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Evolution and Human Sexuality
Peter B. Gray*
Department of Anthropology, University of Nevada, Las Vegas, Las Vegas, NV 89154-5003
KEY WORDS evolution; sexual behavior; sexual function; sexual selection; hominin;
Homo; life course; life history; genetics; neuroendocrine; polygyny
ABSTRACT The aim of this review is to put core fea-
tures of human sexuality in an evolutionary light.
Toward that end, I address five topics concerning the
evolution of human sexuality. First, I address theoretical
foundations, including recent critiques and develop-
ments. While much traces back to Darwin and his view
of sexual selection, more recent work helps refine the
theoretical bases to sex differences and life history allo-
cations to mating effort. Second, I consider central mod-
els attempting to specify the phylogenetic details
regarding how hominin sexuality might have changed,
with most of those models honing in on transitions from
a possible chimpanzee-like ancestor to the slightly polyg-
ynous and long-term bonded sociosexual partnerships
observed among most recently studied hunter-gatherers.
Third, I address recent genetic and physiological data
contributing to a refined understanding of human sex-
uality. As examples, the availability of rapidly increasing
genomic information aids comparative approaches to dis-
cern signals of selection in sexuality-related phenotypes,
and neuroendocrine studies of human responses to sex-
ual stimuli provide insight into homologous and derived
mechanisms. Fourth, I consider some of the most recent,
large, and rigorous studies of human sexuality. These
provide insights into sexual behavior across other
national samples and on the Internet. Fifth, I discuss
the relevance of a life course perspective to understand-
ing the evolution of human sexuality. Most research on
the evolution of human sexuality focuses on young
adults. Yet humans are sexual beings from gestation to
death, albeit in different ways across the life course, and
in ways that can be theoretically couched within life his-
tory theory. Am J Phys Anthropol 57:94–118,
2013. V
C2013 Wiley Periodicals, Inc.
The aim of this review is to put core features of
human sexuality in an integrative, evolutionary light.
Given the central importance of sexuality to the evolu-
tionary imperative—reproductive success—there is
strong theoretical impetus for understanding how evo-
lution has shaped human sexuality in the past, and for
how the influence of past selective forces continue to
manifest in the present. For biological anthropologists,
interdisciplinary social and biomedical scientists, and
even wider audiences, questions about human sexual-
ity can be among the most compelling and meaningful,
another reason to try informing answers to fundamen-
tal questions of human sexuality with the most
scientifically supported and current views. An under-
standing of human sexuality is central to topics as diverse
as predicting the dynamics of a sexually transmitted infec-
tion (STI) outbreak to the reasons why people pour so
much of their time and resources into mating effort.
Human sexuality, as much as any other topical focus
within biological anthropology, warrants ongoing evolu-
tionary scrutiny.
For several reasons, this is arguably an opportune
time for providing a current overview of the evolution of
human sexuality. For one, many of the foundational the-
oretical and empirical touchstones in biological anthro-
pology that focus on the evolution of human sexuality
are dated. Darwin’s (1871) seminal contributions con-
tinue to shape our thinking regarding sexual selection,
sex differences, and evolutionary models of human mat-
ing. So too do important contributions from the 1970s
and 1980s such as Symons (1979) Evolution of Human
Sexuality and Hrdy’s (1981) The Woman that Never
Evolved. Of more recent and prominent works focused
on the evolution of human sexuality, however, many
draw heavily from evolutionary psychology, such as
David Buss’ (2003) The Evolution of Desire. The field
can benefit from a current overview of the evolution of
human sexuality that integrates many of the disparate
strands of work and that helps orient ongoing scholar-
ship. If the field does not do this, others will fill that
gap, leaving open the potential for best-selling but theo-
retically and empirically distorted works such as Ryan
and Jetha’s (2010) Sex at Dawn to shape how people
think human sexuality evolved.
A second reason for a review is new theoretical and
empirical work on the evolution of human sexuality. The
theoretical foundations to sex differences in mating
effort, for example, can also reflect demographic factors
that had been less appreciated in previous theoretical
formulations. New data from neuroimaging, hormonal
studies, and genetics and genomics contribute to an
enriched understanding of the mechanisms of human
sexual behavior. The availability of more recent sex sur-
vey data drawing upon large, international, and even
Internet-based content helps provide new insights into
the patterning of human mating and interest in sexual
stimuli. How these new lines of evidence fit with other
lines of evidence and evolutionary theory is worthy of
investigation. New theory and data could potentially
transform, or slightly modify, or even affirm earlier per-
spectives regarding the evolutionary foundations of
*Correspondence to: Peter B. Gray; Department of Anthropology,
University of Nevada, Las Vegas, 4505 Maryland Parkway, Box 455003,
Las Vegas, NV 89154-5003. E-mail: peter.gray@unlv.edu
DOI: 10.1002/ajpa.22394
Published online in Wiley Online Library
(wileyonlinelibrary.com).
Ó2013 WILEY PERIODICALS, INC.
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 57:94–118 (2013)
human sexuality. As the lead author of a (2013) book—
Evolution and Human Sexual Behavior—that was
designed to be integrative, accessible, and current, I
bring to this review insights from that work; a major
reason why I coauthored that book was to fill a per-
ceived need for a resource that pulled together all of
these strands under one cover for teaching and wider
scholarly purposes.
In the following review, I address five topics concern-
ing the evolution of human sexuality. These are theoreti-
cal foundations, models of hominin sexuality, recent
genetic and physiological data, recent large and rigorous
studies of human sexual behavior, and the relevance of a
life course perspective to human sexuality. These topics
do not exhaust the evolutionary-informed scope of
human sexuality. However, these are areas in which
there have been updated theoretical contributions and
arguably considerable empirical advances, making this a
good time to focus on them. The present review can also
serve as a touchstone to continued interdisciplinary
debates concerning evolution and human sexuality. In
the course of this review, I also highlight hot areas for
scholarly focus; to forecast two examples, the genetic
basis to hominin shifts in sexuality remains poorly speci-
fied, and juvenile sexuality has been understudied rela-
tive to that of young nulliparous adults.
THEORETICAL FOUNDATIONS
OF HUMAN SEXUALITY
The classics: Darwin, Bateman, Trivers, and
Clutton-Brock and Parker
Like so many aspects of evolutionary theory, we begin
the discussion of the evolutionary foundations of human
sexuality with Charles Darwin. His (1871) The Descent
of Man, and Selection in Relation to Sex advanced the
concept of sexual selection, along with emphases on
intrasexual (competition within members of the same
sex) and intersexual selection (competition and coordina-
tion between the sexes). To account for traits such as the
peacock’s train, Darwin suggested that even if a trait
might have apparent survival costs, it could still be
favored by selection if it enhanced mating success. Dar-
win catalogued many examples, and emphasized roles of
male–male competition and female choice as the proc-
esses of sexual selection giving rise to many differences
between the sexes (or sexual dimorphisms), such as the
large antlers of many male deer.
Angus Bateman (1948) conducted classic experiments
with captive fruit flies. He ran a number of trials in
which he mixed multiple females and males together in
glass jars to determine patterns of mating and reproduc-
tive success. He found that males had higher variance in
reproductive success than females. He also found that
the number of mates appeared to mediate sex differences
in reproductive output. For trials 5 and 6 in his experi-
ments, females had similar reproductive success whether
they had mated with one, two, or three males, whereas
males had more offspring the more mates they had. Tri-
als 1–4 also showed, however, that females appeared to
have higher reproductive output if mating with two
males, although that finding garnered less attention.
Overall, Bateman’s observations were consistent with
the idea that the ultimate constraint on female repro-
ductive success tends to be access to sufficient resources
like food, whereas for males the ultimate constraint on
reproductive success tends to be access to reproductive
females. Bateman suggested the sex differences in repro-
ductive constraint originated with sex differences in
gamete size, with females having the larger, more sessile
gametes, and males more mobile and smaller gametes.
Robert Trivers (1972) suggested that relative parental
investment was a more meaningful factor than gamete
size in accounting for sex differences in reproductive
constraint. If females provide more parental investment
than males, as during gestation and lactation among
placental mammals, then females will be the reproduc-
tively limiting sex. Accordingly, females will be careful
to exert choice, and males to compete among themselves
for access to females. In sex role reversal species, by con-
trast, males providing more parental care can become
the choosier sex over whom females compete. Among
jacanas and phalaropes, for example, males provide
more parental care, and females are larger and more col-
orful (see, e.g., Reynolds, 1987).
However, Clutton-Brock and Parker (1991) pointed out
exceptions to Trivers’ framework such as mouth-
brooding frogs, in which males might provide more
parental care than females, and yet females were none-
theless the reproductively limiting sex. They also
pointed out the difficulties of measuring relative paren-
tal investment. Clutton-Brock and Parker (1991) thus
advocated for basing sex differences with respect to
potential reproductive rates: if females have slower
reproductive rates than males within a species, then
females will exert more choice, and males exhibit more
intrasexual competition. Considerable bodies of empiri-
cal work on nonhuman animals (Andersson, 1994) and
humans (Geary, 2010) lend support to general expecta-
tions of sexual selection theory. As an example, across
mammals, male traits that seem to function to enhance
intrasexual competition are more common than are such
female traits, consistent with theoretical arguments that
females tend to be the reproductive limiting sex over
which males compete.
