenotypic

variation, well-deﬁned ecological contexts, and ﬁt-

ness consequences for a particular adaptation.

The purpose of this review is to provide a frame-

work for asking evolutionary, adaptive questions

about human facial expression. First, we establish

human facial expression as a potential behavioral

adaptation, by detailing the phenotypic variation,

ecological contexts, and ﬁtness consequences of fa-

cial behavior. A particular expression, the human

smile, is used as an example of the potential of the

adaptationist approach for understanding human

facial expression in an evolutionary perspective. Fi-

nally, facial behavior is compared to that of nonhu-

man primates to provide some further phylogenetic

perspective on the evolution of facial expression and

its role in the evolution of human social intelligence.

BEHAVIORAL PHENOTYPE SETS

Human universal facial expressions

of emotion

are perhaps the most familiar examples of facial

expression, at least among anthropologists. Six ba-

sic expression categories have been shown to be

recognizable across cultures (see Fig. 1), and this

Notwithstanding Fridlund’s compelling arguments concerning the

use of the alternate term “facial display,”to describe facial actions,

this review will rely on the older term, “facial expression”for the sake

of simplicity. No emotional input is necessarily implied by the use of

the word expression.

Fig. 1. Basic facial expression phenotypes. 1, disgust; 2, fear;

3, joy; 4, surprise; 5, sadness; 6, anger. Posed images from Kanade

et al. (2000).

4YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 44, 2001

ﬁnding is generally accepted by psychologists work-

ing on facial expression. The six basic emotional

expressions,

or facial conﬁgurations associated

with particular emotional situations, have been

shown to be universal in their performance and in

their perception (Ekman and Keltner, 1997), al-

though there is some objection to the idea that these

expressions signal similar emotions in people of dif-

ferent cultures (Fridlund, 1994; Russell and Fernan-

dez-Dols, 1997). The controversy surrounding the

attribution of universal emotions to universal facial

expressions of emotion is important for understand-

ing emotions cross-culturally. Nevertheless, even

those who disagree on the emotion concede the

cross-cultural consistency of the combinations of fa-

cial movements (behavioral phenotypes) that make

up expressions of “disgust,”“fear,”“joy,”“surprise,”

“sadness,”and “anger”(Russell and Fernandez-

Dols, 1997). These expressions were ﬁrst discussed

by Darwin (1872/1998) as universals, and have been

recognized in people from widely divergent cultural

and social backgrounds, as well as in the faces of

individuals born deaf and blind (Darwin, 1872/1998;

Eibl-Eibesfeldt, 1989; Izard, 1977; Ekman and Kelt-

ner, 1997).

In addition to the six basic facial expressions,

there are also coordinated, stereotyped nonverbal

displays that include stereotyped facial expression

components. These include the eyebrow ﬂash, yawn-

ing, startle, the coy display, and embarrassment and

shame displays (Eibl-Eibesfeldt, 1989; Grammer et

al., 1988; Keltner and Buswell, 1997; Keltner and

Harker, 1998; Provine, 1997). These displays typi-

cally combine both facial and postural or gestural

elements and are found in widely distributed popu-

lations, suggesting species rather than cultural

speciﬁcity.

The eyebrow ﬂash is a good example of this kind of

display. The frontalis muscle is consistently used to

raise both medial and lateral parts of the eyebrow

(Ekman and Friesen, 1978; Grammer et al., 1988). A

common repertoire of synchronous facial movements

occurs in combination with the eyebrow ﬂash, in-

cluding most frequently the raising of the lip corners

(smile), and lifting of the upper lid (Grammer et al.,

1988). In addition to consistency in muscle action,

the timing of the eyebrow ﬂash is also consistent

cross-culturally in the three non-Western popula-

tions analyzed. The onset of the eyebrow ﬂash typi-

cally follows a pause in all other facial movements,

and takes about 100 msec, with very little variation

across cultural groups. In addition, coordinated

head movements are found in association with eye-

brow ﬂashes, extending the display beyond the face

itself (Grammer et al., 1988; and see Fig. 2).

In order to produce the eyebrow ﬂash and other

recognizable, universal expressions, humans pre-

sumably use the same facial musculature, and move

it into a similar conﬁguration under similar circum-

stances. Thus each of these coordinated facial dis-

plays can be considered a behavioral phenotype.

Within these facial expression phenotypes, however,

and across individual humans, there is a great deal

of physical variation in structure, movement, and

perception. Universal displays, together with varia-

tion around the basic components of these displays,

comprise what can be considered phenotype sets of

facial expression. Sources of this variation include

anatomical and neurobiological differences, as well

as demographic differences such as sex, age, and

cultural background. In addition, the perception of

facial expression, important for understanding com-

municative adaptations, is also a source of individ-

ual variation.

Anatomical variation and facial expression

The structure of human facial muscles has been

known for some time (Duchenne, 1859/1990; Huber,

1931; and see Fig. 3). The anatomical basis of facial

expression has been described in detail, and an an-

atomically based coding system is available for the

objective study of facial action (Facial Action Coding

System; Ekman and Friesen, 1978). This system

outlines speciﬁc actions produced by particular fa-

cial muscles. The quality of these actions, however,

likely varies with differences in the facial muscles.

Different facial muscles produce different types of

movements, and they are most likely heterogeneous

in their structure and innervation. Goodmurphy and

Ovalle (1999), for example, have shown that muscle

ﬁber types, shapes, and sizes in orbicularis oculi,

pars palpebralis, and corrugator supercilii are sig-

niﬁcantly different, although these two muscles

share the same innervation and embryonic origin

and are found in the same region of the face (lower

eyelid and lower mid forehead, respectively). The

orbicularis oculi consists of 89% fast-twitch ﬁbers,

signiﬁcantly more than the corrugator, implying

some difference in the movements produced by the

two muscles (Goodmurphy and Ovalle, 1999). Zygo-

maticus major and zygomaticus minor muscles are

similar to the orbicularis oculi in their high propor-

tions of fast-twitch ﬁbers, relative to other muscles,

Contempt and embarrassment have also been proposed as univer-

sal facial expressions of emotion, with varying degrees of universality

in expression recognition reported (Haidt and Keltner, 1999).

Fig. 2. Eyebrow ﬂash of greeting (Eibl- Eibesfeldt, 1989).

FACIAL EXPRESSIONS AS ADAPTATIONS 5Schmidt and Cohn]

indicating a possible specialization for fast move-

ments (Stal et al., 1987).

There are also individual differences in the struc-

ture and differentiation of facial muscles. For exam-

ple, a differentiated muscle bundle, the risorius,

thought to be unique to humans, is highly variable.

As many as 22 of 50 specimens in a recent study

lacked this muscle (Pessa et al., 1998a), and Huber

believed that it was absent completely in people of

Melanesian ancestry, although this ﬁnding has not

been replicated (Huber, 1931). Various furrows and

other deformations of the facial skin are produced by

variations in facial muscles, and these may contrib-

ute to individual differences in expression. In most

individuals, the platysma muscle inserts on the skin

over the inferior margin of the mandible, but it is

occasionally observed inserting in the lateral cheek,

causing a vertical depression or furrow to appear

there. The zygomaticus major muscle also varies,

appearing in a biﬁd version with two separate inser-

tion points in 17 of 50 of specimens in an anatomical

study (Pessa et al., 1998a). The tension caused by

the two heads of the muscle at the corner of the

mouth is believed to cause a dimple or small depres-

sion during the contraction of the muscle in smiling

(Pessa et al., 1998b). Changes in facial texture, such

as dimples that appear with a smile in some indi-

viduals, could be of added value in making an ex-

pression noticeable, or in providing information

about the intensity of the expression.

A study of facial musculature in living humans

noted a signiﬁcant sex difference in the thickness of

the zygomaticus major muscle (McAlister et al.,

1998). This study also investigated differences in

musculature, and found no signiﬁcant differences in

either levator labii superioris or zygomaticus major

muscle thickness between Asians and Caucasians

(McAlister et al., 1998). In general, there is not a

great deal of published information on populational

or sex-based variation in facial muscles, and ﬁnd-

ings of populational differences described above

have not been replicated.

The effects of interindividual anatomical varia-

tion, including genetically based variation on facial

expression, are even less well-known. The muscles

themselves are highly variable, with some muscles

appearing in some individuals and not in others

(Pessa et al., 1998b). The presence of anatomical

variation raises important questions about the link

between facial actions and speciﬁc muscles. The re-

lationship between muscle activity and displace-

ment of facial features in expression is individual-

ized to some degree; during posed eyebrow raises,

muscle activity is roughly equal to brow displace-

ment squared. Yet there is wide variation for indi-

vidual brows and left brows rise higher, given the

same amount of muscle activity (Pennock et al.,

1999).

On the other hand, if the action of the face is the

same, although there is variation in the underlying

muscular structure, the resulting facial expressions

may not be meaningfully discriminated. The univer-

sal recognition of some basic expressions indicates

that facial expressions may not depend on a one-to-

one anatomical correspondence in any two facial

signalers. Basic facial expressions are also recogniz-

able in abbreviated form, without the complete set of

facial actions described for the prototype expression.

Regardless of the degree of variation that can be

detected empirically, perceivers may take no notice

of these slight variations (Fridlund, 1997; Shor,

1978), or may categorize them similarly, with high

agreement (Campbell et al., 1999; Cashdan, 1998).

More importantly, it is unknown whether such phe-

notypic variation in facial expression meets these

criteria of “just meaningful difference”(Hauser,

1996) by causing differences in receiver behavior or

judgment to slightly variant displays of the same

type.

Variation in neural control of facial muscles

Neurobiologically, facial expressions are dually

controlled by extrapyramidal and pyramidal tracts,

providing for automatic and voluntary control of

Fig. 3. Muscles of facial expression. 1, frontalis; 2, orbicularis

oculi; 3, zygomaticus major; 4, risorius; 5, platysma; 6, depressor

anguli oris. Original drawing from Huber (1931). (Reprinted by

permission of The Johns Hopkins University Press.)

6YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 44, 2001

facial expression. Based on observations of individ-

uals suffering from various neurological conditions,

Rinn (1984) described both systems of facial move-

ment, along with the differential voluntary control

over upper and lower face, related to greater asym-

metry and voluntary control over the mouth region

than the eyes. This difference is especially apparent

in the facial expressions of people with cortical vs.

extrapyramidal deﬁcits. Those lacking cortical con-

trol produce largely asymmetrical voluntary (posed)

expressions, but symmetrical spontaneous expres-

sions. Extrapyramidal deﬁcits produce the opposite

effect (Rinn, 1984; Ross and Mathiesen, 1998).

Potential asymmetry due to differences in inner-

vation of sides of the face may be related to sex

differences in the brain, and therefore produce vari-

ation in facial expression among individuals. This is

especially true if increased lateralization of cortical

function in males includes more lateralized facial

movement during expressions (Richardson et al.,

2000). Spontaneity of expression also may play a

role, with more spontaneous facial expressions un-

der the control of a different neural pathway, and

therefore more symmetric (Gazzaniga and Smylie,

1990; Rinn, 1984).

Research has not clearly conﬁrmed the predictions

of Rinn (1984) for asymmetry in lower face motion,

especially in spontaneous expressions. Borod et al.

(1998), in a meta-analysis of facial expression and

asymmetry, concluded that facial expressions could

generally be considered left-sided. However, they

did not ﬁnd that men were more likely to have

asymmetry in their facial expressions, although in-

dividual studies had previously suggested this.

Other researchers, using more objective quantita-

tive methods (direct assessment of digitized images,

rather than observer judgments or observer coding)

found that the upper face was much more asymmet-

ric than expected, particularly during spontaneous

expression (Richardson et al., 2000).

In addition, the complex connections that have

been proposed between the experience of positive

and negative emotions and facial expression and

cerebral laterality are still in question (Borod et al.,

1998; Hager and Ekman, 1997). Borod et al. (1998)

found signiﬁcant left-sidedness of facial expression

in studies involving muscle quantiﬁcation and

trained observer ratings (38 of 66 total studies

showed left-sidedness as compared to 3 of 66 show-

ing right-sidedness). The evidence of right-sidedness

for positive expressions (smile), and left-sidedness

for other emotion expressions, was much weaker

(Borod et al., 1998). Although the role of asymmetry

in the quality of facial signals is not clear, asymmet-

ric facial expression is likely to be an important

variable in considering facial displays as adapta-

tions, particularly as it relates to spontaneity or

deliberateness of expression.

Asymmetry in facial display is also probably re-

lated to individual differences in structural asym-

metry that play a powerful role in our perceptions of

human attractiveness and mate quality (Thornhill

and Gangestad, 1994). Interestingly, most studies of

facial expression asymmetry do not take into ac-

count whether or not structural asymmetry or move-

ment asymmetry is responsible for asymmetry of

facial expression. It is possible that asymmetry in

expression is largely determined by asymmetries in

the structure of the face at rest (Smith, 1998). By

necessity, observer judgments of expression inten-

sity can only be collected from faces at the height of

expression, where both structure and movement

have played a role in generating asymmetry. Elec-

tromyographic (EMG) studies of facial expression

seem to support the idea that asymmetry in expres-

sion is simply a result of asymmetry in the structure

of the face, because similar amounts of muscle ac-

tivity are found on both sides of the face (Borod et

al., 1998). In at least one case, the differential effects

of similar muscle activity on facial features has been

demonstrated (Pennock et al., 1999).

In more indirect fashion, the evolved perceptual

preference for symmetry in structure may also ex-

tend to a preference for symmetry in movement.

Spontaneous smiles, for example, are more symmet-

ric than are posed smiles (Frank et al., 1993). They

are also considered more sincere and possibly more

attractive.

Variation within facial expression

phenotype sets

Universal facial expressions, though distinct, are

not uniformly produced or perceived. Asymmetry,

due to neurobiological constraints and the relative

spontaneity of facial movement, is one source of

variation, but there are many others. Part of the

difﬁculty is that expression has too often been stud-

ied opportunistically, without prior expectation or

theoretical outlook on why particular facial move-

ments should be grouped, or how the display as a

whole came to be in the ﬁrst place. Smiling is a good

example of this problem. For example, smiling or the

joy display typically involves upturned lip corners,

and may also involve the squeezing and wrinkling of

skin around the lateral corner of the eye (orbicularis

oculi). Smiles that include both orbicularis oculi and

zygomaticus major activity have been called Duch-

enne smiles in honor of the French anatomist, while

smiles lacking orbicularis oculi activity are non-

Duchenne smiles (Frank et al., 1993). Smiles also

vary in their intensity, in the associated activity of

other facial muscles such as frontalis, and in the

open or closed position of the mouth (Blurton Jones,

1972; Cheyne 1976; Jones et al., 1990; Messinger et

al., 1999) (see Figs. 4, 5). The signiﬁcance of open or

closed mouth smiling is also unknown, although dis-

cussions of nonhuman primate and human homol-

ogy in expression suggest an appeasement function

for closed bared teeth displays (smiles) and a play

readiness function for open mouth displays (laugh-

ter and open mouth smiling) (Preuschoft, 1992; van

Hooff 1972).

FACIAL EXPRESSIONS AS ADAPTATIONS 7Schmidt and Cohn]

Although the anatomical basis of facial expression

in general is fairly well-established, important ques-

tions about the timing and patterning of facial move-

ment also remain unanswered. Spontaneous enjoy-

ment smiles appear to function within time

constraints, typically lasting between 0.5–4 sec and

having smoother transitions between onset, apex,

and offset of movement (Frank et al., 1993). In a

study of adult women’s smiles toward children, var-

ied onset and offset timing of smiles were differen-

tially interpreted by child observers. Smiles with

relatively quicker offset periods were interpreted as

less genuine by these children. Individual adult dif-

ferences were found in patterns of onset and offset

timing of smiles, and approximately 16% (122 of

763) of these smiles had multiple peaks, reinforcing

the difﬁculty in assessing the temporal course of a

smile (Bugental, 1986). Using a quantitative mea-

sure of change in smile appearance, Leonard et al.