Challenges to the classics and greater
appreciation for demographic influences
These frameworks have been subject to critique, with
most debate centering on the methods and interpreta-
tion of Bateman’s fruit fly experiments. Gowaty et al.,
(2012) critiqued Bateman’s methods on statistical
grounds, stating that assumptions of the statistical tests
used were not met; she also conducted an attempt at
replicating his findings, but did not present the data (for
related statistical reasons) to determine whether or not
she found support for his work. In review of several
recent human studies, Brown et al. (2009) found that,
consistent with Bateman’s principle (that males will
have higher variance in reproductive success than
females in species where males exhibit greater mating
competition), human males overall did have greater var-
iance in reproductive success than females. However,
Brown et al. (2009) also noted that this sex difference
was observed in polygynous but not monogamous soci-
eties. An evolutionary-guided look at the specific cases
also suggests that among hunter-gatherer societies (Aka,
!Kung, Ache, Hadza), most of which have low rates of
polygyny, males tend to have higher reproductive suc-
cess than females. In a different review of human stud-
ies, which had partial overlap with Brown et al. in the
EVOLUTION AND HUMAN SEXUALITY 95
American Journal of Physical Anthropology
societies sampled, Betzig (2012) also found that males
had higher variance in reproductive success than
females. Betzig also showed that male reproductive skew
was most pronounced in socially stratified intensive agri-
cultural societies such as the Inka.
Hanna Kokko and colleagues have also deepened our
theoretical understanding of sexuality. She notes that
actual, not potential, reproductive rates are what mat-
ters. That simple observation reminds us to consider the
availability of potential mates (just because males may
gain higher reproductive success than females if having
more mates does not guarantee that mates are avail-
able). Males have lower parental certainty than females,
and fewer males tend to reproduce than females, factors
that result in males gaining fewer benefits and higher
costs to parental care than females (Kokko and Jenn-
ions, 2003, 2008a). These processes may account for the
origins of as well as positive reinforcement of sex differ-
ences in mating and parenting effort. Additionally, how-
ever, demographic factors play an important role in
specifying the gradient of sexual selection (Kokko and
Jennions, 2008b). As an example, if males are subject to
greater mortality as a result of mating competition, then
that lightens to some degree the mating competition the
fewer, surviving males face.
Such critiques have not overturned fundamental prin-
ciples of sexual selection. Kokko’s work refines our theo-
rizing (e.g., to push from sexual selection to parenting
rather than from parenting to sexual selection), but ulti-
mately anticipates sex differences in mating and paren-
tal care observed in the natural world. Additionally,
these kinds of critiques and extensions point to the
importance of socioecological context and demography in
accounting for species and population differences in sex-
ual selection pressures. In populations with more heav-
ily biased sex ratios toward females, we might expect
more female–female competition and enhanced male
choice (e.g., Guttentag and Secord, 1983). Research
shows that in many primate species, including humans,
female reproductive success is highly attuned to varia-
tion in energetic factors such as food availability. Indeed,
related theoretical models help situate socioecological
and demographic considerations: Emlen and Oring
(1977) pointed to the importance of resource distribu-
tions in accounting for variable mating dynamics, and
Mitani et al. (1996) suggested that the operational sex
ratio (ratio of breeding males:females) better represented
the gradient of sexual selection than the actual adult
sex ratio. All said, current sexual selection theory aims
to account for the evolutionary past that shaped over-
arching patterns of sexual selection among species
(including humans), as well as inform an understanding
of population variation in sexual selection pressures.
Female promiscuity, mate choice, and
female–female competition
Part of the challenge to principles of sexual selection
theory stems from data on female mating patterns.
While Darwin and other theoreticians highlighted the
relevance of female choice, how that played out in the
natural world expanded theoretical imaginations. The
advent of genetically based paternity testing among
many avian field studies revealed that a sizable and
variable fraction of offspring were sired by individuals
other than a social partner (Birkhead, 2000). Even
though over 90% of avian species tend to be socially
monogamous (lekking species such as peacocks and tur-
keys among those exceptions), it became clear that social
and sexual relationships were not identical. These obser-
vations may have helped raise more questions about the
traits females sought in a prospective mate, including
how this could give rise to mating with multiple part-
ners (Clutton-Brock and McAuliffe, 2009; Geary, 2010).
Across primates, current theorizing suggests female
mate choice is oriented toward protection of herself and
her offspring. Protection may be useful against would-be
predators, but also against would-be harmful males. In
species in which females mate with multiple males, such
as chimpanzees and rhesus monkeys, Hrdy (1981) has
suggested this represents paternity confusion, designed
to make all males be kinder toward the female and her
offspring. For some species with long-term sociosexual
bonds and paternal care, including primarily small-
bodied South American owl monkeys, tamarins, and
marmosets, females may benefit from male support but
evidence is lacking that females use cues of male invest-
ment (see Dixson, 2012). Females of various species may
also seek to mate with males providing complementary
(e.g., in MHC system) or “good” (e.g., beneficial alleles
against local infectious disease ecology) genes. Since few
primate species have males that provide food resources
(some callitrichid monkeys of South America exceptions)
or have a sexual division of labor, there are no strong
nonhuman primate comparisons of females choosing
males based on economic (e.g., food acquisition and pro-
visioning) criteria.
Although Darwin emphasized female choice and male–
male competition, it is recognized that female–female
competition and male choice occur (Low, 2000; Geary,
2010; Stockley and Bro-Jorgensen, 2011). Given that
female reproductive success is often closely tied to
resources such as food, models of female–female competi-
tion, and cooperation feature reproductively relevant
resources such as ripe fruits among chimpanzees or
resource-bearing males in human agricultural societies.
Studies of nonhuman primates reveal that higher-
ranking female social primates may benefit by having
more surviving offspring and offspring whose reproduc-
tive careers are accelerated; this could be due to prefer-
ential food access often available to higher-ranking
females (reviewed in Pusey, 2012). At the same time,
potential female rank-related variance in female repro-
ductive success should not be oversold; lower-ranking
females may engage in alternative foraging strategies,
resulting in no net differences in reproductive success,
as is also frequently found in primate field studies
(Pusey, 2012). In humans, much of the female–female
competition literature has highlighted competition over
acquiring and maintaining desirable mates (Campbell,
1999; Archer and Coyne, 2005; Geary, 2010). Further,
reproductive competition can lead to attempts to channel
limited resources toward one’s own rather than another
women’s children, as observed with cowife competition
(Jankowiak et al., 2005). As for male choice, even in
multimale, multifemale species such as chimpanzees or
rhesus monkeys, males may still attempt to channel
their mating effort toward more desirable females, such
as maximally fertile females; less fertile and experienced
adolescent females may be relatively shunned as mates,
for example (Manson, 2011; Muller et al., 2006). For
human resource intensive strategies, males may seek to
allocate their limited mating effort toward more desira-
ble mates; that can include preferences for younger
96 P.B. GRAY
American Journal of Physical Anthropology
females of high reproductive value, with whom a man
might have multiple children in a long-standing socio-
sexual relationship, as well as other aspects of attrac-
tiveness indicative of health and relationship
compatibility.
Sexual conflict
One theme of the evolution of mating strategies is
that female and male strategies variably conflict (Chap-
man et al., 2003). That degree of conflict may be mini-
mized in species and contexts in which females and
males form long-term reproductive relationships. If
females and males depend upon each other to maximize
their reproductive success, then they may exhibit less
competition over allocation of resources to offspring or
over competition with other possible mates. Conversely,
when females and males have markedly different mating
strategies, then conflicts between female and male prior-
ities may be more likely. In this vein, an extreme exam-
ple of sexual conflict is sexually selected infanticide, as
observed in several species of one-male polygynous
groups such as gorillas and langur monkeys (Hausfater
and Hrdy, 1984; Van Schaik and Janson, 2000). A male
with a limited social tenure in a polygynous group may
seek to kill nursing infants of a mother to accelerate her
return to fecundity and the possibility of having off-
spring with her; for her part, a female with an infant
may actively attempt to avoid infanticide by a new male,
but if unsuccessful may mate with the new, infanticidal
male.
Sexual conflicts of interest impact other aspects of sex-
ual selection. Male sexual coercion can constrain female
choice (Smuts, 1992; Muller and Wrangham, 2009).
Whereas a female might seek to mate with a given male,
a different male may benefit by preventing her exercis-
ing choice. The empirical data concerning nonhuman
primate and human coercion have accumulated, reveal-
ing that coercive behavior is more common among male
chimpanzees than bonobos, for example. Several phylo-
genetic and adaptive considerations may also be relevant
to the context of human sexual coercion. Terrestrial pri-
mates may be more vulnerable to sexual coercion than
arboreal primates or birds; the larger body sizes of many
terrestrial primates may have been favored in contexts
of male–male contest competition (i.e., fighting), but can
incidentally or adaptively be used for coercive behavior
of females too. In this vein, it has been noted that female
assessments of human male secondary sexual character-
istics such as muscle mass or voice pitch tend to favor
less extreme phenotypes than are favored by males
(Yang et al., 2005; Puts, 2010). These data are consistent
with sexual conflicts of interest: males might favor more
extremes of muscularity in contexts of male–male com-
petition, but females prefer less extremes because male
musculature could also be used for coercion. In this vein,
it can be noted that whereas Miller (2000) has empha-
sized the relevance of ancestral female choice as the
driver of human behavioral evolution, Puts (2010) con-
tends that experimental and observational data are
more compatible with male–male competition serving as
the driver of male characteristics such as upper body
musculature (males have approximately 60% more upper
body lean muscle mass: Lassek and Gaulin, 2009), deep-
ened voices, facial hair, and same-sex physical aggres-
sion. It could be that theoretical models of female choice
borrowed from the avian world, where less sexual
coercion occurs in part because animals can often fly
away from would-be coercive efforts, helped lead to an
underappreciation of the importance of sexual coercion
to the evolution of human sexuality.
Sperm competition and cryptic female choice
One of the few areas of sexual science that Darwin
apparently did not anticipate was postcopulatory selec-
tion. Whereas Darwin’s discussion of sexual selection
concerned processes culminating in mating, we now rec-
ognize that semen within a female’s reproductive tract
continue a dynamic process potentially leading to fertil-
ization. Since Parker’s (1970) seminal studies on insects,
theory and empirical research on sperm competition and
cryptic female choice has progressed rapidly. From the
standpoint of cryptic female choice, the idea is that a
female’s anatomy and physiology can impact which
sperm among those introduced into her reproductive
tract successfully fertilizes her egg (Eberhard, 1996).
From the standpoint of sperm competition, if a female
mates with multiple males, then the sperm of those
males may compete within her reproductive tract to fer-
tilize her egg (Harcourt et al., 1981). Processes of cryptic
female choice and/or sperm competition can be disen-
tangled from those of precopulatory choice and competi-
tion; in practice, some primate species such as wooly
monkeys have extreme sperm competition and minimal
contest competition (Strier, 1990), whereas others such
as chimpanzees exhibit high degrees of both male–male
contest and sperm competition (Dixson, 2012). As we
shall see, the physiological and genetic evidence regard-
ing human cryptic female choice and/or sperm competi-
tion point toward low sperm competition pressures
among our recent ancestors (Dixson, 2009).
Summary
While theoretical foundations underlying the evolution
of human sexuality trace to Darwin, more recent schol-
ars such as Bateman, Trivers, Clutton-Brock and Parker,
and Kokko have all elaborated on the basis of sex differ-
ences in mating and parenting effort. Contrary to some
claims, the discovery of considerable female promiscuity
does not undermine the accuracy or relevance of contem-
porary evolutionary theory. Demographic and socioeco-
logical variation are also inherently important. While
male–male competition and female choice have gar-
nered, for good reason, primary attention, theoretical
and empirical work also focuses on female–female com-
petition and male choice. Sexual conflict is of variable
magnitude, but also of importance to mating systems.
Competition can occur after copulation, giving rise to
sperm competition and cryptic female choice.
EVOLUTIONARY SCENARIOS
In speculating on the evolutionary origins of human
sexual behavior, Charles Darwin noted, in Sexual Selec-
tion, and the Descent of Man, “Humans would probably
have lived, as already stated, as polygamists or tempo-
rarily as monogamists. Their intercourse, judging from
analogy, would not then have been promiscuous. They
would, no doubt, have defended their females to the best
of their power from enemies of all kinds, and would
probably have hunted for their subsistence, as well as
for that of their offspring. The most powerful and able
males would have succeeded best in the struggle for life
EVOLUTION AND HUMAN SEXUALITY 97
American Journal of Physical Anthropology
and in obtaining attractive females.” Darwin’s observa-
tions are consistent with many aspects of contemporary
theorizing concerning the evolution of male mating
strategies; they leave out female strategies, which can
be better elaborated, as we shall see below.
In this section, I discuss evolutionary scenarios con-
cerning hominin sexual behavior. Phylogenetic insights
are drawn from great ape comparisons, specifics of the
hominin fossil and archaeological record, and insights
into recently studied human hunter-gatherer societies,
with overarching theoretical guidance provided by sex-
ual selection theory and socioecological principles. These
scenarios will also later be connected with genetic and
physiological data, contemporary international studies of
human sexual behavior, and insights drawn from a life
course perspective on human sexuality. While there is
ongoing debate about specific evolutionary models of
hominin sexuality, not surprisingly in light of the limita-
tions of available data, it is also worth underscoring that
the models are consistent with these latter bodies of evi-
dence, an indication that there is a coherent and syn-
thetic science here.
Early hominin and australopithecine sexuality
Current genetic and fossil evidence suggests that the
closest living relatives of humans are chimpanzees and
bonobos (Goodman et al., 1998). Genetic data suggest
that chimpanzees and bonobos themselves had a last
common ancestor in Africa about 1 million years ago
(Prufer et al., 2012). Humans had a last common ances-
tor with chimpanzees and bonobos in Africa around 6
million years ago. What was that ancestor like, and
what can we infer about its sexual behavior? As we shall
see, there are clues to help answer these questions, but
there are also uncertainties.
The earliest putative hominin fossils indicate body
sizes comparable to today’s chimpanzees and bonobos
(Senut et al. 2001; Brunet et al., 2005; Lovejoy, 2009).
The finds are all in Africa. The locomotor anatomy sug-
gests the earliest hominins, as well as later gracile and
robust australopithecines, were semiterrestrial bipeds.
They retained both adaptations for spending time mov-
ing in trees (e.g., long finger bones, relatively long arms,
and mobile shoulder joint) and for moving bipedally.
Most models suggest they moved bipedally on the
ground (Lieberman, 2011), although it has also been sug-
gested that bipedalism emerged on tree branches during
foraging (Crompton et al., 2008). The environmental
reconstructions of early and later pre-Homo hominins
suggest they inhabited wooded environments. A few
early hominins, such as a find at Chad dated to approxi-
mately 7 million years ago, appeared to have slightly
larger canine teeth (compared with modern humans, but
less than great apes), though generally among hominins
canine teeth are reduced in size compared with extant
and presumably ancestral great apes. The shift toward
greater bipedalism may have relaxed selection on reten-
tion of larger canine teeth, particularly among males, for
use in male–male contest competition; fighting may have
relied, instead, on punching, grappling, or use of
weapons.
If early hominins were similar to extant great apes,
these early hominins likely mated primarily diurnally
(see Mitani et al., 2012). Among contemporary chimpan-
zees, most matings in the wild occur early morning or
late afternoon when chimpanzee groups are more fused
compared with other times across the day when they
may be more fissioned (Martin Muller, personal commu-
nication). Most sexual behavior was likely in a type of
ventral-dorsal position (e.g., males from behind),
although bonobos and orangutans are recognized to use
a wider array of sexual positions, especially compared
with other primates and mammals (Dixson, 2012). Based
on great ape comparisons, most matings likely took place
on the ground. If early hominin sexuality resembled that
of chimpanzees and bonobos, then it would have entailed
mating in multimale, multifemale groups. Further, early
hominin females might have mated with many males,
while also showing preferential mating access to a domi-
nant male around the time she was most fertile.
Females might have mated during parts of their preg-
nancy, but exhibited profound reductions in sexual
behavior when in a state of postpartum lactational
amenorrhea (i.e., not cycling, but heavily lactating and
caring for an infant). Males may have competed heavily
through alliance formation and contest competition with
other males for social rank, in turn translating higher
rank into greater mating success. At this time, it is diffi-
cult to discern whether chimpanzees, some other ape
such as gorilla, or no extant ape can serve as a strong
referential model for early hominin sexuality. Reasons
for the lack of clarity include questions whether the mul-
timale, multifemale mating system of chimpanzees and
bonobos (and related traits such as exaggerated sexual
swellings and heightened evidence of sperm competition)
is derived or shared with a common ancestor; over
reconstructions of the degree of body size sexual dimor-
phism among early hominins and australopithecines;
and a dearth of relevant genetic analyses that might be
able to address the evolutionary polarity of potential
shifts in hominin sexuality.
Most researchers suggest that australopithecines were
more polygynous than members of the genus Homo
(McHenry, 1994). Debate continues on the number of
gracile australopithecines and their phylogenetic rela-
tionship to earlier hominins and later members of Homo
(Wood and Lonergan, 2008). However, most reconstruc-
tions of the postcranial remains of gracile australopithe-
cines suggest they exhibited greater degrees of body size
sexual dimorphism compared with modern humans and
most other members of Homo, and possibly even more
dimorphism compared with chimpanzees and bonobos
(see Plavcan, 2012). The available postcranial remains
are scanty, are scattered across time and locations, and
entail inferences regarding sex in the first place. Such
considerations have led other researchers to question
whether australopithecine body size dimorphism esti-
mates were exaggerated. It has been suggested the Aust.
africanus might have been less dimorphic than Aust.
afarensis (Harmon, 2009). It has also been suggested by
Reno et al. (2003) that gracile forms were only mildly
dimorphic—comparable with modern humans. If gracile
australopithecines were highly dimorphic, the theoreti-
cal inference would suggest they mated in one-male pol-
ygynous groups, perhaps somewhat similarly to gorillas
(Geary and Flinn, 2001). If they were only mildly dimor-
phic, that suggests, instead, they were mildly polygy-
nous, and perhaps mating in somewhat human-like
ways from this earlier time. The robust australopithe-
cines, which most scholars suggest became evolutionary
side branches rather than serving as human ancestors,
appeared to be highly sexually dimorphic in body size.
Aust. robustus, from southern Africa, may have had
98 P.B. GRAY
American Journal of Physical Anthropology
pronounced sex differences in reproductive maturation,
consistent with one-male polygyny (Lockwood et al.,
2007), and appeared to exhibit sexually dimorphic cra-
niofacial anatomy, also consistent with some degree of
polygyny (Lockwood, 1999). Attempts to draw additional
inferences concerning hominin sexuality have used the
study of digit ratios (Nelson et al., 2011) and other
aspects of craniofacial dimorphism (Schaefer et al.,
2004), but without clear inferences. It can also be noted
that the penis bone (baculum) and homologous clitoral
bone were lost during hominin evolution, a difference
consistent with evolutionary changes in our ancestors’
reproductive anatomy (Dixson, 2009).
Sexuality in the genus Homo
Most evolutionary models of sexuality in the genus
Homo suggest that grade shifts took place during early or
middle Homo evolution. The transition from a presumed
gracile australopithecine form to early Homo remains
murky in phylogenetic and archaeological detail, in part
because of a paucity of fossil material during this time
frame. Scenarios regarding body size estimates and indi-
ces of sexual dimorphism are highly contingent upon
availability of new and still-rare fossil specimens. For
example, 1990s models suggested that Homo erectus/
ergaster exhibited increases in body size, had lower
(human-like) body size sexual dimorphism, and human-
like limb proportions, with much of that inspiration drawn
from the Nariokotome boy (KNM-WT-15000) (Walker and
Leakey, 1993). More recently, a 1.5-Ma pelvis from Gona
has been used to suggest that some females at that time
were still small, and thus that body size dimorphism could
have remained high, and similar to previous scenarios of
highly dimorphic gracile australopithecines (Simpson
et al., 2008). The shift toward human-like body propor-
tions (e.g., relatively shorter arms), along with environ-
mental reconstructions, suggests that Homo inhabited a
wider array of environments than its woodlands-bound
predecessors, and had also become a committed biped (giv-
ing up perhaps sleeping in trees at night). Beginning
around 1.5 Ma, members of Homo began using new stone
tool traditions, marked by the Acheulian (Foley and Gam-
ble, 2009). Estimated brain sizes increased; however, the
limited availability of postcranial remains makes adjust-
ment for body size difficult, and probably thus leaves early
and mid Homo as less encephalized as later forms.