(1991) found that observers perceived the maximum

difference in levels of happiness within the same few

frames in which the smile changed the most. They

interpreted the temporal change in smile, typically

occurring within in a 100-msec window, as a tem-

plate for smile perception. This timing may corre-

spond to the limits of the perceptual system (Frid-

lund, 1994), and is remarkably similar to the timing

of onset in the spontaneous eyebrow ﬂash (Grammer

et al., 1988). Speed of smiling (and possibly other

expressions) is likely a feature of both expression

and perception.

Also of interest are the patterns of coordinated

movements occurring during expression. In the case

of the eyebrow ﬂash, the apex of the expression is

also relatively stable, unaccompanied by the appear-

ance of new facial movements (Grammer et al.,

1988). This suggests that even for the very mobile

and ﬂexible human face, there may be limits on the

types and time course of spontaneous expressions.

Head and eye movements may also occur during

facial expression, and are probably coordinated with

facial movements, as components in a multicompo-

nent display (Ekman, 1979; Niemitz et al., 2000).

Lyons et al. (2000) demonstrated that head position

has signiﬁcant effects on the perception of facial

expression; head position changes were perceived as

facial expression changes, even in the absence of

facial muscle activity.

Given the great diversity of human facial expres-

sion, anthropologists tend to note the ﬂexibility,

complexity, and voluntary nature of facial expres-

sion (Birdwhistell, 1970). Compared even to rela-

tively expressive nonhuman primates, like chimpan-

zees, human facial expression seems to be too

variable to be divided into well-deﬁned phenotype

sets. Voluntary control over facial muscles, espe-

cially over the muscles of the mouth, is a hallmark of

human nonverbal expression, and is likely due to

the articulatory demands of human language (Rinn,

1984). The diversity of voluntary and spontaneous

human facial expression, however, should not be

mistaken for inﬁnite variability in facial expressions

as actually performed. Current research on the con-

ﬁguration and temporal course of facial expression

seems to support this idea, as it has so far shown a

great degree of regularity in expression across cul-

tures and individuals.

Individual differences in observable facial

expression behavior

In addition to underlying physical variation in the

face and in movement, empirically measured facial

behavior varies according to factors such as sex

(Briton and Hall, 1995; Chapell, 1997), age (Chapell,

1997), and cultural background (Ekman, 1973; Kup-

perbusch et al., 1999). Also important in facial ex-

pression are individualized factors, such as sociality

of situation (Fridlund, 1994; Friedman and Miller-

Herringer, 1991; Jakobs et al., 1999) and the emo-

tion-eliciting nature of visual or other stimuli (Cohn

and Tronick, 1983).

Humans vary in their ability and tendency to pro-

duce facial expressions, and this variation is pre-

sumably related to underlying muscular, neurobio-

logical, or social differences. Nevertheless, variation

in the signal itself, the visible changes in the face, is

important to addressing hypotheses of the signaling

value of facial expressions. Dimensions of nonpatho-

logical variability include interpersonal success in

nonverbal communication (Strahan and Conger,

1998) and overall expressiveness (DePaulo, 1992).

There are also sex differences in facial expression,

especially for smiling, with women smiling more

(Briton and Hall, 1995; Chapell, 1997; LaFrance and

Hecht, 1999). LaFrance and Hecht (1999) maintain

that women are not comfortable unless they are

smiling. Women have also been shown to have

thicker zygomaticus major muscles, although it is

unknown whether this is a cause or consequence of

increased smiling (McAlister et al., 1998). Sex dif-

ferences in facial expression are not only frequency-

based, however; there is also some evidence that

women specialize in expressions of happiness, while

men are better performers of angry expressions

(Coats and Feldman, 1996). Individual differences in

Fig. 4. Non-Duchenne and Duchenne smiles. a: Non-Duch-

enne smile. b: Duchenne smile. Images from Kanade et al. (2000).

8YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 44, 2001

facial expression may be apparent even in neonates

(Manstead, 1991). Most of these results are from

industrial societies, and therefore, the range of hu-

man variation is probably not fully represented. Yet

behavioral phenotypes, including universal expres-

sions and facial displays, are clearly variable.

Methods in facial expression research

Hauser (1996) stresses the importance of methods

that allow rigorous comparison among studies of

communication. Currently, there are relatively few

detailed objective methods, although recent work in

facial expression research holds promise for im-

proved expression analysis. The development and

use of the Facial Action Coding System (FACS), an

anatomically based coding system for recording ap-

pearance changes caused by the action of individual

muscles, was the ﬁrst to make possible the collection

of a large body of reliable empirical data on these

expressions (Ekman and Friesen, 1978; Ekman and

Rosenberg, 1997). New methods of facial measure-

ment allow even more objective, quantitative mea-

surement of facial movement (see Fig. 5). Automated

versions of FACS, which will automate the process

of studying facial action, are currently being devel-

oped (Cohn et al., 1999). Other automated methods

have been used for studying nonverbal signals in

general (Grammer et al., 1999), and facial expres-

sions in particular (Richardson et al., 2000; Scriba et

al., 1999). These methods rely mainly on overall

change in images of the face (or entire body) over the

course of nonverbal expression. Amounts of change

in the image, assuming all images are collected in

the same manner, are interpreted as movement pat-

terns (see Fig. 5).

Phenotypic variation in facial perception

Along with the study of the neurobiological and

anatomical and behavioral aspects of facial action,

the perceptual aspects of face recognition and facial

expression recognition have also received a great

deal of attention in the past several decades. In

proposing that patterns of facial movement are com-

municative adaptations, there is also the task of

understanding the coevolution of perceptual mecha-

nisms (Chovil, 1997; Endler 1993; Fridlund, 1997;

Fig. 5. Automated method for quantifying facial movement. a: Posed smile with outlined features tracked automatically (Tian et

al., 2001). b: Movement (radius length relative to mouth center) of right and left lip corners calculated from positional change in

tracked features over ﬁrst 400 msec of expression (Schmidt and Cohn, 2001).

FACIAL EXPRESSIONS AS ADAPTATIONS 9Schmidt and Cohn]

Hauser, 1996). There is good evidence that neurons

in particular areas of the brain have speciﬁc sensi-

tivity to social stimuli, including but not limited to

static faces. The primary evidence for the existence

of separate neurobiological mechanisms of facial ex-

pression recognition comes from studies of disso-

ciable deﬁcits: the ability to recognize faces is re-

tained, while the ability to recognize expressions is

lost in brain damage (Ellis and Young, 1998; Young

et al., 1998) or in cases of autism (Celani et al.,

1999). This evidence supports a theory of neurobio-

logical specialization for social stimuli (Adolphs,

1999; Brothers et al., 1990).

Facial expressions are processed by several dis-

tinct brain regions. The amygdala is largely re-

sponsible for perception of fear and sadness ex-

pressions. Some researchers suggest that the

amygdala may in fact be specialized for perception

of fearful expressions only (Morris et al., 1996).

Other regions, including the somatosensory and

orbito-frontal cortex, are involved in recognizing

emotion blends and anger (Adolphs, 1999; Blair et

al., 1999; Morris et al., 1996). The difference be-

tween explicit processing (subjects instructed to

judge facial expressions) and implicit processing

(subjects instructed to judge sex of actor) may

affect the region of brain used for processing, with

implicit processing handled largely by the amyg-

dala (Critchley et al., 2000).

Differences in facial expression processing likely

relate to differences in adaptive behavioral re-

sponses to expressions. For example, Dimberg

(1997) found that conditioning to faces worked much

better for fear than for any other expression. Neu-

robiological specialization for facial expression pro-

cessing also corresponds to the suggestion that per-

ception of certain signals requires greater

sensitivity (perception of anger at very low levels is

likely to give a ﬁtness advantage; Gosselin et al.,

1997).

While the variation inherent in neurobiological

perceptual systems is unknown, there is evidence for

individual differences in facial expression percep-

tion. The most striking ﬁnding is that women appear

to be more accurate and more sensitive decoders of

facial expression than men, and that this ability

develops in children as young as 3 years (Boyatzis et

al., 1993; Hall, 1984; McClure, 2000). Women in one

study gave higher ratings than men did to happy

expressions and lower ratings to neutral expres-

sions, suggesting they differentiate more among fa-

cial expressions of varying intensities (Katsikitis et

al., 1997). Otta et al (1996) also found slightly

greater discrimination among women, as perceivers

of smiles.

A more speciﬁc ability, detection of deceit, also

varies among individuals, and appears to be based

on a more widely applicable ability—being able to

detect small, extremely rapid changes in facial ex-

pression (Frank and Ekman, 1997). This ability to

detect subtle changes in facial movements may also

be an important individual difference. For example,

some people were better able to judge smiles of en-

joyment accurately, especially those who paid more

attention to the eye region (Frank et al., 1993).

Finally, cultural background plays an important

role in the attribution of personality traits to partic-

ular facial expressions. People with smiling faces

are interpreted as more sociable than people with

neutral faces by both Japanese and American sub-

jects. However, the difference between the sociabil-

ity of neutral and smiling faces is much higher for

Americans than it is for Japanese subjects (Matsu-

moto and Kudoh, 1993).

Diversity among facial expression phenotypes

Although the concept of basic universal expres-

sions has allowed us to consider human facial ex-

pressions as evolved behavioral phenotypes, an em-

phasis on universality obscures the abundance of

variation within and between facial expressions. All

facial expression phenotypes are not created equal.

For example, some expressions are produced much

more often than others (smile; Bavelas and Chovil,

1997), as frequently as two or three separate dis-

plays per minute of face-to-face interaction

(Schmidt, 2000). Others, such as the Duchenne

smile and the fear expression, cannot be as easily

produced voluntarily. Some are quite infrequent

and/or may not be as universally recognizable as

others (disgust, fear, contempt; Fridlund, 1994). Dif-

ferent expressions are also associated with speciﬁc

variation in autonomic functioning, suggesting dif-

ferent roles in emotional, self-regulatory processes.

For example, fear expressions increased heart rate

and sadness expressions decreased it (Levenson et

al., 1990).

Even expressions that seem superﬁcially simi-

lar, such as smiling and laughing (to be discussed

later), are now best studied as separate pheno-

types, given our current knowledge of their forms

and functions (Keltner and Bonanno, 1997;

Preuschoft and van Hooff, 1995; Provine, 1996,

1997). Smiling typically bares the teeth, but does

not necessarily involve vocalization, unlike laugh-

ter, which has a characteristic pattern of open

mouth and exhalation/vocalization associated

with it, and may also include repetitive eye and

mouth closure patterns (Niemitz et al., 2000). Ex-

tensive variation both among and within facial

expression phenotypes clearly requires multiple

adaptive explanations, rather than a single hy-

pothesis of facial expression. The ﬁt between be-

havioral phenotype, ecological context, and ﬁtness

consequences for facial expressions is speciﬁcto

each particular expression phenotype. In general,

however, facial expressions can be considered so-

cial signals, which is a good starting point from

which to diverge into multiple adaptive contexts.

10 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 44, 2001

ECOLOGICAL CONTEXTS AND FITNESS

CONSEQUENCES

Facial expressions as social signals

Facial expression is unambiguously social, in that

the expressions are produced with greater frequency

and intensity in social situations and can be directly

linked to interactive consequences (Fridlund, 1994;

Jancke and Kaufmann, 1994). If facial expressions

(including displays of emotion) are social adapta-

tions, then they are signals, produced primarily with

a social purpose. Whether or not emotion is associ-

ated with most facial expressions, or even any facial

expression, is an issue of great importance in the

psychology of emotion. If facial expressions are sim-

ply outward expressions of emotion, and there is no

speciﬁcﬁtness consequence proposed for the expres-

sions themselves, then they are subject to other

levels of analysis. When facial expressions are con-

sidered as potential adaptations, however, the rela-

tionship between experience of emotion and expres-

sion is not necessarily relevant (Hauser, 1996;

although see Keltner and Gross, 1999, for a func-

tional hypothesis of emotions in general). Forming

hypotheses about the adaptive nature of facial ex-

pressions as social signals requires only that speciﬁc

ecological contexts and ﬁtness consequences of these

behaviors be described. Adaptive explanations of the

signals themselves may or may not include adaptive

explanations of human emotions that underlie much

of facial expression.

Because they occur largely in the interactive con-

text, human facial expressions are generally consid-

ered to be cooperative signaling systems, beneﬁting

both signaler and receiver (Fridlund, 1997). Al-

though it is not generally discussed, facial expres-

sions can probably be considered low cost, as the

energy required to produce an individual spontane-

ous expression is small. Such low-cost, frequent sig-

nals are expected in mutually beneﬁcial interactions

(Krebs and Dawkins, 1984), or within groups of re-

lated individuals (Bergstrom and Lachmann, 1998).

Facial expressions, however, occur in many differ-

ent social contexts, not all of them cooperative. The

complex relationships between social contexts and

shared and conﬂicting interests suggests that hu-

man facial expressions have multiple signaling func-

tions. As extensive modeling, and some empirical

work in nonhuman primates has shown, the ecolog-

ical contexts and ﬁtness consequences of behavior

are critical in determining the nature of the signals

that evolve (Godfray and Johnstone, 2000; Grafen

and Johnstone, 1993; Krebs and Dawkins, 1984).

Socioecological contexts of human facial

expression

Human ecological contexts are undeniably socio-

ecological contexts. Even nonsocial contexts may be

framed in social ways, because of past selective pres-

sures on the design of the mind (Brothers, 1997;

Humphrey, 1976; Kummer et al., 1997). Absent or

semipresent individuals provide enough social con-

text to produce facial expression (speaking with

someone on the phone, or knowing a friend is doing

the same activity in the next room; Fridlund, 1994).

These types of contexts are, of course, unlikely to

have been important until recently in our evolution-

ary history, but they illustrate our readiness and

willingness to produce facial expressions at even the

suggestion of social interaction.

Humans are not completely unresponsive in the

absence of social partners, but facial expressions

vary in their susceptibility to audience effects, with

changes in frequency and/or type of movement with

different receivers (Jancke and Kauffmann, 1994;

Kraut and Johnson, 1979). Some expressions vary

little along social dimensions (startle), while others

vary signiﬁcantly (smile). For example, Fridlund

(1994) and others have found that smiling decreased

monotonically with decreasing social contexts (view-

ing a tape with a friend, viewing a tape that a friend

is watching in the next room, viewing a tape while

friend is off completing tests, viewing a tape alone).

The knowledge that a friend was performing the

same task led subjects to smile signiﬁcantly more.

Kraut and Johnson (1979) observed a similar phe-

nomenon among bowlers, who smiled at friends

more often than they smiled at a spare or strike.

Hess et al. (1995), Jancke and Kaufmann (1994),

and Manstead et al. (1999) found similar effects of

sociality, concurrent with the effects of emotion-elic-

iting stimuli. Obviously, the natural context of facial

expression is social interaction, and anthropological,

adaptive explanations of facial behavior require at-

tention to speciﬁc, naturalistic ecological contexts.

This type of approach has been suggested by

Preuschoft (2000) with respect to the positive emo-

tions, and could be adapted for the positive facial

expressions of emotion. She includes several con-

texts for recategorizing the positive emotions, in-

cluding interacting with babies, interacting with

others playfully, seeking mates, and other social

contexts. Bradbury and Vehrencamp (1998) sug-

gested a similar list: conﬂict resolution, territory

defense, sexual signals, parent/offspring interaction,

social integration, environmental contexts, and au-

to-communication.