Additionally, dental microstructure studies suggest that
the pace of development was more apelike than human-
like (Dean et al., 2001), providing further impetus for see-
ing grade shifts in Homo as still showing considerable
change over time rather than being established quickly
with the earliest forms.
Estimates of both Neandertal and modern human body
size sexual dimorphism suggest about 15% differences in
height and weight (see Plavcan, 2012). This would sug-
gest modest degrees of male–male contest competition,
consistent with slight polygyny/mostly monogamy. If this
pattern held among the common ancestor of Neandertals
and modern humans, then presumably similar aspects of
sexual behavior held among archaic Homo/Homo heidel-
bergensis in Africa by about 600,000 years ago. Whether
similar patterns can be pushed back to earlier Homo
around 1.5 Ma then depends on the issues noted above
regarding early Homo/Homo erectus skeletal material.
A socioecological approach to hominin sexuality
To orient potential evolutionary models of sexuality in
the genus Homo, we can draw upon socioecological princi-
ples. See Figure 1 for graphical illustration of a basic soci-
oecological model. Such principles in turn begin with
Bateman’s observations and additional layers concerning
sex differences in reproductive constraint and reproduc-
tive strategy. As Wrangham (1979) noted, if females are
ultimately constrained by access to resources such as food,
then it makes sense in developing socioecological models
of primate social behavior to ask about how females will
distribute themselves in an environment with respect to
available food resources. Van Schaik (1983) suggests that
predation pressures may also serve as major selective
forces operating on female distributions. Applied to the
transition from a gracile australopithecine to early Homo,
the shift toward committed, terrestrial bipedal lifeways
might have exposed females to greater predation pres-
sures (in terrestrial compared with arboreal environ-
ments). That could favor enhanced female group sizes.
The formation of larger female group sizes could amplify
female foraging competition. That could fuel intensified
female extractive and processing foraging strategies, per-
haps including tools used for digging roots (and tools that,
incidentally, would not be regularly found in the fossil
record, although some microwear studies of stone tools are
also consistent with plant processing). If females are in
larger groups, then they become more difficult for a single
or several males to monopolize. From socioecological first
principles, males are expected to map on to the availability
of fertile females. With a larger number of males within
groups, consequences could be diminished male reproduc-
tive skew, and perhaps male–male egalitarian relation-
ships. The presence of multiple males in groups could
foster lower-ranking males to attempt to mate guard fer-
tile females, as is sometimes observed in chimpanzees, in
which males attempt to coerce females into short-term
liaisons removed from other group members. This kind of
male mate guarding in Homo could give rise to long-term
sociosexual relationships, but within multimale, multife-
male groups.
One aspect of such modeling is that evolutionary shifts
in hominin sexuality are theorized from principles
applied to other primates. This is different from empha-
sizing human-specific arguments. Another aspect of this
approach is that it sees social behavioral changes rooted
in female foraging and male mate-seeking as the
Fig. 1. Socioecological model of hominin sexuality. [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
EVOLUTION AND HUMAN SEXUALITY 99
American Journal of Physical Anthropology
foundation, with subsequent layers of hominin economic
and social behavior building on these. For example, a
pronounced hunter-gatherer sexual division of labor is
viewed as a more recent derivative of the preceding
shifts in sexuality. Table 1 illustrates some leading mod-
els concerning the evolutionary foundations of Homo
sexuality, featuring female perspectives, male perspec-
tives, and commentary concerning the possible timing
and relevance of each (see also Quinlan, 2008).
Hunter-gatherer sexuality
As another source of insight into the evolution of
human sexuality, we can draw upon patterns observed
among recently studied hunter-gatherers (Berndt and
Berndt, 1951; Shoskak, 1981; Hewlett and Hewlett,
2010; Marlowe 2010; Walker et al., 2011). Because the
socioecology of hunter-gatherers is thought to resemble
in some respects (e.g., small group sizes, family relation-
ships) that in which earlier forms of Homo sexuality
evolved, hunter-gatherers can serve as analogies for
more distant patterns. Of course, recently studied
hunter-gatherers are not living fossils; they have differ-
ent foraging tools than more distant ancestors (e.g.,
Marlowe, 2005), among other differences. Yet observa-
tions and patterns among foragers offer insights that
otherwise are not accessible.
From recently studied hunter-gatherers, the most
common marital system is slight polygyny (Marlowe,
2005; Walker et al., 2011). That is, among foragers, most
TABLE 1. Models of the evolution of sexuality in the genus Homo
Model Female perspective Male perspective Timing and relevance
Mate guarding
(Hawkes, 2004)
No benefit; constrained
choice
Aids paternity certainty Some relevance across Homo;
may have been the original
basis of long-term Homo socio-
sexual partnerships, since in a
wider phylogenetic scope male
mate guarding is one of the
best predictors of social
monogamy (Lukas and
Clutton-Brock, 2013); female
hunter-gatherers favor specific
mates and often seek to main-
tain a bond
Hired Gun (Mesnick,
1997)
Reduced predation and/or
coercion by others of self
and offspring
Benefit from sexual access in
exchange for offering
protection
Some relevance across Homo;
while male partners tend to
be the most coercive of female
partners, those same male
partners may also deter addi-
tional coercion by other males
and have helped reduce preda-
tion upon females and their
offspring
Infanticide avoidance
(van Schaik and
Dunbar, 1990)
Reduced risk of infanticide if
bonded to a protective
male
May benefit from sexual access
and higher offspring survival
Some relevance if early homi-
nins or Australopithecus lived
in one-male groups, in which
infanticide risk tends to be
higher; little evidence of
infanticide risk by unrelated
males among hunter-
gatherers
Food-for-sex (Fisher,
1982)
Benefit from food Benefit from sexual access May be part of the sex-specific
resource exchange within
long-term unions and within a
sexual division of labor
Male provisioning
(Kaplan et al., 2000)
Gain from food resources in
a sexual division of labor
Gain from mating access with a
long-term partner within a
sexual division of labor
Relevance to recently studied
hunter-gatherers, but unlikely
to be the starting point of
Homo shifts to long-term soci-
osexual bonds
Cooking hypothesis
(Wrangham et al.,
1999)
Benefit from reduced theft of
cooked foods
Gain mating access in exchange
for protection
Evidence of regular use of fire/
cooking aligns best with
archaic Homo/Homo heidel-
bergensis; evidence for hunter-
gatherer concern over food
theft is lacking; could be con-
sistent with other protective
services
Exchange of mates
under parental con-
trol (Apostolou, 2007;
Chapais, 2008)
Expands social ties and pro-
vides a mate, but with
potential parent-offspring
conflict
Gains expanded social ties and a
mate, but with some potential
for parent-offspring conflict
Relevant to recently studied
hunter-gatherers; unlikely to
project beyond modern
humans given cognitive and
demographic constraints
underlying mate exchange
100 P.B. GRAY
American Journal of Physical Anthropology
societies allow polygyny, but typically only a few men
are married polygynously, with the vast majority of men
monogamously partnered. The regional exception is
among Australian aborigines, where higher rates of pol-
ygynous marriage were observed, but that may be in
part due to cultural transmission from New Guinea (see
O’Connell and James 2007). Across hunter-gatherers,
the men most likely to have two wives tend to have
higher status, achieved through hunting success or from
shamanic activities (Smith, 2004). Divorce is variably
common, especially among younger couples (Blurton
Jones et al., 2000; Winking et al., 2007). Marriages,
especially first marriages, are often arranged by older
family members, although the married partners may
have influence on whether those arrangements proceed
(Apostolou, 2007). While polyandrous mating systems
tend to be rare in mammals generally, including
humans, it is occasionally observed among hunter-
gatherers, and typically when there is a shortage of men
(Starkweather and Hames, 2012). Affairs also occur,
with love triangles (e.g., men competing over a woman)
a regular context of competition among foraging societies
such as the !Kung and Hadza (Gat, 2006).
What other aspects of hunter-gatherer sexuality stand
out? A sexual division of labor is observed, whereby
females tend to undertake subsistence activities yielding
reliable food availability and in ways compatible with
having young children (Bliege Bird, 1999; Marlowe,
2007). Males tend to undertake riskier subsistence activ-
ities, including foraging for more difficult-to-acquire but
highly relished foods such as larger game animals and
honey. Foods may be shared between partners, with a
female’s provision of reliable resources helping make
possible a male’s riskier undertakings. Resources males
acquire appear designed to serve multiple ends: partly
as provisioning family members, but with the acquisition
and sharing of large game also enhancing male status in
ways that may yield political benefits or mating opportu-
nities (Marlowe, 2010). It has also been suggested that
male resources are most useful during a “critical period”
of provisioning a partner who has a young nursling
(Marlowe, 2003).
Also among hunter-gatherers, females spend the bulk
of their reproductive years pregnant or in a state of lac-
tational amenorrhea, meaning that they are nursing but
not cycling (Short, 1976; Ellison, 2001). This latter
observation has dramatic impacts on the evolutionary
foundations of human sexuality: it is comparatively rare
for a woman to be cycling, and the true female baseline
is one of fluctuations in sexuality across reproductive
states. Among foragers, there are also few unattached
and fertile females. Individuals are familiar, and fre-
quently off-limits as potential mates because they are
close family members or circumscribed for other reasons
that may be designed to extend coalitions (e.g., wife
exchange). One of the distinguishing features of hunter-
gatherer sexuality (and among humans more generally)
compared with other primates is that sex is typically
covert. While in some other primates surreptitious mat-
ings may take place to reduce same-sex conflict or coer-
cion (e.g., baboons: Smuts 1985), this is a much greater
human concern. Ethnographic descriptions include cases
of couples having sex at night near a fire, within short
distance of children and other campmates (but the poor
light providing some cover); in other cases, a couple may
seek to have sex during the day away from camp. One
interpretation of the pattern of seeking to shield sexual
behavior from the eyes of others is that it may reduce
sexual jealousy and mating competition. The unusual
human mating pattern of forming long-term sociosexual
bonds within multimale, multifemale groups is vulnera-
ble to such competition. This privacy-seeking also con-
trasts with the sexual behavior of other apes, in which
group members often have abilities to witness the sexual
behavior of group-mates. Additionally, same-sex sexual
behavior has been observed among some, but not all,
hunter-gatherer societies (Hewlett and Hewlett, 2010).
Few details of forager sexuality across the life course are
available: sex play has been reported in several foraging
societies, including the !Kung, Hadza, and Aka, while
among the Aka older couples have sex less frequently
(Hewlett and Hewlett, 2010).
Summary
Evidence from other primates, the hominin fossil and
archaeological record, and studies of human hunter-
gatherers can be combined with socioecological princi-
ples to depict the evolution of hominin sexuality. There
are a range of theoretical models to draw upon in this
effort, such as models emphasizing the benefits of male
protection or care to long-term bond formation. Some
reconstructions of early hominin sexuality draw upon
comparisons with chimpanzees because they and bono-
bos are our closest living relatives and the body sizes of
early hominins are chimpanzee-sized. However, debate
continues whether features of the multimale, multife-
male mating system of chimpanzees are derived or
shared with an early hominin. Many reconstructions of
gracile australopithecine sexuality feature pronounced
body size sexual dimorphism, which could be consistent
with gorilla-like one-male polygyny; other reconstruc-
tions emphasize more modest dimorphism. Most scenar-
ios suggest that body size dimorphism decreased in early
or mid Homo evolution, with the inference that these
hominins experienced lower male–male contest competi-
tion and more socially monogamous long-term bonds.
The transition toward slightly polygynous/more monoga-
mous Homo may have originated with changes in female
foraging and possibly increased group sizes, attended by
male mate guarding. During later Homo, male care, in
particularly provisioning, may have arisen, reinforcing
benefits such as higher female lifetime reproductive suc-
cess and male sexual access to forming long-term bonds.
Among modern humans, older adults may have taken on
greater and evolutionarily novel roles in mate exchange.
Across the existence of long-term hominin sociosexual
bonds, male protection against harassment by other
males may have been relevant, although aspects of the
infanticide and cooking models are less supported. Key
features of human sexuality thus arose in a mosaic fash-
ion during hominin evolution, with different selective
scenarios having relevance at different times.
RECENT GENETIC, GENOMIC AND
PHYSIOLOGICAL DATA
An explosion of recent data from genetics, genomics,
and physiology can inform an understanding of human
sexuality. These sources of data can provide tests of
evolutionary-based understandings of human sexuality.
For example, human and chimpanzee Y chromosomes
have diverged rapidly, likely in part due to species differ-
ences in mating systems (Hughes et al., 2010). Equally
important, an evolutionary perspective can help account
EVOLUTION AND HUMAN SEXUALITY 101
American Journal of Physical Anthropology
for the patterning of genetic data and how proximate
mechanisms underpinning romantic and sexual behavior
are expressed. Many of the recent physiological data on
human sexuality have been generated without explicit
evolutionary theorizing and interpretation, even though
an understanding of proximate mechanisms and adapt-
ive function are complementary ways to investigate the
system (Tinbergen, 1963).
Recent genetic and genomic data
related to the evolution of
human sexuality
From the standpoint of genetics and genomics, several
approaches guide the application of new data to evolu-
tionary questions. One is a candidate gene approach, in
which tests for signatures of selection are undertaken to
determine whether a specific gene thought to be impli-
cated in some aspect of human sexuality shows evidence
of this. As an example, the vasopressin 1a receptor
(AVPR1a) has been implicated in vertebrate male spe-
cies- and individual differences in mating and paternal
behavior; one report noted that humans and bonobos
had a distinct AVPR1a compared with chimpanzees,
raising the prospect of this genetic difference serving as
a possible contributor to species differences in male
social behavior (Hammock and Young, 2005). However,
subsequent comparative genomic work has not impli-
cated selection upon this gene during hominin or homi-
nid evolution. Another approach is to study genetic
information from other forms, such as herpes or HIV, to
infer possible evolutionary-related scenarios regarding
sexually transmitted disease. Still another is to draw
comparisons of genomic sequences (or even entire
genomes rather than a single or several genes) between
humans and other species, between human populations,
or within individuals of a human population in order to
identify genetic differences that might be related to sex-
ually relevant phenotypic differences. In a related vein,
quantification of sex-specific genetic contributions to a
population can inform an understanding of mating
dynamics (e.g., specifying the relative variance in male
and female reproductive success: Heyer et al., 2012). As
an example, relatively less male genetic variation was
found among the Yoruba in Nigeria compared with a
Chinese sample, consistent with recognized differences
in marital behavior (Labuda et al., 2010).
The availability of a draft human genome in 2003, and
of the chimpanzee in 2005, has helped fuel interest in
the growing field of comparative genomics (Chimpanzee
Sequencing and Analysis Consortium, 2005). The avail-
ability of orangutan, gorilla, and bonobo genomes now
means that the genomes of all great ape species can be
compared to discern similarities and differences, and in
turn raise questions about the genetic bases to species
differences in sexuality. The publication of draft genomes
from Neandertal and Denisova extends the comparisons
to two extinct and closely related hominins (Disotell,
2012). Given that Neandertals and Denisovans are appa-
rently sister groups, and that modern humans, apart
from a small degree of population-specific interbreeding
with them, last shared common ancestry with Neander-
tals and Denisovans around 600,000 years ago, compari-
sons with these extinct hominins can help shed light on
the phylogenetic structure of hominin sexuality. What
insights have emerged thus far in such comparisons
(O’Bleness et al., 2012)?
Genetically based insights into the evolution of
human sexuality
One of the most rapidly evolving regions of the human
and these other hominids’ genomes involves male repro-
ductive genes (Clark and Swanson, 2005). Given that
the bottom line of evolution is reproductive success, it is
not surprising that genes involved in spermatogenesis or
semen parameters might be rapidly evolving. As an
example, the SEMG1 and SEMG2 genes, thought to aid
the coagulation of semen, appear to have undergone a
loss of function in gorillas and gibbons (which have low
sperm competition pressures), while being subject to
adaptive evolution in chimpanzees (which have high
sperm competition pressures); “Interestingly, the obser-
vation that humans are in a sense intermediate in their
rates of seminal protein evolution between chimpanzees
and gorillas parallels that of anatomical and physiologi-
cal correlates of mating systems.” (Carnahan and
Jensen-Seamen 2008: 946). However, a wider compari-
son of the ejaculate components across humans, chim-
panzees, bonobos, and gorillas found that effective
population size rather than sperm competition pressure
appeared to best explain overall species differences in
genes expressed in ejaculate (Good et al., 2013). When
related analyses compared subsets of genes involved in
different ejaculate functions, there was some evidence of
enhanced selection on genes involved in immune
responses and protease, suggesting different components
of the semen may evolve at different rates. Good et al.
(2013) also note that the genetic bases underlying spe-
cies differences in human and African ape male repro-
ductive function may rely more on gene expression than
protein differences and that gamete differences may be
more important than semen components.
Differences between human androgen receptor and
that of primates have also been noted (Mubiru et al.,
2012). Humans have more CAG repeats in the androgen
receptor promoter region than other primates although
it is not clear whether the pattern represents phyloge-
netic inertia or lineage-specific selective effects. A 2011
paper noted a deletion upstream of the human androgen
receptor after comparison with other primates including
chimpanzee (McLean et al., 2011). In vivo work suggests
that the genetic deletion results in loss of penile spines
situated on the glans penis. Further, Neandertal has the
same version as humans (Hawks, 2011), suggesting this
deletion may have occurred among a common ancestor,
possibly in association with some derived aspect of homi-
nin mating. As to other genetic insights into hominin
sexuality, Gentry et al. (1988) note that oral and genital
herpes strains have separated long ago and remained
that way, suggesting that oral sex may not have
occurred regularly among our ancestors (since those
strains of herpes would have merged rather than
remained distinct).
From a candidate gene approach, some work has
investigated potential human between- and within-
population genetic differences related to sexually rele-
vant phenotypic differences. Population differences in
dopamine receptor (DRD4) genetic polymorphisms exist,
with possible explanations advanced to account for those
differences including genetic drift, adaptive migration,
and mating behavior (Harpending and Cochran, 2002).
A U.S. study found that polymorphisms in this same
system were associated with differences in sexual prom-
iscuity in a university sample (Garcia et al., 2010).
102 P.B. GRAY
American Journal of Physical Anthropology
Individual differences in the oxytocin system, including
the oxytocin receptor, have been observed (Prichard
et al., 2007). However, these have not been clearly linked
with sexuality related phenotypic differences. A Swedish
study of AVPR1a polymorphisms identified small differ-
ences in male partnering behavior (Walum et al., 2008).
Overall, the cases suggesting genetic polymorphisms
associated with differences in partnering or sexual
behavior have small effect sizes, and as of yet have
failed to identify candidate genes playing large potential
roles in accounting for the hominin evolutionary shifts
in sexuality reconstructed from comparative, fossil,
archaeological, and other lines of evidence. Although
human mating is expected to entail the expression of
many genes, it remains a puzzle why so few genes of
even small effect sizes have not been identified as con-
tributing to such behavior.