In this paper, we propose the following set of so-

cioecological contexts for a beginning exploration of

facial expression as adaptation: infant/caregiver in-

teraction, cooperative interactions, speech, and in-

traspeciﬁc competitive interactions. We consider

Adaptive explanations of facial expressions as emotions are based

primarily on the adaptiveness of experiencing emotions. Expressing

emotion is an important part of signaling hypotheses of expression.

The exploration of relationships between internally experienced emo-

tion and facial expression is an important part of adaptive hypotheses

of emotions, but the link between expressions of emotion and ﬁtness

consequences does not necessarily require that other adaptive conse-

quences of that emotion be explained.

FACIAL EXPRESSIONS AS ADAPTATIONS 11Schmidt and Cohn]

each separately deﬁned ecological context as a

source of variation in costs and beneﬁts of facial

signaling, with separate ﬁtness consequences for

each.

Infant/caregiver interaction

Both cooperative and competing interests of care-

givers (parents) and offspring are probably reﬂected

in the signaling systems that develop in the context

of this interaction (Trivers, 1974). Infants may pro-

duce honest signals of need (Grafen, 1990; Godfray

and Johnstone, 2000). Another possibility is that

infant signals of need are exploitative, or represent

the outcome of sibling conﬂict (Godfray and John-

stone, 2000). Infant facial expressions can be inter-

preted in these signaling contexts.

Infants display basic facial expressions that are

similar to or the same as those of adults (Fig. 1).

Some of the component movements of facial expres-

sion are probably innate (Izard and Malatesta, 1987;

Meltzoff, 1996). In addition, studies of infant facial

expression are most accessible to adaptive perspec-

tives, in that researchers are unable to rely on self-

report and must instead look at expression ethologi-

cally and with an eye toward consequences of

behavior.

Human infant displays have been discussed in the

literature from a signaling perspective. Infant cry-

ing is generally interpreted as an honest signal of

need, as expected theoretically (Fridlund, 1997;

Hauser, 1996). Facial expressions associated with

crying are part of this multicomponent signal, and

crying is effective in getting attention.

Infant smiling as a display is also coordinated

with caregiver behavior. Infants look at their moth-

ers whether the mother is attentive or not, but they

only give the signal (smile) if she is attentive (Jones

et al., 1991). The probability that an infant will

smile at its mother is greatest when the mother is

most attentive and also smiling (Cohn and Tronick,

1987). Infant bouts of smiling end quite abruptly

when the mother ceases smiling (Cohn and Elmore,

1988). These results suggest that infants’production

of smiles is sensitive to the potential costs and ben-

eﬁts of making this expression. Since infants are

subject to both maintenance and growth needs, they

could be expected to modify even these seemingly

inexpensive signals, depending on the potential for

positive response. Positive responses, in this case,

would be directly related to positive ﬁtness conse-

quences, including increased opportunities to inter-

act, leading to earlier and better performance in

social interaction. The function of infant smiling and

mother infant interaction in the development of se-

cure attachment is also signiﬁcant in that secure

attachment in infancy has been demonstrated to

predict social competence later in childhood, while

insecure attachment has negative consequences

(Belsky and Nezworski, 1988).

One of the positive consequences of facial expres-

sion seems simply to get attention focused on one-

self. Faces and facial expressions draw attention

(Vuilleumier, 2000). Much of the work on this most

simple effect of facial expression comes from the

study of infant interaction. Human infants use facial

expression to induce their mothers to pay attention,

and mothers typically vocalize or smile in response

(Jones et al., 1990). The bared teeth smile seems

particularly useful in this context; Jones et al. (1990)

found bared teeth smiles are more likely to be pro-

duced in social situations, when the infant is paying

attention to its mother and not to toys. These bared

teeth smiles subsequently elicit responsive and at-

tentive behavior from the mother, a positive ﬁtness

consequence. The ﬁtness effects of parental atten-

tion for the human infant are potentially great, con-

sidering the intense social and nutritional needs of

the infant, as well as possible risks associated with

lack of maternal attention, including failure to

thrive, physical danger, and at the extreme, death

from neglect or abandonment. Whether or not these

social signals are honest signals of need or not, how-

ever, is difﬁcult to assess given our current knowl-

edge of infant smiling and attentional needs. The

relationship between infant facial expressions and

honest signals of need could be investigated, if need

were operationalized as continuing ﬁtness costs

(length of time since last feeding, distance from

mother, presence of sibling competitors), and signal

intensity and frequency were operationalized for

speciﬁc expressions.

As infants develop, they more often initiate bouts

of smiling (Cohn and Tronick, 1987). They coordi-

nate facial displays with their caregivers (parents),

signaling only in what are considered appropriate

contexts. This coordination of signals may represent

a coordination of interests among healthy offspring

and healthy parents (those with incentive to con-

tinue investment). Among older children, it becomes

apparent that ability in sending and receiving facial

signals varies, and is associated with positive ﬁtness

consequences of personal and social adjustment

(Nowicki and Duke, 1994). As children get older,

they use their perceptions of smiles to ﬁne-tune

their expressions until their deliberate posed smiles

are relatively indistinguishable from their sponta-

neous smiles. There is strong evidence that this

process relies on visual observation. Deliberate

smiles of blind children were distinguishable from

their spontaneous smiles, when viewed by untrained

observers, while those of sighted children were not

(Castanho and Otta, 1999). As adults, persons blind

from birth produce voluntary expressions of emotion

that are signiﬁcantly less recognizable than those of

sighted individuals (Galati et al., 1997).

The specialization for infant signals of need is also

speciﬁc to target receivers of the signal. Infants vary

their signals, depending on the parent (Forbes et al.,

2000). Infant signals are differentially perceived by

men and women, and infant smiles increase accu-

racy in infant recognition in women only (Jones,

1984). Jones (1984) suggested that the smile was a

12 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 44, 2001

signal speciﬁcally directed toward women. Infants

may vary signaling, based on experience of in-

creased beneﬁts for the behavior. For example, in

infant smiling during parent/infant interaction, in-

fants often begin with a non-Duchenne smile and

then convert this smile to a Duchenne smile, by

adding the contraction of the orbicularis oculi

around the lateral edge of the eye (60% of all infant

Duchenne smiles observed; Messinger et al., 1999).

It is not inconceivable that this change in expression

could be related to dynamic changes in the infant’s

perception of beneﬁts of the energetically more ex-

pensive Duchenne smile.

Long-term cooperative social interaction

Infants’and children’s competence in producing

and perceiving facial expressions eventually devel-

ops into adult skill at using facial expression with

peers and other members of the social group. Rela-

tionships among relatives and other group members

are characterized by speech and also by extensive

facial expression behavior. The complexity of rela-

tionships in these groups means that contexts for

facial expression will vary from interaction to inter-

action, on a frequent basis.

Because of their unusual stability and tendency to

involve long-term relationships, human groups are a

likely context in which reciprocal altruism can de-

velop (Trivers, 1971). Reciprocal altruism would

clearly be greatly facilitated by the use of signals for

willingness to reciprocate or negative sanction of

failure to reciprocate. It is in this context of repeated

long term interactions that patterns of repeated fa-

cial expression can provide interactants with exactly

the information they need with regard to others’

intentions, altruistic or otherwise (Brown and

Moore, 2000; Silk et al., 2000). Brown and Moore

(2000) hypothesized that in some cases, an “altruist

detector”might be more valuable than a “cheater

detector”mechanism. Presumably this detector

would depend on input from stimuli such as facial

expression, tone of voice, and other nonverbal sig-

nals. In this context a conventional signal of rela-

tively small magnitude, but repeated performance

would be predicted (Dawkins, 1993). If a pattern of

signaling that corresponds with reliable altruism

can be maintained, then the pattern could become

ritualized as a representation of the sender’s altru-

istic intent. Continuous performance of this expres-

sion pattern might indicate that others can rely on

the actor to act in an altruistic fashion. Zahavi

(1993) notes that these types of reliable conventional

signals can work in the following way: “a signal

which seems to be performed alike by all individuals

in a set is in fact a ‘standard’against which the

quality of different individuals of the same set is

judged”(Zahavi, 1993, p. 228).

This explanation of facial expression in coopera-

tive interaction is congruent with the suggestion by

DePaulo (1992) that the primary goal of facial ex-

pression, conscious or unconscious, is self-presenta-

tion. According to DePaulo (1992), although decep-

tion could be adaptive, people are not trying to make

themselves appear as something they are not. In-

stead, people typically try to enhance the accuracy of

qualities they already display (DePaulo, 1992;

Schmidt et al., 2000). For positive facial expressions,

in particular, it could be proposed that they are

trying to appear more altruistic (Brown and Moore,

2000).

Several human facial expression patterns support

this view of altruistic signaling. Social status in

housemates was associated with smiling, with high

status housemates smiling more (Cashdan, 1998). A

closed mouth bared-teeth display in nonhuman pri-

mates is typically interpreted as a submissive or

appeasement display (van Hooff, 1972; Preuschoft,

1992; and see discussion below). This kind of display

is not expected to be most frequent in the highest

status individuals. However, another perspective,

based on cooperative signaling, could explain these

data. The individuals with high social status could

be signaling their high potential for, or willingness

to reciprocate (Brown and Moore, 2000). In fact,

smiling in this study was quite frequent, for both

males and females (smiling about 10% of the time

they were observed; Cashdan, 1998).

Grammer et al. (1988) found a similar frequent

signaling pattern in eyebrow ﬂashes, with the added

point that initial eyebrow ﬂash was typically the

most intense (getting attention of partner), followed

by conversational eyebrow ﬂashes (regularly signal-

ing to attentive partner; Grammer et al., 1988).

One problem with this explanation is the possibil-

ity of nonreciprocators using repeated low-cost con-

ventional signals, such as smiles, in a deceptive

manner. Then somehow these repeated signals must

be costly, or at least so costly as to negate the beneﬁt

for a cheater that uses them correctly, but decep-

tively (Zahavi and Zahavi, 1997). Or the signals, if

cheaply produced, could represent coordinating ac-

tivity among individuals whose interests are some-

times in conﬂict, but who beneﬁt by coordinating

activities nonetheless (Silk et al., 2000).

A signal has to truthfully represent the interac-

tant’s intentions to reciprocate: honest signals are

hard to fake (Keltner and Bonanno, 1997). An auto-

matic, nonconscious response (electromyographi-

cally measured response of the receiver’s own facial

muscles) to the smiling of others, occurs only to

Duchenne smiles that are more energetically expen-

sive and harder to fake (Surakka and Hietanen,

1998). A similar response occurs to the nonverbal

vocal components of speech. The corrugator muscle

activated in the case of anger, and the orbicularis

oculi muscle activated when hearing contented

voices (Hietanen et al., 1998). Contrary to Fridlund

(1997), these reﬂexive displays of emotion may be

quite adaptive, especially in contexts where auto-

matic, quick response is critical. Emotional readout

can be a powerful social tool, especially if it brings

about positive ﬁtness consequences for senders and

FACIAL EXPRESSIONS AS ADAPTATIONS 13Schmidt and Cohn]

receivers. For example, facial expressions play a role

in creating and supporting empathy, an emotional,

yet also adaptive phenomenon (Brothers, 1989;

Buck and Ginsburg, 1997; Keltner and Bonanno,

1997). Other positive ﬁtness consequences could in-

clude the likelihood of receivers sharing food and

other resources with the signaler, beneﬁts that arise

from the reciprocally altruistic relationship in gen-

eral.

The predicted characteristics of spontaneous

smiles in the context of long-term cooperators, there-

fore, include honest signals such as the Duchenne

sign (orbicularis oculi activity) and characteristic

timing (see Fig. 5). We know that spontaneous

smiles differ in quality, with more simultaneous

muscle actions that are more coordinated, as com-

pared to deliberate, posed smiles (Gosselin et al.,

1997; Hager and Ekman, 1997; Hess et al., 1995).

The contrast between Duchenne and non-Duchenne

smiles is often interpreted as the difference between

truly felt and less than sincere expressions of emo-

tion. Ekman and Friesen (1982) maintain that the

Duchenne smile is not easily faked, although a da-

tabase of 105 posed smiles contained as many as

67% Duchenne smiles (Kanade et al., 2000; and see

Fig. 4). Whether or not the signal can be faked, it is

still consistent with an adaptive signal hypothesis of

smiling. More honest signals tend to be more costly

signals, and the Duchenne smile, to the extent that

it is more sincere, is certainly more costly (involves

at least two muscles instead of just one; see Fig. 3).

In addition, Bugental (1986) found that longer, and

therefore more costly smiles, were interpreted as

more sincere. For laughter, Niemitz et al. (2000)

found that longer laugh expressions, with more dy-

namic eye and mouth movements, were judged to be

the most positive and sincere expressions by adult

observers.

The range of contexts under which spontaneous

smiles occur, and the resulting degrees of honesty

expected, have not been considered. More research

into the quality of nonverbal signals may be the key

to distinguishing expressions along a continuum

from more honest facial signals to those that are less

than sincere (Grammer et al., 1997). For example,

observer response toward spontaneous Duchenne

and non-Duchenne smiles from the same individual

would be predicted to differ, with the longest spon-

taneous Duchenne smiles generating the most pos-

itive responses.

Yet this perspective also suggests that even over-

learned or automatic signals, like the non-Duchenne

or “social smile,”could be important signals of coop-

erative intention, rather than emotion. If social

smiles are performed according to a regular pattern,

then signaling of positive intention could consist of a

regular pattern or script for signaling, and devia-

tions from the expected expression pattern would

signal negative intentions. Social smiles, and other

expressions, are coordinated and timed with atten-

tion to the listener. They typically occur as an alter-

nate response to verbal “back channels,”such as

“uh-huh”and “yeah,”that signal continued partici-

pation in a conversation (Bavelas and Chovil, 1997;

Brunner, 1979).

We propose a new perspective on the costs associ-

ated with these social signals. In addition to the

energetic costs of repeated signaling that add up

during a social encounter, there are signiﬁcant costs

to the signaler if he is to maintain the signaling

pattern properly. These costs are attentional. Tim-

ing requires attention, and attention requires the

redirection of the sender’s neural processing and

perception toward one interactant and away from

others. Only by paying attention to the receiver and

the course of the social interaction, can the sender

continue to signal correctly. The smile, for example,

is not a randomly performed signal. The small

amount of evidence available shows that smiling is

highly coordinated with the meaning as well as the

nonverbal aspects of speech. Smiling “too early”or

“too late”can cause an individual to appear insin-

cere (Bugental, 1986; Ekman and Friesen, 1982).

Smiling when the topic of speech prohibits smiling is

a signiﬁcant social faux pas.

For non-Duchenne smiling then, the cost of the

signal is the cost of the attention paid to the inter-

actant, rather than the added physiological costs

associated with a spontaneous Duchenne smile. This

prediction could be tested by observing the rate and

timing of social smiles and other facial expressions

during conditions in which attention is disrupted.

Alternatively, one could measure the loss of atten-

tion to outside events during a typical interaction, as

an approximation of the risk in focusing on one

individual while other potentially important events

or interactions are taking place. The level of involve-

ment in an interaction as measured by number or

intensity of social signals or gaze, among other mea-

sures, is predicted to correlate with decreased atten-

tion to other stimuli in the vicinity.

If regular signaling that one focuses on the

speaker is accomplished by smiling, then a violation

could be detrimental to individual ﬁtness. Self-pre-

sentation is arguably more important among poten-

tial reciprocators, i.e., people whom one already

knows. Regular smiling, then, could be a sign of

altruistic intentions and would be expected to be

more frequent around friends (Jakobs et al., 1999).