Neuroendocrine bases of human
partnering and sexual behavior
How do scholars gain insight into potential physiologi-
cal contributions to an evolutionary understanding of
human sexuality? Much of the inspiration stems from a
rapidly growing body of research on the neuroendocrine
bases of human partnering and sexual behavior. This
body of work, in turn, represents a cross-fertilization
with comparative and animal model research. Through
recent developments in brain imaging, identification of
increased brain activity associated with specific mating
stimuli helps reveal brain processes involved in human
sociosexuality. The measurement of baseline or reactive
hormone concentrations in relation to partnering or sex-
ual behavior also sheds light on the endocrine bases to
human sociosexuality. The neuroendocrine system takes
in, integrates, and puts out information that helps orient
an individual’s anatomy, physiology, and behavior. The
way its mechanisms work both constrain and reflect age-
, sex-, individual-, and context-specific variation in ways
that may enhance reproductive success. As an example,
pubertal increases in male testosterone levels are bound
by tissue-specific androgen receptor in ways that may
enhance an individual’s motivation to engage in same-
sex competition and courtship behavior.
Imaging romance and sex
With respect to brain imaging, foundational studies on
partnering have been conducted by Bartels and Zeki
(2000) and Aron et al. (2005) using fMRI. These kinds of
studies typically rely on assessing brain activity in
response to subjects looking at photos of, for example, a
romantic partner, subtracting out brain activity from
control stimuli in order to reveal brain areas of height-
ened activity. Bartels and Zeki’s (2000) study revealed
increased activity in the hypothalamus and ventral teg-
mental area. Activation of the hypothalamus is of note
because of the linkage to potential peripheral hormone
release (e.g., release of hormones to reach the pituitary,
and in turn throughout the body). These are areas
known to be rich in oxytocin and vasopressin receptors
and involved in reward pathways, suggesting linkages
with these other systems and the positively reinforcing
experience of reflecting upon a romantic partner. Fur-
thermore, decreases in brain activity associated with
social judgment and negative emotion were also noted.
Aron et al.’s (2005) study of romantic love showed both
similarities and differences with Bartels and Zeki’s find-
ings, with differences potentially due to Aron et al.’s sub-
jects reporting higher levels of passionate love for their
partners and overall shorter relationship durations com-
pared with the earlier study. The ventral tegmental area
also showed heightened activity. Among subjects in
longer-term relationships, some areas such as the ante-
rior cingulate cortex were more highly activated. This
could suggest development of connections between more
emotional and higher cognitive associations with longer-
term relationships. In a related study of the brain activ-
ity of rejected participants, many of the same brain
areas such as the ventral tegmental area were activated
as in those subjects in love. However, a few areas were
differentially activated too, including parts of the fore-
brain that have been implicated in drug-use addiction
(Fisher et al., 2006). Additionally, Zeki and Romaya
(2010) found that patterns of brain activity of subjects in
love did not differ depending on sexual orientation,
either for males or females.
Brain imaging studies relying upon fMRI or PET scan-
ning have also been conducted on women and men
engaged in sexual behavior. Such studies typically sit-
uate an individual inside of a brain scanner, asking the
individual or a partner to stimulate to orgasm. As an
example, Beverly Whipple and colleagues have investi-
gated the responses of women with spinal cord injuries
to cervical self-stimulation, showing both brain activity
associated with the behavior while also implicating the
vagus nerve in transmitting genital information to the
brain (see Komisaruk and Whipple, 2005). In a PET
study enabling direct comparisons between women and
men’s brain responses to genital stimulation (by mastur-
bation) and orgasm, Georgiadis et al. (2009) found dis-
tinct sex differences during the stimulation phase of the
sexual response but similarities between the sexes dur-
ing the experience of orgasm. As examples, during geni-
tal stimulation, men experienced greater deactivation of
the right amygdala, greater activation of the right mid-
dle temporal gyrus, and women experienced greater acti-
vation of the right inferior parietal lobe. During orgasm,
by contrast, men and women shared an array of deacti-
vated brain areas such as the left inferior frontal gyrus,
while among the few differences women showed more
pronounced activity in the right insula. The observation
that many brain areas were deactivated during orgasm
is itself notable, marking an experience in which individ-
uals are less aware of their wider surroundings. Another
observation from such imaging studies is that the brain
mechanisms of sexual behavior are distinct from those
activated while imagining a romantic partner. This dis-
tinction is consistent with other findings that romantic
love and sexual behavior can be behaviorally separated
too; while someone may both experience romantic pas-
sion and sex with the same individual, ethnographic
reports note that these can be separated (e.g., when hav-
ing sex without love, or loving someone without having
a sexual relationship: Jankowiak, 2008).
Hormonal bases of partnering and sexual
behavior
The hormonal bases of partnering have been explored
in the U.S., but also a growing number of international
settings (van Anders and Gray, 2007; Gray and Camp-
bell, 2009). Drawing upon comparative research, much
of the focus has been on oxytocin and male testosterone
with respect to partnering, although relations between
EVOLUTION AND HUMAN SEXUALITY 103
American Journal of Physical Anthropology
partnering and other hormones such as cortisol have
also been investigated. In one oxytocin study of 38 cou-
ples, Grewen et al. (2005) found that blood oxytocin lev-
els were higher throughout a 10 min behavioral session
among both women and men reporting greater partner
support. In an experimental study in which women and
men were either administered intranasal oxytocin
(which is known to exert effects in the brain) or intra-
nasal placebo spray, those given oxytocin showed rela-
tively higher ratios of positive to negative partner-
directed behavior during a behavioral interaction session
(Ditzen et al., 2009). Studies of both young and older
couples’ blood cortisol levels have been conducted in
relation to marital discussion, a paradigm in which cou-
ples report to a lab and are asked to talk about topics
known to elicit conflict (such as sex or finances) (Kiecolt-
Glaser et al., 2003). As one of the main findings from
these studies, women in more positive marital relation-
ships had rapidly reduced cortisol profiles during the
discussion compared with women in less positive marital
relationships (Robles et al., 2006).
Among studies investigating male testosterone levels
and partnering, nearly all conducted in North America
have found that men involved in committed, romantic
relationships have lower testosterone levels than their
unpartnered counterparts (Gray and Campbell, 2009). A
longitudinal study of U.S. military veterans showed that
men’s testosterone levels increased after divorce, provid-
ing evidence that changes in relationships can precede
changes in men’s testosterone levels (Mazur and Micha-
lek, 1998). However, international studies have been less
consistent in findings linking men’s testosterone and
relationship status. In most international studies,
becoming a father in a social context in which men pro-
vide paternal care is associated with lower testosterone
levels, but a number of studies found no difference in
the testosterone levels between partnered and unpart-
nered childless men. As an example, in a sample of 126
men aged 21–38 in Beijing, fathers had lower testoster-
one levels than both partnered childless men and single
men, with the testosterone levels of partnered men with
and without children similar (Gray et al., 2006). A longi-
tudinal study of several hundred men in Cebu City, Phil-
ippines, revealed that becoming a father, particularly of
infants and associated with greater paternal care,
caused men’s testosterone levels to decrease (Gettler
et al., 2011). Four studies have also investigated men’s
testosterone levels with respect to polygynous marriage.
In the first of these, conducted among Swahili men in
Kenya, men married to two wives had higher testoster-
one levels than other men, whether monogamously mar-
ried or unmarried (Gray, 2003). The interpretation of
these and the several other studies suggest that age and
social context matter, and it is also difficult to disentan-
gle potential confounding influences such as develop-
mental impacts of diet and physical activity.
A growing number of studies have investigated hormo-
nal responses to sexual behavior, with emphasis upon
intercourse or solitary masturbation culminating in
orgasm. Some of these studies have become more techni-
cally feasible with the use of minimally invasive mea-
surement of hormone concentrations from saliva,
whereas assessments of peptides typically rely upon
blood draws. Some of the effects appear sex-specific,
whereas others, involving peptide release, appear simi-
lar between females and males. Among males, a consid-
erable literature has tested whether men’s testosterone
levels acutely increase in response to sexual stimuli
(reviewed in Archer, 2006; Van Anders and Watson,
2006). In studies of solitary masturbation, men’s testos-
terone levels do not consistently change, whereas expo-
sure of men to audiovisual sexual stimuli more reliably
increases testosterone levels. In a naturalistic study of
men’s testosterone responses conducted at an adult club,
men’s testosterone levels increased more when engaging
in rather than observing sexual behavior (Escasa et al.,
2011). In other studies, among men who masturbated,
prolactin levels did not change (Kruger et al., 2003),
whereas in men and women who engaged in partnered
intercourse their prolactin levels showed acute increases
(reviewed in Bancroft, 2005). In both men and women,
oxytocin levels tend to increase during sexual behavior;
it may be that oxytocin is involved in smooth muscle
contraction associated with orgasm as well as the
rewarding experience of sexual behavior (Carmichael
et al., 1987). Studies have typically found no changes in
women’s estradiol, but in some cases increases in wom-
en’s testosterone levels during sexual behavior (see Van
Anders et al., 2009). Summarizing much of this litera-
ture on hormonal responses to sexual behavior, major
features of the system are homologous across a wide
phylogenetic scope, and both sex similarities and differ-
ences have been recognized.
Human reproductive physiology and anatomy in
comparative context
Closing out this section on physiological data, several
recent syntheses warrant attention. One is Alan Dixson’s
(2009) compilation of human reproductive physiology
and anatomy placed within an explicitly comparative
perspective, and the conclusion from multiple lines of
evidence that humans bear signatures of low sperm com-
petition pressures. While classic comparative research
on relative primate testis size reached a similar conclu-
sion (Harcourt et al., 1981), the earlier work had a
human sample size of four individuals from which tes-
ticle size was measured in contrast to over 7,000 men in
more recent analyses. Dixson summarizes data indicat-
ing humans have low sperm quality, ejaculate volume,
sperm reserves, modestly sized seminal vesicles, and
prostate gland, and relatively small sperm midpiece vol-
ume. Human females also have relatively short oviduct
length (i.e., Fallopian tube), consistent with relatively
low sperm competition pressures. This conclusion is at
odds with views espoused in Baker and Bellis (1994) or
Ryan and Jetha (2010). The evolutionary significance of
population and individual human variation in these and
related reproductive physiological and anatomical
parameters, however, remains unclear. As examples, dif-
ferences in human sperm counts (Dixson, 2009) and
female vaginal flora (Ravel et al., 2011) have been noted.