The social smile, with its high frequency, could be

the redundant signal that Johnstone (1997) pro-

poses is the best for getting across a message to

conspeciﬁcs. Krebs and Dawkins (1984) predicted

that cooperative signals should be small and rela-

tively inexpensive. An analysis of EMG or other

measure of facial exertion would be predicted to

show that this type of smile is much cheaper than

other expressions, like anger, fear, or disgust, which

corresponds with the fact that it is used frequently

in cooperative interactions.

14 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 44, 2001

Positive ﬁtness consequences

In proposing that smiling, or any other facial ex-

pression, is an adaptive social signal, it is important

to establish the positive ﬁtness consequences of such

a behavioral phenotype. There is evidence to suggest

that facial expressions function to increase coopera-

tion and afﬁliation during interaction. The positive

ﬁtness consequences of facial expression include the

promotion of social acceptance and afﬁliation, and

the moderation of the effects of socially negative

actions. While the effects of facial expressions, and

smiling in particular, have not typically been opera-

tionalized as ﬁtness consequences in the past, a

review of recent studies indicates that these conse-

quences are signiﬁcant, at least with regard to social

variables such as status. Smiling affects perception

of the smiler’s other important characteristics, by

increasing attributions of intelligence (Otta et al.,

1993, 1996), happiness (Otta et al., 1996), and social

status (LaFrance and Hecht, 1999). Although not

typically associated with grief, smile displays during

conversation, especially those that include orbicu-

laris oculi activity around the lateral edge of the eye,

increase perceiver sympathy for bereaved persons

(Keltner and Bonanno, 1997). People associate smil-

ing with happiness and with positive intentions di-

rected toward them (liking) (Floyd and Burgoon,

1999). Smiling, in both the United States and Japan,

is associated with increased sociability (Matsumoto

and Kudoh, 1993).

These responses are mostly determined by self-

report in psychological studies, but there is also

direct physiological evidence that facial displays can

bring about positive responses, even in the absence

of overt emotional response. For example, a study of

the effects of viewing Ronald Reagan’s reassuring

smile display (raised brows, tilted head, relaxed

open mouth smile) brought about physiological signs

of positive response, even in viewers who said they

did not support Reagan politically (Sullivan and

Masters, 1991).

Clearly, smiling displays can bring about positive

ﬁtness consequences, and perhaps even convince

others of ones’willingness to reciprocate. There may

be some individuals who fail to signal regularly,

however. In this case, the failure to signal socially is

in itself a signal (Bergstrom and Lachmann, 1998).

The failure to smile correctly, or to produce an ap-

propriate level of facial expression during coopera-

tive social interaction, may be at the root of some of

the social difﬁculties of people with ﬂat affect as a

result of schizophrenia, Parkinson’s disease, and de-

pression, or facial paralysis (Mueser et al., 1997;

Sakamoto et al., 1997; VanSwearingen et al., 1999).

Signaling altruism also implies the presence of

remedial social signals. For individuals who fail to

act in an altruistic manner, and get caught, there

are also embarrassment and shame displays. Inter-

estingly, in embarrassment displays, there is also a

smile, although here it is interpreted as an appease-

ment signal (Keltner and Buswell, 1997). Shame

displays are slightly different, and do not typically

include a smile (Keltner, 1997). Although there is no

consensus on whether or not the smile is an honest

signal or not, the blushing response seems much

closer to the deﬁnition of an honest costly signal.

Also a component of the shame display, blushing

actually works, with signiﬁcant differences in the

perceptions of interactants depending on whether or

not an individual blushed. Blushing, which can be

part of a voluntary shame display including glancing

around and lowering the head, actually reduces neg-

ative evaluations of actor behavior (deJong, 1999).

As a remedial gesture, blushing may be particularly

potent, because it is a gesture that is generally be-

lieved to be impossible to fake. It is not surprising

then that it produces a better response in observers

than does the glancing around display, another re-

medial gesture (deJong, 1999). Keltner et al. (1997)

interpreted both shame and embarrassment dis-

plays as appeasement displays, and supported this

hypothesis with evidence that they are negatively

correlated with aggression in adolescent boys.

Facial expressions during speech

Facial expression during social interaction is pos-

sibly an honest signal of afﬁliation, or willingness to

reciprocate. Among humans, however, social inter-

action almost invariably involves speech, and there

are unique considerations in the adaptiveness of the

relationship between facial expression and speech.

Facial expression is coordinated with speech at sev-

eral levels: the use of muscles of facial expression to

articulate speech sounds (Massaro, 1998), the con-

tribution of facial expressions to the syntactic struc-

ture and meaning of particular utterances (Bavelas

and Chovil, 1997; Ekman 1979), and graded or qual-

itative conversational signals that apply to the over-

all meaning of speech (Ekman 1979).

In addition to its functions for the speaker, facial

expression is also an important part of listener ac-

tivity (as many as 20% of conversational expressions

were back channel cues, performed while the indi-

vidual was not talking; Bavelas and Chovil 1997).

Smiles are produced with the same timing as other

back channel conversational cues, such as “uh-huh,”

that signal continued participation in a conversation

(Brunner, 1979), and are also produced at the end of

utterances (Schmidt, 2000). Laughter occurs along

with speech in a coordinated pattern (Provine,

1997). Ekman (1979) detailed the multiple patterns

of association of brow movements with speech: as

“batons”stressing a particular word, as question

marks, or as “underliners”emphasizing a sequence

of words, among others. Although the role of facial

expressions and other gestures in language evolu-

tion may be limited, understanding the coevolution

of language and the “gesture-call”system is critical

to understanding human speaking behavior (Burl-

ing, 1993, with comment by Blount, p. 38 –39).

FACIAL EXPRESSIONS AS ADAPTATIONS 15Schmidt and Cohn]

Facial expressions might also be related to the

social functions of speech. Although humans prac-

tice some social grooming (Schievenhovel, 1997),

Dunbar (1996) suggests that the role of grooming in

human society has largely been taken over by con-

versation. If nonverbal signals, including facial ex-

pressions, are coordinated with speech, they might

also assist in the grooming function of speech. An-

drew (1962) notes that lip smacking and other lip

movements are intention signals for grooming in

nonhuman primates. It is also possible that humans,

while potentially “grooming”partners with speech,

have retained these intentional signals in the form

of lip sucking, lip wipes and lip biting (action units

28, 37, and 32, Ekman and Friesen, 1978). Fridlund

(1997) refers to lip biting as a sign of agitation, but

it may also be a sign of desire to groom which is often

is associated with agitation. It is an open question

whether or not these movements are more frequent

during “vocal grooming”than in other human social

settings.

Courtship and facial expression

Facial expressions are only a small proportion of

the large number of human courtship signals. They

are typically embedded in coordinated displays, in-

cluding both whole body and whole head move-

ments, as well as other signals. As such, they have a

role in signaling both interest and mate quality.

Signaling mate quality is accomplished by a number

of physical trait-based characteristics such as sym-

metry, glossiness of hair and skin, and healthiness

of eyes (Gangestad and Thornhill, 1997).

Because of the numerous available indicators of

potential mate quality, one might argue that facial

expression was unnecessary. However, it could also

be expected that facial expression would act in such

a way to enhance positive traits, while disguising or

concealing less positive traits (enhancing self-pre-

sentation as a potential mate). Eibl-Eibesfeldt

(1989) describes how clothing and makeup in many

cases, accomplish these goals, but facial expression

could provide an intermediate behavioral level be-

tween cultural modiﬁcation and physical, biological

constraints.

The description of the courtship displays of young

German and Japanese women, for example, shows

the importance of both facial expression and other

nonverbal movements in signaling attraction (will-

ingness to mate or to consider mating). Grammer et

al. (1999) found that during mixed sex interactions

between young men and women, a woman’s interest

in her partner was related to the regularity of her

nonverbal signals. Smaller, more regular move-

ments in women’s nonverbal displays were associ-

ated with increased levels of interest (Grammer et

al., 1999). This could be the case for facial expression

as well. In the case of opposite sex interactants,

repeated signaling with honest signals could also be

interpreted as a sign of willingness to invest in a

mate, and therefore increase the likelihood of repro-

ductive opportunities for the signaler. While there is

little positive evidence to support this idea, there is

some negative evidence in that the lack of coordina-

tion of facial movements in people with schizophre-

nia can cause signiﬁcant social problems for people

with schizophrenia (Dworkin, 1992).

With regard to symmetry and mate selection, it is

possible that symmetric facial expressions (typically

spontaneous and difﬁcult to produce voluntarily)

evoke attractiveness in the same way that struc-

tural facial symmetry (also very difﬁcult to fake)

does (Gangestad and Thornhill, 1997; Grammer and

Thornhill, 1994). Finally, facial and other nonverbal

signals during courtship can be expected to be rele-

vant to other signals of mate quality and to coordi-

nate with them. The ﬂirtatious hair ﬂip (Grammer

et al., 1999), for example, shows dynamically the

quality of the hair, while open-mouthed expressions

may reveal the quality of the teeth, as well as the

tissues inside the mouth.

Strangers, competitors, and conﬂicts of interest

A variety of individual ecological contexts are in-

cluded here, mainly because of the general lack of

empirical information on facial expressions in these

different contexts. In contrast to previously de-

scribed situations, however, interactions with

strangers and other interactions where potential

conﬂicts of interest are likely to share some basic

signaling properties.

Krebs and Dawkins (1984) suggest signals to

those whose interests potentially conﬂict with the

signaler should be stereotyped and clear, rather

than hidden. Facial expressions are expected to re-

vert to conventional forms that reveal as little extra

information as possible (Wagner and Lee, 1999). In a

classic study of Japanese and American students

responding to ﬁlms, facial expressions differ least in

the presence of an interviewer, demonstrating the

increased regularity of facial signals in a socially

risky situation (with interviewer, rather than alone;

Ekman and Friesen, 1978; Wagner and Lee, 1999).

Signalers have some conscious perception of their

signaling patterns: students can predict, based on

context, their own likelihood of smiling at a joke told

by a professor as compared to a joke told by another

student (Nagashima and Schellenberg, 1997). Ste-

reotyped nonverbal bodily displays are also charac-

teristic of interactions in which strangers meet

(Grammer et al., 1997).

The sender may modify normally beneﬁcial honest

expressions in these contexts, depending on poten-

tial costs or beneﬁts of revealing information. When

meeting strangers, humans sometimes attempt to

hide or suppress expression. For example, children

and young adults from a variety of cultures re-

sponded to a friendly stranger with a similar pattern

of direct gaze with expression, followed by hiding the

face, and then another glance (Eibl-Eibesfeldt,

1989). Concealment also occurs in competitive inter-

actions with known individuals. In a study of indi-

16 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 44, 2001

vidual differences of the tendency to self-monitor

facial expressions, some individuals were found to

conceal expressions of joy after winning. These

smiles were concealed either by covering with a

hand, or by twisting other muscles in the face into

positions that minimized the appearance of the

smile (Friedman and Miller-Herringer, 1991).

Yawns and laughs are also sometime concealed by

covering (Provine, 1997).

According to Fridlund (1997), concealment of emo-

tion is highly desirable, and coordinates with the

“dictate of privacy”in social interaction. This is mis-

leading, though, in that the desirability of conceal-

ment necessarily varies with the particular socioeco-

logical context. Only those facial signals that depend

on privacy between signaler and receiver for positive

consequences are expected to be concealed from the

rest of the group. Although there are likely to be

contexts in which people seek to hide their expres-

sions from others, facial expressions toward strange

conspeciﬁcs in many cases are expected to be clear

and easily readable, whether they signal internal

state or not (Endler, 1993).

Meetings between strangers represent an obvious

opportunity for deception, in that the altruistic ten-

dencies of the signaler are unknown to the recipient.

However, actively deceptive signals are expected to

disappear eventually, because they do not supply

the receiver with useful information (Zahavi and

Zahavi, 1997). Actively deceptive signals of this type

can be contrasted with concealment of basically hon-

est signals, as in the effective suppression of facial

expression in contexts where food resource, personal

reputation, or other positive outcome is at stake

(Ekman et al., 1997; Mitchell, 1999). A third strat-

egy, the modiﬁcation of internal states associated

with expressions (emotional states), may be the

most adaptive form of deception. Expressions gener-

ated under these conditions of self-deception would

then appear spontaneous and automatic (Alexander,

1987). Emotional readout is not necessarily an hon-

est signal: it may be the result of self-deception, and

so appear honest when it is not.

Various facial behaviors associated with deceit

have been described and tested empirically (Ekman,

1985; Ekman et al., 1997). These studies provide

interesting information on social intelligence and

Machiavellian intelligence in particular. However,

the same researchers also found that there is sub-

stantial variation in the ability of human observers

to detect these nonverbal signs of deceit. Although

some individuals have this capability, many, even

those with extensive experience or training (e.g.,

police ofﬁcers), are unable to reliably detect cheating

individuals using these cues (Frank and Ekman,

1997). In fact, at least for facial expression, human

cheater detection mechanisms seem to be remark-

ably bad at detecting cheating, at least among

strangers (DePaulo, 1992; Ekman, 1985; Gosselin et

al., 1997). Either these mechanisms do not provide a

clear ﬁtness beneﬁt, and have therefore not been

subject to selection pressure (Bradbury and Vehren-

camp, 2000; Zahavi, 1993), or else the relevant con-

text of cheater detection is found not among strang-

ers, but among social group members. Cheater

detection ability may depend on an almost statisti-

cal knowledge of the normal facial expression pat-

tern for a given interaction partner, rather than a

stranger.

Cheater detection mechanisms are undoubtedly

an important feature in the evolution of human so-

cial intelligence, but in the case of facial expression

may not be very ﬁne-tuned (DePaulo, 1992). The

answer to these puzzling ﬁndings probably lies in

the structure of human social interaction, especially

in the distant past. Cheater detection would be ex-

pected for individuals with whom we have the most

contact (strangers would likely not have been

trusted in any case). We hypothesize that detection

of deception by observation of nonverbal behavior

may be limited to those individuals that have regu-

lar contact with the deceiver, and might not be ex-

pected anyway in those who have no prior knowl-

edge of this person (such as observers described in

Ekman, 1985 and Ekman et al., 1997). We are not

aware of any studies of the relative difference in

cheater detection performance based on the facial

expression of known and unknown subjects, but pre-

dict that cheater detection would be signiﬁcantly

better for known individuals.

If facial expressions are generally used as honest

signals, as part of an ordered sequence of actions

appropriate to a particular social context, relatively

infrequent use of these signals for deceitful purposes

is a possibility. Mitchell (1999) describes these reg-

ular sequences of events as social scripts, where

deception is dependent upon successful mainte-

nance of behavior patterns (including facial expres-

sions), that signal cooperation while individuals act

deceitfully in their own self-interest. For example,

the maintenance of friendly facial expression, fol-

lowed by negative actions toward others, constitutes

the deceptive use of a social script, facial expressions

associated with afﬁliative interaction.

Signal properties of facial displays

Facial expressions are usually thought of as inti-

mate, based in dyads such as parent/infant interac-

tions, or conversations. More attention, however,

needs to be drawn to the differential properties of

facial signals, such as their clarity across relatively

large distances (as far as 45 m; Hager and Ekman,

1979) and from angles other than directly face-to-

face. Although facial expressions are typically ob-

served at close range, their signal properties may

allow for accurate reading by individuals in the pe-

riphery (eavesdroppers). The detectability of signals

is an important factor in their ability to provide

positive ﬁtness consequences for signalers (Endler,

1993). A comparison between the distance of facial

expression recognition and average social distance

in other species would be interesting in this regard.

FACIAL EXPRESSIONS AS ADAPTATIONS 17Schmidt and Cohn]

The functions of facial expression

By using these adaptive frameworks, and clarify-

ing phenotype, ecological context, and ﬁtness conse-

quences of facial signaling, it becomes possible to

investigate facial expression from an evolutionary

perspective (Fridlund, 1994). Both social and emo-

tional expressive functions of facial expression can

be investigated from an evolutionary perspective

and represent different levels of analysis (Hauser,

1996, p.495; see Jakobs et al., 1999, for a discussion

of this issue from the psychological perspective). Fit-

ness consequences are particularly hard to detect,

especially given the cross-sectional nature of many

psychological studies of facial expression. A single

facial expression, with few exceptions, cannot be

expected to provide a dramatic ﬁtness beneﬁt or

cost. Rather, repeated signaling of intention or emo-

tion is probably the biggest contributor to ﬁtness

accruing from facial expression.

In addition, the critical factor in the ﬁtness of a

particular expression such as a smile may be the

difference between it and the average smile of the

actor. Psychological studies generally take a cross-

sectional approach, measuring only a few minutes of

expression in a large number of individuals. Longi-

tudinal studies of facial expression are few and far

between (but see Messinger et al., 1999). Still, we

can begin to get a picture of the importance of facial

expression to ﬁtness by considering some of the re-

sults of facial expression research.

PHYLOGENETIC PERSPECTIVES ON FACIAL

EXPRESSION

Homology in facial expressions

Game theoretic models of facial signaling assume

the actors have the alternative of not signaling at

all, and may refer to a past generation where sig-

naling did not yet exist (Bradbury and Vehrencamp,

2000). Although these issues are theoretically im-

portant, the phylogeny of humans and the long pre-

history of sociality in the Primate order make it

somewhat unlikely that signaling with human facial

expression would disappear completely in humans.

This is immediately apparent when considering the

drastic and deeply damaging social consequences of

facial paralysis (VanSwearingen et al., 1999). These

and other forms of complete facial paralysis have

such negative social consequences that it is difﬁcult

to imagine the lack of facial signaling as an alterna-

tive. In some cases, the amount of depression asso-

ciated with facial paralysis is directly related to the

degree of disability in producing a prototypical smile

(VanSwearingen et al., 1999). It remains to be dem-

onstrated, however, whether human facial expres-

sions function adaptively, or whether they are sim-

ply remnants of a prelinguistic past (Darwin, 1872/

1998).

Clearly, though, there are differences in the fre-

quency and intensity of facial expression across nor-

mal individuals. Facial expressions are not always

produced when they would be advantageous, and

this may lead to negative ﬁtness consequences that

are less dramatic than those discussed above, but

still potentially costly in social interaction, depend-

ing on cultural and social context. Negative ﬁtness

consequences here are conceptualized as reduced

access to cooperative relationships that tend to en-

hance survival and reproductive potential. Given

the long history of sociality in our lineage and the

ubiquity of facial expression in observations of nat-

uralistic social interaction, we hypothesize that a

certain level of facial expression must be obtained,

or the individual risks losing the ﬁtness beneﬁts

acquired during earlier interactions.

The homology of human and nonhuman primate

facial expression may illustrate continuity among

facial expression phenotypes, especially between

apes and humans. Preuschoft and van Hooff (1995)

note the remarkable stereotypy of facial expression

across an order that is usually known for its behav-

ioral ﬂexibility. Production of basic facial expres-

sions, such as the fear grimace in rhesus macaques,

also appears to be highly canalized (Geen, 1992). It

would be surprising if the level of stereotypy and

homology in nonhuman primate expression did not

extend to at least some patterns of human expres-

sion.

Particular expressions, notably the silent bared

teeth and relaxed open mouth displays, have been

compared to the human smile and laughter (Cheva-

lier-Skolnikoff, 1973; Darwin, 1872/1998; Preuschoft,

1992; Preuschoft and van Hooff, 1995; van Hooff,

1972). Given the similar structure of the facial mus-

cles and adaptations for focusing on the face, homol-

ogy between nonhuman primate and human facial

expressions has been suggested (van Hooff, 1972).

The smile has been proposed as a homologue to the

silent bared teeth display (SBT), and the laugh as

homologue to relaxed open mouth displays of mon-

keys and apes (van Hooff, 1972; Preuschoft, 1992;

Preschoft and van Hooff, 1995). Preuschoft (2000)

suggests that the apparent contrast between the

smile of humans (upward lip corners) and the SBT of

nonhuman primates may simply be the result of

similar muscles stretched over a very different-

shaped muzzle (Preuschoft, 2000; see Fig. 6).

In addition, there are common neurobiological

bases among humans and other primates in the

control of facial expression. A recent study of projec-

tions from the cortex to the facial nerve nucleus in

rhesus macaques found a pattern similar to that

described by Rinn (1984). Bilateral cortical projec-

tions to facial nuclei control frontalis and orbicularis

oculi muscles, and contralateral projections to the

opposite facial nucleus, control the muscles around

the mouth, allowing hemispheric differences in ex-

pressiveness to inﬂuence the lower face in particular

(Morecraft et al., 2001). Researchers have begun to

demonstrate asymmetry in facial expressions of

monkeys, indicating that these underlying mecha-

nisms of control also produce similar effects in non-

18 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 44, 2001

human primates. Hook-Costigan and Rogers (1998)

found that positive social calls and expressions of

marmosets were lateralized to the right, while neg-

ative social expressions (fear) were lateralized to the

left. Hauser and Akre (2001) also found that expres-

sions were seen earlier on the left side of the face in

rhesus macaques, although they found no different

between expressions interpreted as socially positive

or negative, expanding on the original ﬁnding by

Hauser (1996) that rhesus expressions were left-

sided.

Although at least some degree of neurobiological

and physical homology of expression seems likely,

there is also the problem of differential function.

Can human smiles really be homologous to the SBT

display when human smiles signal joy and nonhu-

man primate smiles signal appeasement or fear?

From the perspective of human facial expression,

the roles of the smile continue to expand with new

research, and the equation of human smiles with

happiness has been called into question (Ekman,

1985; Fridlund, 1994). Nonhuman primate bared

teeth displays (as in Fig. 6) may be similar in mean-

ing and function to human smiles, depending on the

socioecological context.

Human anger expressions and ape anger/fear ex-

pressions may also be homologous (Chevalier-

Skolnikoff, 1973). Embarrassment has been pro-

posed as a homologue of primate appeasement

displays, because it shares characteristics such as

withdrawal, minimizing appearance, and smile with

downward glance (Keltner and Buswell, 1997). Ho-

mology has also been proposed for the yawn, which

shows evidence of being a social signal of transition

between activity states both in humans and in non-

human primates (Deputte, 1994; Provine, 1997).

Homology may also apply to aspects of hiding or

minimizing expression. There are several examples

of apes preventing conspeciﬁcs from seeing their

expression by covering the face with a hand (Toma-

sello and Call, 1997), and one report of a chimp that

was able to control a grimacing mouth by pressing

down on his lips with his hand (Mitchell, 1999).

Humans also move their hands to the face to hide

expressions, possibly because such expressions

would be detrimental to their interests if openly

recognized (Keltner and Buswell, 1997) for embar-

rassment; see Provine (1997) for yawning. Humans

have the additional ability of hiding their expres-

sions with the actions of other facial muscles, espe-

cially around the mouth where the downward pull of

the depressor anguli oris muscle can somewhat con-

ceal the rise of the lip corners due to the action of

zygomaticus major (Ekman, 1985). Spontaneous ex-

pressions may have elements of concealment associ-

ated with them, such as “twisting”the smile to avoid

appearing too pleased in front of others (Friedman

and Miller-Herringer, 1991). In the same study, the

authors noted that hand contact increased with in-

creased smiling. It could be that the action of other

facial muscles in some cases was inadequate to the

task of concealing the increasingly obvious smile

signal.

Consideration of nonhuman primate facial expres-

sion, of course, is necessarily be speciﬁc to each

individual species. Divergent facial signaling sys-

tems, as well as some homology, are to be expected.

For the relationship between overall amount of ex-

pression and the structure of the facial nerve nu-

cleus, Peburn et al. (2000) found variation between

species, with the relatively more expressive ma-

caques (Macaca fascicularis) showing signiﬁcantly

larger and better deﬁned facial subnuclei than the

relatively less expressive patas monkeys (Erythroce-

bus patas). Aotus, the only nocturnal anthropoid,

has virtually no facial expression, and its facial mus-

culature is also relatively less differentiated (Chev-

alier-Skolnikoff, 1973; Huber, 1931).

Likewise, many features of human facial expres-

sion, although universal, are speciﬁc to our species.

The relative hairlessness of the human face, com-

pared to the majority of nonhuman primates, sug-

gests that the combination of exposed skin and re-

tained hair may have some signaling value. Brows

are an important component of human greeting and

surprise displays, as well as some smiles. Upturned

Fig. 6. Homologous displays in human and nonhuman pri-

mates. a: Rhesus macaque submissive display. Photograph by

Frans DeWaal, 1989.(Silent bared teeth display.) b: Human

smile. From Kanade et al.,2000. (Silent bared teeth display.)

c: Bonobo play face. Photograph by Frans DeWaal, 1988. (Re-

laxed open mouth display.) d: Human play face, from Forbes et

al., 2000. (Relaxed open mouth display.)

FACIAL EXPRESSIONS AS ADAPTATIONS 19Schmidt and Cohn]

inner brows also signal sadness or grief (Eibl-

Eibesfeldt, 1989; Ekman, 1979). Results of an in-

triguing study in which cross-species perception of

facial expression was tested, the brows, a highly

visible signal on the hairless human face, are not

used by macaques seeking to identify sad human

facial expressions. They rely instead on other, more

phylogenetically conserved traits, such as cheek

movement (Kanazawa, 1998). Retention of facial

hair in adult male humans may also play a role in

accentuating or concealing expression. Unfortu-

nately, there has been no research on male facial

hair as it relates to facial expression.

Functions of facial expressions in nonhuman

primates

Like humans, many nonhuman primate species

pay close attention to the faces of other group mem-

bers. The complexity of face perception and facial

expression among the great apes, for example, is

just beginning to be demonstrated. Chimpanzees

can use facial features to recognize both kin and

unfamiliar conspeciﬁcs, and to categorize facial ex-

pressions of other chimpanzees (Parr et al., 1998,

2000; Parr and deWaal, 1999). Chimpanzees can

follow the gaze of another individual by paying at-

tention to its face (Povinelli and Eddy, 1996). Goril-

las, chimpanzees, and bonobos have all been re-

ported to monitor the gaze of others, in ways that

suggest they may be able to anticipate the impact of

their facial expressions on others’behavior (Mitch-

ell, 1999). Clearly, facial expressions are important

signals in the social lives of most nonhuman pri-

mates, just as they are for humans.

By monitoring facial expressions, the intentions of

others can be inferred. Because of their relatively

stable existence, social groups provide an environ-

ment in which low-cost, redundant signals can help

primates to keep track of the other individual, pos-

sibly similar to that which Chadwick-Jones (1998)

described for baboons. Cheater males may fail to

give continuous signals, and so must be monitored.

The failure to adhere to social scripts, including

facial expression patterns, draws attention to unco-

operative individuals (Mitchell, 1999). Groups of

nonhuman primates may also utilize redundant,

“cheap”vocal signals in order to coordinate their

activities (Silk et al., 2000). Haslam (1997) provided

a list of social grammars for primate interaction,

including communal sharing, authority ranking,

equality matching, and market pricing. These are

behavioral contexts of the type that could corre-

spond to functional variation in primate facial ex-

pression. As in humans, considering facial expres-

sions as adaptations would require speciﬁcation of

relevant phenotypes, ecological contexts, and posi-

tive and negative ﬁtness consequences.

CONCLUSIONS

Current functional approaches to facial expres-

sion are largely framed in terms of the proximate

functions of emotions associated with that expres-

sion (Keltner and Gross, 1999; Keltner and Haidt,

1999; Yik and Russell, 1999). Fridlund (1994) has

argued from a critical psychological perspective for

the consideration of all facial behavior in a behav-

ioral ecological or evolutionary framework. Evolu-

tionary approaches, however, are necessarily based

on the study of facial expressions as biological adap-

tations, and much of the recent work on facial ex-

pression has been conducted outside this perspec-

tive. Nevertheless, results from this type of

functional study of facial expression provide a basis

on which to base evolutionary hypotheses of facial

behavior. By detailing the phenotypic variation in

facial expression, clearly deﬁning the ecological con-

texts of facial expression, and describing some of the

positive ﬁtness consequences of these behaviors, we

have outlined an evolutionary approach to the study

of human facial expressions as adaptations.

By proposing testable hypotheses about the na-

ture of facial expression, we can also provide a ﬁrm

biological basis to other anthropological studies of

communication. The unfortunate, and probably un-

intended, view of nonverbal expression as represent-

ing an ancestral-primate primitive ability, while

language is uniquely specialized in humans, can

also be challenged from this perspective (Eibl-

Eibesfeldt, 1989). Instead of just assuming that non-

verbal expression has much in common with pri-

mate relatives, but is also culturally patterned, we

may be in a position to specify more clearly the

adaptive role of facial expression in human social

interaction.

Human social intelligence is obviously a major

contributor to human brain evolution and intellec-

tual abilities. Our most specialized and adaptive

system of social signaling is human language

(Pinker, 1994). Remarkable in its complexity and

also because of its seeming discontinuity with non-

human primate vocalization and communication,

human language has clear adaptive consequences

for human evolution (Burling, 1993; Pinker, 1994).

There is no doubt that language has been a driving

force in the evolution of human behavior, and the

social context is dominated by language (Dunbar,

1996). Yet there is more to social intelligence than

language. Deﬁcits in other social skills, including

nonverbal skills, make it very clear that language

alone is not sufﬁcient for successful social interac-

tion, and the positive consequences of these skills

are only now being described. This argues for in-

creased attention to the evolution of nonverbal sig-

naling systems, including facial expression, from the

perspective of physical anthropology.

ACKNOWLEDGMENTS

We thank John S. Allen and Melissa Panger for

careful reading and helpful criticism of this article.

This research was supported by NIH grant MH12579

to K. Schmidt and by NIH grant MH56193 to J.

Cohn.

20 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 44, 2001

LITERATURE CITED

Adolphs R. 1999. Social cognition and the human brain. Trends

Cogn Sci 3:469 –479.

Alexander RD. 1987. The biology of moral systems. Hawthorne,

NY: Aldine deGruyter.

Andrew RJ. 1962. The origin and evolution of the calls and facial

expressions of the primates. Behaviour 20:1–107.

Bavelas JB, Chovil N. 1997. Faces in dialogue. In: Russell JA,

Fernandez-Dols JM, editors. The psychology of facial expres-

sion. New York: Cambridge University Press. p 334 –346.

Belsky J, Nezworski T. 1988. Clinical implications of attachment.

In: Belsky J, Nezworski T, editors. Clinical implications of

attachment. Hillsdale, NJ: Lawrence Erlbaum Associates. p

3–17.

Bergstrom CT, Lachmann M. 1998. Signaling among relatives.

III. Talk is cheap. Proc Natl Acad Sci USA 95:5100 –5105.

Birdwhistell RL. 1970. Kinesics and context. Philadelphia: Uni-

versity of Pennsylvania Press.

Birdwhistell RL. 1975. Background considerations to the study of

the body as a medium of “expression”. In: Benthall J, Polhemus

T, editors. The body as a medium of expression. New York: E.P.

Dutton and Co., Inc. p 36 –58.

Blair R, Morris J, Frith C, Perrett D, Dolan R. 1999. Dissociable

neural responses to facial expressions of sadness and anger.

Brain 122:883–893.

Blurton Jones NG. 1972. Non-verbal communication in children.