That variation helps provide a basis on which selection
can act, but it is not clear if observed population differ-
ences in, for example, testis size can be traced to
population-specific selective pressures rather than other
evolutionary or developmental processes.
The way in which genital responses align (or not) with
subjective sexual arousal tends to differ between males
and females. Through measurement of vaginal blood
flow or penile tumescence in response to exposure to
audiovisual stimuli, meta-analysis reveals that women’s
responses less consistently align (r5.26) with subjective
reports of arousal compared with men’s responses
104 P.B. GRAY
American Journal of Physical Anthropology
(r5.66) (Chivers et al., 2010). As one extreme illustra-
tion, women showed vaginal blood flow responses when
watching video of bonobos having sex, even though
women reported not being aroused. How can these pat-
terns be situated within larger evolutionary concepts?
One interpretation is that a low threshold of female sex-
ual response, particularly vaginal lubrication, can be
adaptive because it avoids injuries during coerced or
unarousing sex. Another interpretation recognizes that
female sexual desire is highly contextualized, helping
inform how subjective and objective measures of
response can be separated. Still another interpretation
could recognize that males tend to have a more procep-
tive, and females a more receptive sexuality, with the
result that it makes more sense for male subjective and
objective responses to align to best facilitate the motiva-
tion and capacity to engage in sexual behavior, while
females can respond sexually even to a sexual encounter
in which they have little motivation to initiate. As such
interpretations indicate, new physiological data continue
to call for interpretation in light of an overarching evolu-
tionary perspective. There are also potential prospects to
expand the comparative and human socioecological scope
(e.g., investigating brain activity in polygynous contexts)
of such work.
Summary
New genetic, genomic, and physiological data concern-
ing human sexuality are available and need to be inte-
grated with contemporary evolutionary theory and
models depicting the evolution of hominin sexuality. The
heritable basis underlying shifts toward human long-
term slightly polygynous bonding can be investigated
from candidate gene or comparative genomic
approaches, but to date has been unsuccessful. While
multiple lines of evidence converge in indicating that
humans are adapted for low sperm competition pres-
sures, the genetically based attempts (e.g., SEMG1 and
other genes expressed in semen) to specify the evolution-
ary origins and trajectory of this pattern in hominins
also remain murky. Evolutionary theory tells us that
reproductively relevant genes should be hotspots of
selective action; that expectation is partly born out,
although subject to caveats such as whether gene regu-
lation or protein coding is more important.
Brain imaging and hormonal studies help elucidate
the shared and derived mechanisms by which long-term
human bonding and sexual behavior occur. The brain
imaging data, in particular, derive from a very narrow
cultural scope. Nonetheless, various brain processes in
humans show homology with mechanisms from other
long-term bonded species such as prairie voles. There
are some distinct brain processes implicated in romantic
bonding from sexual behavior, as would be expected
from other lines of evidence (e.g., in other species that
engage in sexual behavior without long-term bonds, or
in humans the ability to separate sex from love). Brain
imaging and hormonal studies of romantic bonding show
evidence of sex-similarities (e.g., in involvement of oxyto-
cin in both sexes); that is expected since the mechanisms
of long-term sociosexual bonds may in part piggyback on
those related to mammalian maternal behavior. At the
same time, evidence of lower male testosterone levels
within contexts of long-term intimate family relation-
ships speaks to processes that appear more sex-specific
(i.e., males decreasing the hormonally based investment
in mating effort). The data on sex differences in the coin-
cidence of objective and subjective measures of sexual
arousal indicates that—as Darwin and other theorists
would anticipate—sex differences exist. One overarching
interpretation is that males have been equipped with a
more proceptive sexuality and females a more receptive
sexuality.
RECENT AND LARGE DATA SETS ON AMERICAN
AND INTERNATIONAL HUMAN SEXUALITY
Alfred Kinsey’s interview-based studies of U.S. human
sexuality were foundational contributions. The data he
and collaborators collected and published in the mid-
20th century provided quantitative insight into the sex
lives of approximately 12,000 Americans (Kinsey et al.,
1948, 1953). He found, for example, that premarital and
extramarital sex was common, and that more men than
women reported same-sex sexual behaviors. As impor-
tant as these works were, however, others criticized the
findings on methodological grounds. The sample was
nonrandom, including an inordinate number of men in
prisons, and short on members of groups (e.g., from
churches) who less often agreed to participate in the
research (factors that could have overestimated the fre-
quency of certain sexual behaviors). It was not until the
1990s that a nationally representative probability sam-
ple of U.S. adult sexual behavior was conducted (Lau-
mann et al., 1994). In the first part of this section, we
consider what this 1994 publication, along with several
other large and rigorous, U.S. data sets on sexuality
have found, effectively updating Kinsey’s insights with
more current and arguably better-designed studies.
These studies are important to an evolutionary perspec-
tive on human sexuality for several reasons. One is that
they enable rigorous tests of expectations derived from
relevant theory, such as whether males report higher
sexual desire than females, as would be expected if
females are the reproductively limiting sex. These data
can serve as the basis for productive, empirically
grounded discussions concerning the patterning of
human sexuality, a contrast to choosing a single study
suiting one’s preferred interpretation or ignoring alto-
gether data on the subject. Another reason is that an
evolutionary perspective can be brought to bear on such
rich data sets, something which is lacking in the major-
ity of the original publications describing these large
human sexuality studies.
Before presenting findings from U.S. and international
sexuality data, it is important to comment on methodo-
logical constraints inherent to the study of human sex-
ual behavior. The study of human sexual behavior is one
of the most emotionally, politically, and socially charged
domains of human experience, complicating its study.
Direct observations of human sexual behavior are rarely
possible. While a primatologist can observe nonhuman
primates in the wild or captivity engaging in various
sexual behaviors (Dixson, 2012), few humans will toler-
ate the same intrusions into their sex lives. Accordingly,
the vast amount of information on human sexual behav-
ior derives from questionnaire responses and interviews.
Individuals may be asked to complete a survey, some-
times anonymously and confidentially, on a paper-and-
pencil version, or perhaps via a computer interface. Col-
lecting data by these means raises questions of validity.
How do we know whether individuals are accurately
describing their sexual behavior, especially when there
EVOLUTION AND HUMAN SEXUALITY 105
American Journal of Physical Anthropology
may be temptations to downplay (e.g., affairs) or exag-
gerate (in the attempt to impress others) claims? Some
cross-checks are feasible, such as aligning data between
partners or the sexes (e.g., regarding coital frequency
within partnerships) or other sources of clinical or com-
mercial information (e.g., patterns of STIs and knowl-
edge of contraception use should be consistent with
reports of sexual behaviors). The appropriate caution is
thus warranted when evaluating data on human sexual
behavior.
Sex in large U.S. surveys
From Ed Laumann and colleagues’ (1994) study of
U.S. sexuality, approximately 5,000 individuals aged 18–
59 years participated in in-person interviews. Outcomes
included frequency and types of various sexual behav-
iors, and were provided with respect to sociodemographic
variables including age group, sex, sexual orientation,
ethnicity, and educational attainment. Data on the fre-
quency of sexual behavior the past year are presented
with respect to age group and sex in Table 2. As shown
in this table, the percentage of women and men report-
ing having sex four or more times the past year tends to
diminish with age, while the percentage of women and
men not having sex the past year is highest among
young and old age groups.
This same U.S. survey provided a quantitative snap-
shop of other features of adult sexuality. Across all age
groups, men engage in more frequent masturbation than
do women. As an example, at ages 18–24, 29% of men
and 9% of women reported masturbating at least once a
week. Masturbation rates also declined with age, with
10% of men and 2% of women aged 55–59 reporting
weekly masturbation. Men reported more often thinking
about sex and using autoerotic material such as watch-
ing erotic videos; for example, 54% of men and 19% of
women reported thinking about sex at least daily. Extra-
marital affairs were noted among a minority of respond-
ents: “Over 90 percent of the women and over 75 percent
of the men in every cohort report fidelity within their
marriage, over its entirety.” (Laumann et al., 1994: 214).
Men reported a higher frequency over various time
frames engaging in same-sex sexual behavior than did
women with, for example, 2.0% of men and 0.8% of
women specifying having sex with same-sex partners
over the past five years. With respect to sexual identity,
2.8% of men and 1.4% of women reported either bisexual
or homosexual identity. Women (22%) were much more
likely to report being forced to have sex than were men
(1%), with most of women’s forced sex occurring with
known individuals such as someone with whom the
woman was in love or someone she knew well.
More recently, a nationally representative probability
sample of U.S. sexuality was conducted in 2009 by schol-
ars based at Indiana University, thus continuing Kin-
sey’s legacy of work there. This study relied upon
computer-aided survey. A sample of 5,865 men and
women aged 14–94 participated. Findings from the sur-
vey were published in a special edition of the Journal of
Sexual Medicine in 2010. The study presented data on
various sexual practices such as intercourse, masturba-
tion, and oral sex with respect to age, sex, and a host of
social variables including sexual orientation, ethnicity,
education, income, and geographical region. Among
some of the take-home findings from the study, males
engaged in higher rates of masturbation than females
across the sample; age-related patterning in sexual
behaviors showed that masturbation was relatively more
common early and late in adult life, while vaginal sex
relatively more common during the prime reproductive
years; and for both men and women partnered sexual
behaviors declined in frequency in the 50s onward,
showing even steeper declines around the 70s.
A quantitative cross-cultural approach
to human sexuality
Apart from U.S. studies, we also want to know about
the wider cultural record of human sexuality. What can
be said about the sexual behavior of people around the
world? Charles Darwin himself tried to collect as much
information on mate preferences and other features of
sexuality as he could by compiling the observations of
travelers and others with international experience.