In: Hinde RA, editor. Non-verbal communication. New York:

Cambridge University Press. p 271–295.

Borod JC, Koff E, Yecker S, Santschi C, Schmidt JM. 1998. Facial

asymmetry during emotional expression: gender, valence and

measurement technique. Psychophysiology 36:1209 –1215.

Boyatzis, CJ, Chazan E, Ting CZ. 1993. Preschool children’s

decoding of facial emotions. J Genet Psychol 154:375–382.

Bradbury JW, Vehrencamp SL. 1998. Principles of animal com-

munication. Sunderland, MA: Sinauer Associates, Inc.

Bradbury JW, Vehrencamp SL. 2000. Economic models of animal

communication. Anim Behav 59:259 –268.

Briton NJ, Hall JA. 1995. Gender-based expectancies and ob-

server judgments of smiling. J Nonverb Behav 19:49 –65.

Brothers L. 1989. A biological perspective on empathy. Am J

Psychiatry 146:10 –19.

Brothers L. 1997. Friday’s footprint: how society shapes the hu-

man mind. New York: Oxford University Press.

Brothers L, Ring B, Kling A. 1990. Response of neurons in the

macaque amygdala to complex social stimuli. Behav Brain Res

41:199 –213.

Brown D. 1990. Human universals. New York: McGraw-Hill, Inc.

Brown WM, Moore C. 2000. Is prospective altruist-detection an

evolved solution to the adaptive problem of subtle cheating in

cooperative ventures? Supportive evidence using the Wason

selection task. Evol Hum Behav 21:25–37.

Brunner LJ. 1979. Smiles can be back channels. J Pers Soc

Psychol 37:728 –734.

Buck R, Ginsburg B. 1997. Communicative genes and the evolu-

tion of empathy. In: Ickes W, editor. Empathic accuracy. New

York: Guilford Press. p 17–43.

Bugental DB. 1986. Unmasking the “polite smile”: situational

and personal determinants of managed affect in adult-child

interaction. Pers Soc Psychol Bull 12:7–16.

Burling R. 1993. Primate calls, human language, and nonverbal

communication. Curr Anthropol 34:25–53.

Byrne RW. 1995. The ape legacy: the evolution of Machiavellian

intelligence and anticipatory interactive planning. In: Goody

EN, editor. Social intelligence and interaction. New York: Cam-

bridge University Press. p 37–52.

Campbell R, Woll B, Benson PJ, Wallace SB. 1999. Categorical

perception of face actions: their role in sign language and in

communicative facial displays. Q J Exp Psychol [A] 52:67–95.

Cashdan E. 1998. Smiles, speech, and body posture: how women

and men display sociometric status and power. J Nonverb Be-

hav 22:209 –228.

Castanho AP, Otta E. 1999. Decoding spontaneous and posed

smiles of children who are visually impaired and sighted. J Vis

Impair Blind 93:659 –662.

Celani G, Battacchi M, Arcidiacono L. 1999. The understanding of

the emotional meaning of facial expressions in people with

autism. J Autism Dev Disord 29:57–66.

Chadwick-Jones J. 1998. Developing a social psychology of mon-

keys and apes. East Sussex, UK: Psychology Press, Ltd.

Chapell MS. 1997. Frequency of smiling across the life span.

Percept Mot Skills 85:1326.

Chevalier-Skolnikoff S. 1973. Facial expression of emotion in

nonhuman primates. In: Ekman P, editor: Darwin and facial

expression. New York: Academic Press. p 11–90.

Cheyne JA. 1976. Development of forms and functions of smiling

in preschoolers. Child Dev 47:820 –823.

Chovil N. 1997. Facing others: a social communicative perspec-

tive on facial displays. In: JA Russell JA, Fernandez-Dols JM,

editors. The psychology of facial expression. New York: Cam-

bridge University Press. p 321–333.

Coats EJ, Feldman RS. 1996. Gender differences in nonverbal

correlates of social status. Pers Soc Psychol Bull 22:1014 –1022.

Cohn JF, Elmore M. 1988. Effect of contingent changes in moth-

ers’affective expression on the organization of behavior in

3-month-old infants. Infant Behav Dev 11:493–505.

Cohn JF, Tronick EZ. 1983. Three-month-old infants’reaction to

simulated maternal depression. Child Devel 54:185–193.

Cohn JF, Tronick EZ. 1987. Mother-infant interaction: the se-

quence of dyadic states at three, six, and nine months. Dev

Psychol 23:68 –77.

Cohn JF, Zlochower A, Lien J, Kanade T. 1999. Automated face

analysis by feature point tracking has high concurrent validity

with manual FACS coding. Psychophysiology 36:35–43.

Cosmides L, Tooby J, Barkow J. 1992. Evolutionary psychology

and conceptual integration. In: Cosmides L, Tooby J, Barkow J,

editors. The adapted mind. New York: Oxford University Press.

p3–18.

Critchley H, Daly E, Phillips M, Brammer M, Bullmore E, Wil-

liams S, Van Amelsvoort T, Robertson D, David A, Murphy D.

2000. Explicit and implicit neural mechanisms for processing of

social information from facial expressions: a functional mag-

netic resonance imaging study. Hum Brain Mapp 9:93–105.

Darwin, C. 1872/1998. The expression of the emotions in man and

animals. 3rd ed. Ekman P, editor. New York: Oxford University

Press.

Dawkins MS. 1993. Are there general principles of signal design?

Philos Trans R Soc Lond [Biol] 340:251–255.

deJong PJ. 1999. Communicative and remedial effects of social

blushing. J Nonverb Behav 23:197–217.

DePaulo BM. 1992. Nonverbal behavior and self-presentation.

Psychol Bull 111:203–243.

Deputte BL. 1994. Ethological study of yawning in primates:

quantitative analysis and study of causatiion in 2 species of

Old-World monkeys (Cercopithecus albigena and Macaca fas-

cicularis). Ethology 98:221–245.

DeWaal FBM. 1988. The communicative repertoire of captive

bonobos(Pan paniscus), compared to that of chimpanzees. Be-

haviour 106:183–251.

DeWaal FBM. 1989. Peacemaking among primates. Cambridge,

MA: Harvard University3 Press.

Dimberg U. 1997. Psychophysiological reactions to facial expres-

sions. In: Segerstrale U, Molnar P, editors. Nonverbal commu-

nication: where nature meets culture. Mahwah, NJ: Lawrence

Erlbaum Associates. p 27–60.

Duchenne GBA. 1859/1990. The mechanism of human facial ex-

pression. New York: Cambridge University Press.

Dunbar R. 1996. Gossip, grooming and the evolution of human

language. Cambridge, MA: Harvard University Press.

Dunbar R. 1998. The social brain hypothesis. Evol Anthropol

6:178 –190.

Dworkin RH. 1992. Affective deﬁcits and social deﬁcits in schizo-

phrenia: what’s what? Schizophr Bull 18:59 –64.

Eibl-Eibesfeldt I. 1989. Human ethology. New York: Aldine de

Gruyter.

FACIAL EXPRESSIONS AS ADAPTATIONS 21Schmidt and Cohn]

Ekman P. 1973. Cross-cultural studies of facial expression. In:

Ekman P, editor. Darwin and facial expression. New York:

Academic Press. p 169 –222.

Ekman P.1979. About brows: emotional and conversational sig-

nals. In: von Cranach M, Foppa K, Lepenies W, Ploog D, edi-

tors. Human ethology: claims and limits of a new discipline.

New York: Cambridge University Press. p 169 –222.

Ekman P. 1985. Telling lies: clues to deceit in the marketplace,

politics, and marriage. New York: W.W. Norton.

Ekman P, Friesen WV. 1978. Facial action coding system. Palo

Alto: Consulting Psychologists Press.

Ekman P, Friesen W. 1982. False, felt, and miserable smiles. J

Nonverb Behav 6:238 –252.

Ekman P, Keltner D. 1997. Universal facial expressions of emo-

tion: an old controversy and new ﬁndings. In: Segerstrale U,

Molnar P, editors. Nonverbal communication: where nature

meets culture. Mahwah, NJ: Lawrence Erlbaum Associates. p

27–46.

Ekman P, Rosenberg E. 1997. What the Face Reveals. New York:

Oxford University Press.

Ekman P, Friesen WV, O’Sullivan M. 1997. Smiles when lying.

In: Ekman P, Rosenberg E, editors. What the face reveals. New

York: Oxford University Press. p 201–216.

Ellis HD, Young AW. 1998. Faces in their biological and social

context. In: Young AW, editor. Face and mind. New York:

Oxford University Press. p 67–95.

Endler JA. 1993. Some general comments on the evolution and

design of animal communication systems. Philos Trans R Soc

Lond [Biol] 340:215–225.

Fessler DMT. 1999. Toward an understanding of the universality

of second order emotions: biocultural approaches to the emo-

tions. New York: Cambridge University Press. p 75–116.

Floyd K, Burgoon JK. 1999. Reacting to nonverbal expressions of

liking: a test of interaction adaptation theory. Commun Monogr

66:219 –239.

Forbes E, Cohn JF, Lewinsohn P, Moore GA. 2000. Mother-father

differences in parent and infant affect. International Confer-

ence on Infant Studies, Brighton, England.

Frank MG, Ekman P. 1997. The ability to detect deceit general-

izes across different types of high-stake lies. J Pers Soc Psychol

72:1429 –1429.

Frank MG, Ekman P, Friesen WV. 1993. Behavioral markers and

recognizability of the smile of enjoyment. J Pers Soc Psychol

64:83–93.

Fridlund A. 1994. Human facial expression: an evolutionary view.

New York: Academic Press.

Fridlund AJ. 1997. The new ethology of human facial expressions.

In: Russell J, Fernandez-Dols JM, editors. The psychology of

facial expression. New York: Cambridge University Press. p

103–129.

Friedman H, Miller-Herringer T. 1991. Nonverbal display of emo-

tion in public and in private: self-monitoring, personality, and

expressive cues. J Pers Soc Psychol 61:766 –775.

Galati D, Scherer KR, Ricci-Bitti PE. 1997. Voluntary facial ex-

pression of emotion comparing congenitally blind with nor-

mally sighted encoders. J Pers Soc Psychol 73:1363–1379.

Gangestad SW, Thornhill R. 1997. Human sexual selection and

developmental stability. In: Simpson JA, Kenrick DT, editors.

Evolutionary social psychology. Mahwah, NJ: Lawrence Erl-

baum Associates. p 169 –196.

Gazzaniga MS, Smylie CS. 1990. Hemispheric mechanisms con-

trolling voluntary and spontaneous facial expressions. J Cogn

Neurosci 2:239 –245.

Geen TR. 1992. Facial expressions in socially isolated nonhuman

primates: open and closed programs for expressive behavior. J

Res Pers 26:273–280.

Godfray HCJ, Johnstone RA. 2000. Begging and bleating: the

evolution of parent-offspring signaling. Philos Trans R Soc

Lond [Biol] 355:1581–1591.

Goodall J. 1986. The chimpanzees of Gombe. Cambridge, MA:

Cambridge University Press.

Goodmurphy C, Ovalle W. 1999. Morphological study of two hu-

man facial muscles: orbicularis oculi and corrugator supercilii.

Clin Anat 12:1–11.

Gosselin P, Kirouac G, Dore FY. 1997. Components and recogni-

tion of facial expression in the communication of emotion by

actors. In: Ekman P, Rosenberg E, editors. What the face re-

veals. New York: Oxford University Press. p 243–270.

Grafen A. 1990. Biological signals as handicaps. J Theor Biol

144:517–546.

Grafen A, Johnstone RA. 1993. Why we need ESS signalling

theory. Philos Trans R Soc Lond [Biol] 340:245–250.

Grammer K, Thornhill R. 1994. Human (Homo sapiens) facial

attractiveness and sexual selection: the role of symmetry and

averageness. J Comp Psychol 108:233–242.

Grammer K, Schiefenhovel W, Schleidt M, Lorenz B, Eibl-

Eibesfeldt I. 1988. Patterns on the face: the eyebrow ﬂash in

crosscultural comparison. Ethology 77:279 –299.

Grammer K, Filova V, Fieder M. 1997. The communication par-

adox and possible solutions: towards a radical empiricism. In:

Schmitt A, Atzwanger K, Grammer K, Schaefer K, editors. New

aspects of human ethology. New York: Plenum Press. p 91–120.

Grammer K, Honda M, Juette A, Schmitt A. 1999. Fuzziness of

nonverbal courtship communication unblurred by motion en-

ergy detection. J Pers Soc Psychol 77:487–508.

Hager JC, Ekman P. 1979. Long-distance transmission of facial

affect signals. Ethol Sociobiol 1:77–82.

Hager JC, Ekman P. 1997. The asymmetry of facial actions is

inconsistent with models of hemispheric specialization. In: Ek-

man P, Rosenberg E, editors. What the face reveals. New York:

Oxford University Press. p 40 –62.

Haidt J, Keltner D. 1999. Culture and facial expression: open-

ended methods ﬁnd more expressions and a gradient of recog-

nition. Cogn Emot 13:225–266.

Hall JA. 1984. Nonverbal sex differences: communication accu-

racy and expressive style. Baltimore: Johns Hopkins Univer-

sity Press.

Haslam N. 1997. Four grammars for primate social relations. In:

Simpson JA, Kenrick DT, editors. Evolutionary social psychol-

ogy. Mahwah, NJ: Lawrence Erlbaum Associates. p 297–316.

Hauser MD. 1996. The evolution of communication. Cambridge,

MA: MIT Press.

Hauser MD, Akre K. 2001. Asymmetries in the timing of facial

and vocal expressions by rhesus monkeys: implications for

hemispheric. specialization. Anim Behav 61:391–400.

Hess U, Banse R, Kappas A. 1995. The intensity of facial expres-

sion is determined by underlying affective state and social

situation. J Pers Soc Psychol 69:280 –288.

Hietanen JK, Surakka V, Linnankoski I. 1998. Facial electromyo-

graphic responses to vocal affect expressions. Psychophysiology

35:530 –536.

Hook-Costigan MA, Rogers LJ 1998. Lateralized use of the mouth

in production of vocalizations by marmosets. Neuropsychologia

36:1265–1273.

Huber E. 1931. Evolution of facial musculature and facial expres-

sion. Baltimore: The Johns Hopkins Press.

Humphrey NB. 1976. The social function of intellect. In: Bateson,

PPG, Hinde RA, editors. Growing points in ethology. Cam-

bridge: Cambridge University Press. p 303–317.

Izard CE. 1977. Human emotion. New York: Plenum Press.

Izard CE, Malatesta CZ. 1987. Perspectives on emotional devel-

opment: Differential emotions theory of early emotional devel-

opment. In: Osofsky JD, editor. Handbook of infant develop-

ment. New York: Wiley and Sons. p 494 –554.

Jakobs E, Manstead ASR, Fischer AH. 1999. Social motives and

emotional feelings as determinants of facial displays: the case

of smiling. Pers Soc Psychol Bull 25:424 –435.

Jancke L, Kaufmann N. 1994. Facial EMG responses to odors in

solitude and with an audience. Chem Senses 19:99 –111.

Johnstone RA. 1997. The evolution of animal signals. In: Krebs

JR, Davies NB, editors. Behavioural ecology: an evolutionary

approach. London: Blackwell Science, Ltd. p 155–178.

Jones S. 1984. Adult recipients of infant communications: a sex

difference in the salience of early “social”smiling. Infant Behav

Dev 7:211–221.

Jones SS, Raag T, Collins KL. 1990. Smiling in older infants: form

and maternal response. Infant Behav Dev 13:147–165.

22 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 44, 2001

Jones S, Collins K, Hong H-W. 1991. An audience effect on smile

production in 10-month-old infants. Psychol Sci 2:45–49.

Kanade T, Cohn JF, Tian Y. 2000. Comprehensive database for

facial expression analysis. In: Proc Fourth IEEE International

Conference on Automatic Face Gesture Recognition (FG’00)

Grenoble, France. March 2000. p 46 –53.

Kanazawa S. 1998. What facial part is important for Japanese

monkeys (Macaca fuscata) in recognition of smiling and sad

faces of humans (Homo sapiens). J Comp Psychol 112:363–370.

Katsikitis M, Pilowsky I, Innes JM. 1997. Encoding and decoding

of facial expression. J Gen Psychol 124:357–370.

Keltner D. 1997. Signs of appeasement: evidence for the distinct

displays of embarrassment, amusement and shame. In: Ekman

P, Rosenberg E, editors. What the face reveals. New York:

Oxford University Press. p. 133–160.

Keltner D, Bonanno GA. 1997. A study of laughter and dissocia-

tion: distinct correlates of laughter and smiling during bereave-

ment. J Pers Soc Pscyhol 73:687–702.

Keltner D, Buswell BN. 1997. Embarrassment: its distinct form

and appeasement functions. Psychol Bull 122:250 –270.

Keltner D, Gross JJ. 1999. Functional accounts of emotions. Cogn

Emot 13:467–480.

Keltner D, Haidt J. 1999. Social functions of emotions at four

levels of analysis. Cogn Emot 13:505–521.

Keltner D, Harker L. 1998. The forms and functions of the non-

verbal signal of shame. In: Gilbert P, Andrews B, editors.

Shame: interpersonal behavior, psychopathology, and culture.

New York: Oxford University Press. p. 78 –98.

Keltner D, Mofﬁtt TE, Stouthamer-Loeber M. 1997. Facial ex-

pressions of emotion and psychopathology in adolescent boys.

In: Ekman P, Rosenberg E, editors. What the face reveals. New

York: Oxford University Press. p 434 –452.

Kraut RE, Johnson RE. 1979. Social and emotional messages of

smiling: an ethological approach. J Pers Soc Psychol 37:1539 –

1553.

Krebs JR, Dawkins R. 1984. Animal signals: mind-reading and

manipulation. In: Krebs JR, Davies NB, editors. Behavioural

ecology. London: Blackwell Scientiﬁc Publications. p 380 –402.

Kummer H, Daston L, Gigerenzer G, Silk J. 1997. The social

intelligence hypothesis. In: Weingart, P, Mitchell S, Richerson

P, Maasen S, editors. Human by nature: between biology and

the social sciences. Mahwah, NJ: Lawrence Erlbaum Associ-

ates. p 157–179.

Kupperbusch C, Matsumoto D, Kooken K, Loewinger S, Uchida

H, Wilson-Cohn C, Yrizarry N. 1999. Cultural inﬂuences on

nonverbal expressions of emotion. In: Philippot P, Feldman RS,

Coats EJ, editors. The social context of nonverbal behavior.

New York: Cambridge University Press. p 17–44.

LaBarre W. 1947. The cultural basis of emotions and gestures. J

Pers 16:49 –68.

LaFrance M, Hecht MA. 1999. Option or obligation to smile: the

effects of power and gender on facial expression. In: Philippot,

P, Feldman RS, Coats EJ, editors. The social context of non-

verbal behavior. New York: Cambridge University Press. p

45–70.

Leonard CM, Voeller KKS, Kuldau JM. 1991. When’s a smile a

smile? Or how to detect a message by digitizing the signal.

Psychol Sci 2:166 –172.

Levenson RW, Ekman P, Friesen W. 1990. Voluntary facial action

generates emotion-speciﬁc autonomic nervous system activity.

Psychophysiology 27:363–384.

Lyons MJ, Campbell R, Plante A, Coleman M, Kamachi M, Aka-

matsu S. 2000. The Noh mask effect: vertical viewpoint depen-

dence of facial expression perception. Proc R Soc Lond Biol

267:2239 –2245.

Manstead ASR. 1991. Expressiveness as an individual difference.

In: Feldman RS, Rime B, editors. Fundamentals of nonverbal

behavior. New York: Cambridge University Press. p 285–328.

Manstead ASR, Fischer AH, Jakobs EB. 1999. The social and

emotional functions of facial displays. In: Philippot, P, Feldman

RS, Coats EJ, editors. The social context of nonverbal behavior.

New York: Cambridge University Press. p 287–313.

Massaro DW. 1998. Perceiving talking faces. Cambridge, MA:

MIT Press.

Matsumoto D, Kudoh T. 1993. American-Japanese cultural dif-

ferences in attributions of personality based on smiles. J Non-

verb Behav 17:231–243.

McAlister R, Harkness E, Nicoll J. 1998. An ultrasound investi-

gation of the lip levator musculature. Eur J Orthod 20:713–720.

McClure EB. 2000. A meta-analytic review of sex differences in

facial expression processing and their development in infants,

children, and adolescents. Psychol Bull 126:424 –453.

Meltzoff AN. 1996. The human infant as imitative generalist: a

20-year progress report on infant imitation with implications

for comparative psychology. In: Heyes CM, Galef BM, editors.

Social learning in animals: the roots of culture. San Diego, CA:

Academic Press, Inc. p 347–370.

Messinger D, Fogel A, Dickson KL.1999. What’s in a smile? Dev

Psychol 35:701–708.

Mitchell RW. 1999. Deception and concealment as strategic script

violation in great apes and humans. In: Parker ST, Mitchell

RW, Miles HL, editors. The mentalities of gorillas and orangu-

tans. New York: Cambridge University Press. p 295–315.

Morecraft RJ, Louie JL, Herrick JL, Stilwell-Morecraft KS 2001.

Cortical innervation of the facial nucleus in the non-human

primate—a new interpretation of the effects of stroke and re-

lated subtotal trauma on the muscles of facial expression.

Brain 124:176 –208.

Morris JS, Frith CD, Perrett DI, Rowland D, Young AW, Calder

AJ, Dolan RJ. 1996. A differential response in the human

amygdala to fearful and happy facial expressions. Nature 383:

812–815.

Mueser KT, Valentiner DP, Agresta J. 1997. Coping with nega-

tive symptoms of schizophrenia: patient and family perspec-

tives. Schizophr Bull 23:329 –339.

Nagashima K, Schellenberg JA. 1997. Situational differences in

intentional smiling: a cross-cultural exploration. J Soc Psychol

137:297–301.

Niemitz C, Loi M, Landerer S. 2000. Investigations on human

laughter and its implications for the evolution of hominid vi-

sual communication. Homo 51:1–18.

Nowicki SJ, Duke MP. 1994. Individual differences in the non-

verbal communication of affect: the diagnostic analysis of non-

verbal accuracy scale. J Nonverb Behav 18:9 –35.

Otta E, Lira BBP, Delevati OP, Pires CSG. 1993. The effect of

smiling and of head tilting on person perception. J Psychol

128:323–331.

Otta E, Abrosio FFE, Hoshino RL. 1996. Reading a smiling face:

messages conveyed by various forms of smiling. Percept Mot

Skills 82:1111–1121.

Parr L, deWaal FBM.1999. Visual kin recognition in chimpan-

zees. Nature 399:647–648.

Parr L, Hopkins WD, deWaal FBM.1998.The perception of facial

expressions by chimpanzees, Pan troglodytes. Evol Commun

2:1–24.

Parr L, Winslow JT, Hopkins WD, deWaal, FBM. 2000.Recogniz-

ing facial cues: Individual discrimination by chimpanzees (Pan

troglodytes) and rhesus monkeys (Macaca mulatta). J Comp

Psychol 114:47–60.

Peburn TA, Sheridan KA, Kheck NM, Hof PR, Gasdogas J, Erwin

J, Gannon PJ. 2000. Differences in the brainstem facial motor

nucleus in Erythrocebus patas and Macaca fascicularis: a qual-

itative and morphometric analysis. Am J Phys Anthropol

[Suppl] 30:247–248.

Pennock JD, Johnson PC, Manders EK, VanSwearingen JM.

1999. Relationship between muscle activity of the frontalis and

the associated brow displacement. Plast Reconstr Surg 104:

1789.

Pessa J, Zadoo V, Adrian EJ, Yuan C, Aydelotte J, Garza J.

1998a. Variability of the midfacial muscles: analysis of 50

hemifacial cadaver dissections. Plast Reconstr Surg 102:1888 –

1893.

Pessa J, Zadoo V, Garza P, Adrian EJ, Dewitt A, Garza J. 1998b.

Double or biﬁdzygomaticus major muscle: anatomy, incidence,

and clinical correlation. Clin Anat 11:310 –313.

Pike GE, Kemp RI, Towell NA, Phillips KC. 1997. Recognizing

moving faces: the relative contribution of motion and perspec-

tive information. Vis Cogn 4:409 –437.

FACIAL EXPRESSIONS AS ADAPTATIONS 23Schmidt and Cohn]

Pinker S. 1994. The language instinct. New York: Morrow.

Povinelli DJ, Eddy TJ .1996. Chimpanzees: joint visual attention.

Psychol Sci 7:129 –135.

Preuschoft S. 1992. Laughter and smile in Barbary macaques

(Macaca sylvanus). Ethology 91:220 –236.

Preuschoft S. 2000. Primate faces and facial expressions. Soc Res

67:245–271.

Preuschoft S, van Hooff JARAM. 1995. Homologizing primate

facial displays: a critical review of methods. Folia Primatol

(Basel) 65:121–137.

Provine RR. 1996. Contagious yawning and laughter: signﬁcance

for sensory feature detection, motor pattern generation, imita-

tion, and the evolution of social behavior. In: Heyes CM, Galef

BG, editors. Social learning in animals: the roots of culture.

San Diego, CA: Academic Press, Inc. p 179 –208.

Provine RR. 1997. Yawns, laughs, smiles, tickles, and talking:

naturalistic and laboratory studies of facial action and social

communication. In: Russell JA, Fernandez-Dols JM, editors.

The psychology of facial expression. New York: Cambridge Uni-

versity Press. p 158 –175.

Reeve HK, Sherman PW. 1993. Adaptation and the goals of

evolutionary research. Q Rev Biol 68:1–32.

Richardson C, Bowers D, Bauer R, Heilman K, Leonard CM.

2000. Digitizing the moving face during dynamic displays of

emotion. Neuropsychologia 38:1028 –1039.

Rinn WE. 1984. The neuropsychology of facial expression: a re-

view of the neurological and psychological mechanisms for pro-

ducing facial expressions. Psychol Bull 95:52–77.

Ross RT, Mathiesen R. 1998. Volitional and emotional supranu-

clear facial weakness. N Engl J Med 338:1515.

Russell JA, Fernandez-Dols JM. 1997. What does a facial expres-

sion mean? In: Russell JA, Fernandez-Dols JM, editors. The

psychology of facial expression. New York: Cambridge Univer-

sity Press. p 3–30.

Sakamoto S, Nameta K, Kawasaki T, Yamashita K, Shimizu A.

1997. Polygraphic evaluation of laughing and smiling in schizo-

phrenic and depressive patients. Percept Mot Skills 85:1291–

1302.

Scheifenhovel W. 1997. Universals in interpersonal interactions.

In: Segerstrale U, Molnar P, editors. Nonverbal communica-

tion: where nature meets culture. Mahwah, NJ: Lawrence Erl-

baum Associates. p 61–86.

Schmidt KL. 2000. Variation in the timing and display of the

human smile. Am J Phys Anthropol [Suppl] 30:272.

Schmidt KL, Cohn JF. 2001. Dynamic modeling of human facial

expression. Am J Phys Anthropol [Suppl] 31:132.

Schmidt KL, Johnson FYA, Allen JS. 2000. Cross-cultural anal-

ysis of eventfulness in the lives of people with schizophrenia.

Schizophr Bull 26:825–834.

Scriba H, Stoeckli SJ, Pollack A, Veraguth D, Fisch U. 1999.

Objective evaluation of normal facial function. Ann Otol Rhinol

Laryngol 108:641–644.

Shor RE. 1978. The production and judgment of smile magnitude.

J Gen Psychol 98:79 –96.

Silk JB, Kaldor E, Boyd R. 2000. Cheap talk when interests

conﬂict. Anim Behav 59:423–432.

Smith WM. 1998. Hemispheric and facial asymmetry: faces of

academe. J Cog Neurosci 10:663–667.

Stal P, Eriksson PO, Eriksson A, Thornell, LE. 1987. Enzyme-

histochemical differences in ﬁbre-type between the human ma-

jor and minor zygomatic and the ﬁrst dorsal interosseus mus-

cles. Arch Oral Biol 32:833–841.

Strahan E, Conger AJ. 1998. Social anxiety and its effects on

performance and perception. J Anx Disord 12:293–305.

Sullivan DG, Masters RD. 1991. Facial displays and political

leadership: some experimental ﬁndings. In: Schubert G, Mas-

ters RD, editors. Primate politics. Carbondale: Southern Illi-

nois University Press. p 188 –206.

Surakka V, Hietanen JK. 1998. Facial and emotional reactions to

Duchenne and non-Duchenne smiles. Int J Psychophysiol 29:

23–33.

Thornhill R, Gangestad S. 1994. Fluctuating asymmetry and

human sexual behavior. Psychol Sci 5:297–302.

Tian Y, Kanade T, Cohn JF. 2001. Recognizing action units for

facial expression analysis. IEEE Transactions on Pattern Anal-

ysis and Machine Intelligence 23:97–115.

Tomasello M, Call J. 1997. Primate cognition. New York: Oxford

University Press.

Trivers R. 1971. The evolution of reciprocal altruism. Q Rev Biol

46:35–57.

Trivers R. 1974. Parent-offspring conﬂict. Am Zool 14:249 –264

van Hooff JARAM. 1972. A comparative approach to the phylog-

eny of laughter and smiling. In: Hinde RA, editor. Non-verbal

communication. New York: Oxford University Press. p 209 –

242.

VanSwearingen JM, Cohn JF, Bajaj-Luthra A. 1999. Speciﬁc

impairment of smiling increases the severity of depressive

symptoms in patients with facial neuromuscular disorders.

Aesthet Plast Surg 23:416 –423.

Vuilleumier P. 2000. Faces call for attention: evidence from pa-

tients with visual extinction. Neuropsychologia 38:693–700.

Wagner H, Lee V. 1999. Facial behavior alone and in the presence

of others. In: Philippot P, Feldman RS, Coats EJ, editors. The

social context of nonverbal behavior. New York: Cambridge

University Press. p 262–286.

Yik MSM, Russell JA. 1999. Interpretation of faces: a cross-

cultural study of a prediction from Fridlund’s theory. Cogn

Emot 13:93–104.

Young AW, Newcombe F, deHaan EHF, Small M, Hay DC. 1998.

Dissociable deﬁcits after brain injury. In: Young AW, editor.

Face and mind. New York: Oxford University Press. p 181–208.

Zahavi A. 1993. The fallacy of conventional signaling. Philos

Trans R Soc Lond [Biol] 340:227–230.

Zahavi A, Zahavi A. 1997. The handicap principle. New York:

Oxford University Press.

24 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 44, 2001

İnsansı robot ile bazı Türkçe işaret dili ifadelerinin gerçekleştirilmesiDemonstration of some Turkish sign language expressions

Article

Dec 2022

Mehmet GÜL

Dünya genelinde 466 milyon Türkiye’de ise yaklaşık 1.5 milyon işitme ve konuşma engelli bireyin kendi aralarında ve diğer bireyler ile iletişimlerini sağlamak için kullandıkları dil işaret dilidir. El, yüz ve vücut mimikleri ile gerçekleştirilen bu dil, ülkeden ülkeye ve konuşulduğu ülkelerde ise bölgeden bölgeye değişiklik gösterebilen dinamik bir dildir. Bireyin daha küçük yaşta işaret dili öğrenmesi, bireyin hem bilişsel ve entelektüel gelişimi hem de akademik gelişimi açısından önemli olması bu dilin önemini gözler önüne sermektedir. Bu nedenle işaret dili öğrenimi konusunda teknolojinin kullanılmasına dönük geliştirilen bir dizi çalışma bulunmaktadır. Geliştirilen insansı robotların işaret dili öğreniminde başarılı bir şekilde icra edildiği kanıtlandı. Ülkemizde işaret dili öğrenimi üzerine geliştirilen herhangi bir insansı robotun bulunmaması ve bu alanda yapılan güncel çalışmaların literatürde yer edinmesi üzerine Türkçe İşaret Dili ifadelerini gerçekleştiren insansı robot çalışması yapıldı. Çalışma kapsamında insansı robotun geliştirme aşamaları detaylı bir şekilde tartışılmaktadır.

Systematic Review of Affective Computing Techniques for Infant Robot Interaction

Article

Full-text available

Mar 2023

Research studies on social robotics and human-robot interaction have gained insights into factors that influence people’s perceptions and behaviors towards robots. However, adults’ perceptions of robots may differ significantly from those of infants. Consequently, extending this knowledge also to infants’ attitudes toward robots is a growing field of research. Indeed, infant-robot interaction (IRI) is emerging as a critical and necessary area of research as robots are increasingly used in social environments, such as caring for infants with all types of disabilities, companionship, and education. Although studies have been conducted on the ability of robots to positively engage infants, little is known about the infants’ affective state when interacting with a robot. In this systematic review, technologies for infant affective state recognition relevant to IRI applications are presented and surveyed. Indeed, adapting techniques currently employed for infant’s emotion recognition to the field of IRI results to be a complex task, since it requires timely response while not interfering with the infant’s behavior. Those aspects have a crucial impact on the selection of the emotion recognition techniques and the related metrics to be used for this purpose. Therefore, this review is intended to shed light on the advantages and the current research challenges of the infants’ affective state recognition approaches in the IRI field, elucidates a roadmap for their use in forthcoming studies as well as potentially provide support to future developments of emotion-aware robots.

InMyFace: Inertial and Mechanomyography-Based Sensor Fusion for Wearable Facial Activity Recognition

Preprint

Full-text available

Feb 2023

Recognizing facial activity is a well-understood (but non-trivial) computer vision problem. However, reliable solutions require a camera with a good view of the face, which is often unavailable in wearable settings. Furthermore, in wearable applications, where systems accompany users throughout their daily activities, a permanently running camera can be problematic for privacy (and legal) reasons. This work presents an alternative solution based on the fusion of wearable inertial sensors, planar pressure sensors, and acoustic mechanomyography (muscle sounds). The sensors were placed unobtrusively in a sports cap to monitor facial muscle activities related to facial expressions. We present our integrated wearable sensor system, describe data fusion and analysis methods, and evaluate the system in an experiment with thirteen subjects from different cultural backgrounds (eight countries) and both sexes (six women and seven men). In a one-model-per-user scheme and using a late fusion approach, the system yielded an average F1 score of 85.00% for the case where all sensing modalities are combined. With a cross-user validation and a one-model-for-all-user scheme, an F1 score of 79.00% was obtained for thirteen participants (six females and seven males). Moreover, in a hybrid fusion (cross-user) approach and six classes, an average F1 score of 82.00% was obtained for eight users. The results are competitive with state-of-the-art non-camera-based solutions for a cross-user study. In addition, our unique set of participants demonstrates the inclusiveness and generalizability of the approach.

To Know Them, Remove Them: An Outer Methodological Approach to Biophysics and Humanities

Preprint

Apr 2021

Arturo Tozzi

Set theory faces two difficulties: formal definitions of sets/subsets are incapable of assessing biophysical issues; formal axiomatic systems are complete/inconsistent or incomplete/consistent. To overtake these problems reminiscent of the old-fashioned principle of individuation, we provide formal treatment/validation/operationalization of a methodological weapon termed “outer approach” (OA). The observer’s attention shifts from the system under evaluation to its surroundings, so that objects are investigated from outside. Subsets become just “holes” devoid of information inside larger sets. Sets are no longer passive containers, rather active structures enabling their content’s examination. Consequences/applications of OA include: a) operationalization of paraconsistent logics, anticipated by unexpected forerunners, in terms of advanced truth theories of natural language, anthropic principle and quantum dynamics; b) assessment of embryonic craniocaudal migration in terms of Turing’s spots; c) evaluation of hominids’ social behaviors in terms of evolutionary modifications of facial expression’s musculature; d) treatment of cortical action potentials in terms of collective movements of extracellular currents, leaving apart what happens inside the neurons; e) a critique of Shannon’s information in terms of the Arabic thinkers’ active/potential intellects. Also, OA provides an outer view of a) humanistic issues such as the enigmatic Celestino of Verona’s letter, Dante Alighieri’s “Hell” and the puzzling Voynich manuscript; b) historical issues such as Aldo Moro’s death and the Liston/Clay boxing fight. Summarizing, the safest methodology to quantify phenomena is to remove them from our observation and tackle an outer view, since mathematical/logical issues such as selective information deletion and set complement rescue incompleteness/inconsistency of biophysical systems.

Cada cabeza su sentencia

Book

Full-text available

Mar 2016

Thiago Moraes

Desarrollamos el modelo teórico M8 - Modelo de ocho dispositivosadaptativos del cerebro social ? donde incluímos el módulo de puniciónentre los siete que ya estaban previstos en modelos anteriores de cogniciónsocial (Kanazawa, 2004 ). Definimos que el Big five ? 5 grandes rasgos de personalidad , que resultan de la acción estratégica de los módulos . Para testear la existencia del módulo cognitivo de punición, estudiamos elinterés popular por la pena de muerte y la punición en el mundo, conénfasis en los Estados Unidos (2004-2013). Buscamos correlaciones entrecuatro fuentes: 1) datos del Gallup institute ; 2) dados del trabajo de PeterJason Rentfrow (2010); 3) datos del The Democracy Ranking of the Quality of Democracy sobre la calidad de la democracia en los países del mundo y4) datos de Big Data creados a partir de la herramienta Google Trends .Trazamos 4 hipótesis para realizar el estudio: 1) la influencia del M8 (ymás precisamente del módulo de punición), el interés por la punición y la pena de muerte es universal y no varía mucho en su distribución, incluso enescenarios estruturales distintos; 2) existe una fuerte correlación entre altosniveles de extroversión e interés por la pena de muerte en los EstadosUnidos; 3) hay una sólida relación entre interés por la pena de muerte einterés en campañas electorales en los Estados Unidos; 4) existe unatendencia para que niveles similares en cuanto al interés por la pena demuerte aparezca en grupos con preferencias distintas (política y religión),en tanto haya semejanza en cuanto a la distribución de los rasgos de la personalidad (con variaciones en los mismos rasgos). Nuestros resultadosapuntan que la primera y la segunda hipótesis son partidarias tanto en países de alto como de bajo s core democrático, hay incidencia de interés por la punición y la pena de muerte; en los Estados Unidos hay una fuertísima correlación entre interés por la pena de muerte y la extroversión( r = 0,472; p = 0,001). La tercera hipótesis tiene también alta adhesión,existe una correlación altísima entre interés electoral e interés por la penade muerte ( r = 0,336; p < 0,001). Además de eso, encontramos un patrón deestacionalidad de interés consistente, inédito en la literatura. La cuartahipótesis muestra relativa adhesión, puesto que escenarios donde haydistribución de personalidad semejante (con variaciones en los mismosrasgos), los individuos tienden a mostrar niveles de interés semejante por la pena de muerte. La corroboración de las hipótesis 1, 2 y 3 dan base para laexistencia de un módulo punitivo propio del modelo M8 de cognición social.

The Hitchhiker’s Guide to a Credible and Socially Present Robot: Two Meta-Analyses of the Power of Social Cues in Human–Robot Interaction

Article

Full-text available

Jan 2023

Social cues have been construed as an important concept in human–robot interaction, as they can be manipulated to reflect robots’ perceived genders, personalities, emotions, identities, and so on. This study seeks to understand the overall effects of social cues and applies two meta-analyses to explore a hierarchy of social cues that elicits different degrees of users’ social responses. A total of 25 and 44 effect sizes were calculated to represent the respective magnitudes of the effects of social cues on users’ social presence (N = 2498) and trust in social robots (N = 4147). Results suggested that although the overall effects of social cues were small, manipulating social robots’ facial and kinetic cues can induce medium-to-large-sized effects on users’ social presence and trust. In addition, the overall positive effect sizes of social cues indicated that designing humanlike, natural, and lifelike cues was effective in evoking users’ social presence and trust in social robots. The results of the two meta-analyses can contribute to the theoretical implications of the Computers are Social Actors paradigm and the practical and methodological design of human–robot interaction.

Clinical Assessments of Fracture Healing and Basic Science Correlates: Is There Room for Convergence?

Article

Full-text available

Dec 2022
Curr Osteoporos Rep

Purpose of Review The purpose of this review is to summarize the clinical and basic science methods used to assess fracture healing and propose a framework to improve the translational possibilities. Recent Findings Mainstays of fracture healing assessment include clinical examination, various imaging modalities, and assessment of function. Pre-clinical studies have yielded insight into biomechanical progression as well as the genetic, molecular, and cellular processes of fracture healing. Efforts are emerging to identify early markers to predict impaired healing and possibly early intervention to alter these processes. Summary Despite of the differences in clinical and preclinical research, opportunities exist to unify and improve the translational efforts between these arenas to develop and optimize our ability to assess and predict fracture healing, thereby improving the clinical care of these patients.

Exploring the Intricacies of Human Character: A Cutting-Edge Platform for Personality Detection

Article

Jun 2023

Abhinav T V

This research introduces a platform for personality detection that leverages artificial intelligence (AI) and machine learning (ML) principles. By employing facial recognition, eye detection, body language analysis, speech recognition, and heart rate detection, the platform aims to accurately discern and analyse an individual's personality traits. The platform analyses various cues emitted by an individual, including facial expressions, micro expressions, eye movements, postures, gestures, speech patterns, and heart rate fluctuations. Through the integration of these AI and ML techniques, the platform offers comprehensive insights into an individual's emotional state, communication style, and personality characteristics. This technology has promising applications in personalized marketing, customer analysis, mental health assessment, and team dynamics optimization. By harnessing the power of AI and ML, this platform has the potential to revolutionise industries and pave the way for tailored experiences and enhanced human interactions. Preface: This research endeavours to present a ground-breaking idea for copyright protection. Recognizing the importance of safeguarding intellectual property in the digital age, this study emphasizes the need for a comprehensive and robust approach to copyright enforcement. To realise the practical implementation of this idea, a well-developed research team with expertise in technology, law, and digital rights is essential. Furthermore, successful integration and collaboration with renowned IT companies are vital to harness their resources, technical capabilities, and industry influence. By bringing together the collective knowledge and expertise of such a team and leveraging the support of renowned IT companies, this research aims to pave the way for a more effective and efficient copyright protection framework that ensures the rights of creators and innovators in the modern digital landscape.

Ballistic Timing of Smiles is Robust to Context, Gender, Ethnicity, and National Differences

Conference Paper

Oct 2022

Social Petworks

Chapter

Nov 2022

Andreas Ervik

Chapter 5, “Social Petworks,” examines how cuteness diverts. Cuteness is rendered a process of retaining juvenile features and behavior into adulthood. The chapter analyzes some exceptionally famous social media profiles, including the Facebook celebrity dog Boo. Social networks are framed as social petworks, where users share animal selfies. This frame opens for exploring how technological development is predicated on and continues mammalian evolutionary processes, as technology is cutened and humans are domesticated to screen-bound living with algorithm-determined feeds. Cuteness comes with a range of creative potential, examined here through theorizing YouTube videos of birds parroting pop songs. Cuteness allows mammals to turn evolution from struggle to snuggle for survival, to increased investment into comforting and soothing distress. The intensity of this in contemporary social petworks is examined through care-giving of the disabled celebrity cat Lil Bub. The chapter concludes by offering a novel way of understanding what is known as cute aggression, a common yet paradoxical desire to squeeze or crush that which is perceived as cute without harmful intent.

The evolution of reciprocal altruism

Article

Full-text available

Jan 1971

Robert Trivers

Peacemaking among Primates

Book

Dec 1989

The ape legacy: the evolution of Machiavellian intelligence and anticipatory interactive planning

Chapter

Mar 1995

Richard W. Byrne

There is a growing view that intelligence evolved as a product of social interdependence. The unique development of human intelligence was probably linked to the use of spoken language, but language itself evolved in the context of social interaction, and in its development it has shaped - and been shaped by - social institutions. Taking as their starting-point the social production of intelligence and of language, scholars across a range of disciplines are beginning to rethink fundamental questions about human evolution, language and social institutions. This volume brings together anthropologists, linguists, primatologists and psychologists, all working on this new frontier of research.

Deception and concealment as strategic script violation in great apes and humans

Chapter

Aug 1999

Robert Mitchell

Research on the mental abilities of chimpanzees and bonobos has been widely celebrated and used in reconstructions of human evolution. In contrast, less attention has been paid to the abilities of gorillas and orangutans. This 1999 volume aims to help complete the picture of hominoid cognition by bringing together the work on gorillas and orangutans and setting it in comparative perspective. The introductory chapters set the evolutionary context for comparing cognition in gorillas and orangutans to that of chimpanzees, bonobos and humans. The remaining chapters focus primarily on the kinds and levels of intelligence displayed by orangutans and gorillas compared to other great apes, including performances in the classic domains of tool use and tool making, imitation, self-awareness, social communication and symbol use. All those wanting more information on the mental abilities of these sometimes neglected, but important primates will find this book a treasure trove.

What does a facial expression mean?

Chapter

Mar 1997

This reference work provides broad and up-to-date coverage of the major perspectives - ethological, neurobehavioral, developmental, dynamic systems, componential - on facial expression. It reviews Darwin's legacy in the theories of Izard and Tomkins and in Fridlund's recently proposed Behavioral Ecology theory. It explores continuing controversies on universality and innateness. It also updates the research guidelines of Ekman, Friesen and Ellsworth. This book anticipates emerging research questions: what is the role of culture in children's understanding of faces? In what precise ways do faces depend on the immediate context? What is the ecology of facial expression: when do different expressions occur and in what frequency? The Psychology of Facial Expressions is aimed at students, researchers and educators in psychology anthropology, and sociology who are interested in the emotive and communicative uses of facial expression.

Recognizing facial cues: Individual discrimination by chimpanzees (Pan troglodytes) and rhesus monkeys (Macaca mulatto)

Article

Jan 2000
J Comp Psychol

Faces are one of the most salient classes of stimuli involved in social communication. Three experiments compared face-recognition abilities in chimpanzees (Pan troglodytes) and rhesus monkeys (Macaco mulatto). In the face-matching task, the chimpanzees matched identical photographs of conspecifics' faces on Trial 1, and the rhesus monkeys did the same after 4 generalization trials. In the individual-recognition task, the chimpanzees matched 2 different photographs of the same individual after 2 trials, and the rhesus monkeys generalized in fewer than 6 trials. The feature-masking task showed that the eyes were the most important cue for individual recognition. Thus, chimpanzees and rhesus monkeys are able to use facial cues to discriminate unfamiliar conspecifics. Although the rhesus monkeys required many trials to learn the tasks, this is not evidence that faces are not as important social stimuli for them as for the chimpanzees.

Darwin and cross-cultural studies of facial expression

Article