Those cross-cultural anecdotes are included in his 1871
book. As far as more rigorous cross-cultural study of
human sexuality, the standard bearer has been Ford
and Beach’s (1951) Patterns of Sexual Behavior. This is a
compilation of cross-cultural accounts drawn from the
human relations area files (HRAF). In Ford and Beach’s
book, sections are devoted to a variety of sexual prac-
tices, from subadult sexuality to adolescence, and adult
sexual behaviors ranging from intercourse positions to
extramarital affairs. Variation is recognized and in some
cases patterned; for example, longer taboos against post-
partum female sexual behavior are found in polygynous
societies, consistent with the idea that a wife with cow-
ives can abstain longer from sex after having had a baby
when there is another sexual partner available to her
husband. Additionally, Ford and Beach concluded that
cross-cultural standards of attractiveness varied,
although female appearance garnered more attention
than male appearance, and male social success often
mattered more than attractiveness in determining male
marital and reproductive success. In this section, we
move beyond Ford and Beach’s book to present more
recent international studies of human sexual behavior.
Some of these more recently published studies are deriv-
atives of the HRAF data set, while others rely on nation-
ally representative probability samples of sexual
behavior from various countries around the world.
The most widely used quantitative measures of cross-
cultural sexuality tend to rely upon marital patterns.
Societies are variously described as monogamous, with
TABLE 2. Frequency of sex in the past year from early 1990s
U.S. national sex survey
Age
group
Not
at all
A few
times
per year
A few times
per month
2–3 times
per week
4 or more
times
a week
Men
18–24 15 21 24 28 12
25–29 7 15 31 36 10
30–39 8 15 37 23 6
40–49 9 18 40 27 5
50–59 11 22 43 20 2
Women
18–24 11 16 32 9 12
25–29 5 10 38 37 10
30–39 9 16 36 33 6
40–49 15 16 44 20 5
50–59 30 22 35 12 2
106 P.B. GRAY
American Journal of Physical Anthropology
mild polygyny, generalized polygyny, and occasionally
polyandry based on the most prevalent patterns of mar-
riage. As noted in the more than 1,200 societies compris-
ing the HRAF, approximately 85% have some degree of
polygyny (Murdock, 1949). A related sample, restricted to
the so-called standard cross-cultural sample (Murdock
and White, 1969), viewed as a more representative (with
respect to linguistic and geographic distribution) cross-
cultural sample of 186 societies, shows similarly that
mild polygyny is the most common marital pattern, with
around 15% of societies classified as monogamous. The
use of marital patterns as a proxy for aspects of sexuality
has its pluses and minuses. On the favorable side, this
measure is more readily available in the literature, most
of partnered human sexual behavior occurs within long-
term relationships such as marriage, and the relations
between marital and other quantifiable variables such as
paternal involvement enables testing for links between
sexuality and other factors. On the downside, use of
societal-level marital patterns ignores premarital and
extramarital sex, reliance upon prostitution, and ignores
within-societal variation in sexuality as well as that aris-
ing in other contexts such as across the full life course.
Evolutionary psychologists led by David Buss and
David Schmitt have conducted some of the largest inter-
national surveys related to human sexuality. These
efforts relied upon survey administration to samples in
various countries of the world, frequently relying upon
international university student samples but in other
cases also including community samples. These samples
thus lack the small-scale hunter-gatherer, horticultural,
agricultural and pastoralist emphases of the cross-
cultural anthropological surveys drawing upon the
HRAF or SCCS. That said, these studies have yielded
rich findings that speak to sex, age-related, and other
aspects of human sexuality.
From the effort led by David Buss (1989, 2003), mate
preferences were surveyed among 37 societies around
the world. Among his findings, love, emotional stability
or maturity, and a dependable character were ranked
the highest among traits preferred in a mate by both
men and women around the globe. These preferred traits
accord with what one would expect for maintenance of a
successful long-term sexual and reproductive relation-
ship. Buss also found sex differences in mate choice.
Men valued female attractiveness more than women did,
whereas women valued status and resources in a part-
ner more than did men. Men preferred younger part-
ners, whereas women preferred older partners. Variation
across societies also emerged. Chastity in a mate was
highly valued in some countries such as India and Indo-
nesia, but not in other countries such as the Nether-
lands and France.
David Schmitt spearheaded an international project
focused on questionnaire-based investigation of human
sociosexuality through the International Sexuality
Description Project (Schmitt, 2003, 2005). This sample
drew upon approximately 16,000 participants from 52
countries. Among the central findings were that men
consistently expressed more interest in having multiple
sexual partners than women, and across variable
lengths of time (e.g., over the next month to next 30
years). Similarly, men consistently had higher sociosex-
uality scores (e.g., expressing more open rather than
restricted sexual views) than did women across the sam-
ple. Yet in both desired number of sexual partners and
sociosexuality there was considerable international vari-
ation observed. In East Asian samples, 17.9% of men
and 2.6% of women said they wanted more than one sex-
ual partner over the next month, whereas in South
America 35.0% of men and 6.1% of women wanted more
than one sexual partner over the next month. Across
countries, several variables were correlated with socio-
sexuality scores. The operational sex ratio (OSR) was
correlated with sociosexuality scores, whereby societies
with a relatively higher proportion of women tended to
have less restricted sociosexuality. Fertility rates were
inversely related to sociosexuality, such that societies
with higher rates of fertility had more restricted socio-
sexuality. Further, scores on the Human Development
Index (a measure of societal quality of life) were posi-
tively associated with more open sociosexuality.
An increasing number of countries, apart from the
U.S., have conducted nationally representative probabil-
ity samples of sexual behavior. To illustrate, the first
probability sample of sexual behavior in China was con-
ducted in 1999–2000 (Parish et al., 2007). This Chinese
study included more than 3,700 adults aged 20–64 who
reported to a secure neighborhood location to complete
an interview and items on a laptop computer. As an
illustration of the findings, approximately 70% of men
and 30% of women under the age of 30 reported mastur-
bating, rates that were higher than previous reports in
China based on smaller samples, and consistent with
recent changes in sexual behavior. Additionally, approxi-
mately 50% of men and 30% of women in the late 1990s
reported engaging in premarital sex, rates that were
markedly higher than data from previous decades, espe-
cially the 1950s. Rates of same-sex sexual behavior were
higher for men than women, a pattern also found cross-
culturally (Kirkpatrick, 2000; Poiani, 2010).
The largest compilation of international sexual behav-
ior data yet was published by Kaye Wellings and col-
leagues in 2006. They drew upon data from 59
countries, from Kenya to Ghana, Bangladesh to Turkey.
All studies were published between 1995 and 2005,
included some longitudinal analyses, and featured
nationally representative samples. The compilation was
facilitated by the availability of Demographic and Health
Surveys from many developing countries, conducted in
order to provide baseline data informing reproductive
health agendas but also providing an unprecedented
availability of quantitative, international data.
A number of take-home points emerge from the analy-
sis, showing both patterns observed across all samples
as well as variation that itself invites explanation. In all
countries, married men and women report a higher like-
lihood of engaging in sexual behavior in the past four
weeks compared with unmarried respondents. This pat-
tern reinforces the context of most human sexual behav-
ior, occurring within the context of a long-term
relationship such as marriage. Across samples, men typi-
cally report more often having had multiple sex partners
the past year than do women. This sex difference
appears across an array of contexts, including premari-
tal, extramarital or marital (as in polygynous marriages)
sexual behavior. The countries in which a higher preva-
lence of women report multiple sex partners the past
year tend to be in Europe or North America, and largely
reflect premarital behavior. Age discrepancies in mar-
riages reveal that husbands tend to be older than wives,
although the age difference is lowest in countries such
as the U.S. (2.2 years) and Australia (1.9 years), and
highest in African countries such as Burkina Faso (14.7
EVOLUTION AND HUMAN SEXUALITY 107
American Journal of Physical Anthropology
years) and Mali (13.2 years). The age of first intercourse
varies globally. In some countries, women have an ear-
lier sexual debut and higher intercourse rates compared
with men; examples include a number of west African
countries and Bangladesh, in which women marry at
younger ages and the pattern reflects having sex within
marriages. The prevalence of premarital sex among
women is highest among countries such as the U.S. and
in Europe, but quite low in other countries such as Mali,
Nepal, and Indonesia. In the latter cases, less than 25%
of women report engaging in intercourse before mar-
riage. In other countries, such as Brazil, men tend to
have younger ages of sexual debut than women, with
this reflecting higher rates of male premarital sex,
including in some cases with prostitutes. Data compiled
by Wellings and colleagues only tabulate use of prosti-
tutes by men, a recognition that prostitution is largely
an industry servicing men; most male prostitutes service
other men rather than women. Yet the percentage of
men who say they paid for sex in the past year—a mea-
sure of use of prostitutes—varies around the globe.
Higher rates are found in China and East and West
Africa (around 10%) than in Latin America, North Amer-
ica, Europe, and North Africa (less than 3%).
Sex on the Internet
New sources of data on human sexuality continue to
arise. The Internet is one opening novel avenues by
which measures of human sexuality can be obtained and
interpreted. This is reflected in multiple ways, including
the diversification of websites catering to all kinds of sex-
ual and romantic interest groups. Online dating has
become a burgeoning industry and an increasingly com-
mon means by which partners in countries such as the
U.S. meet. The quantitative analysis of search contents
has also opened up fresh ways to explore human sexual
expression. The most ambitious and current account to
quantitatively explore sexuality related Internet search
contents was published in Ogas and Gaddam’s (2011) A
Billion Wicked Thoughts. They report the results of over
400 million web searches from more than 2 million users,
primarily from the U.S., but also other countries such as
the U.K., India, Nigeria, and Canada. They analyzed
search data from 2009–2010 as well as a supplementary
AOL search database on more than 650,000 users. Much
of their effort honed in on potential differences in search
content according to sex and sexual orientation.
Summarizing what they found, “On the Web, men pre-
fer images. Women prefer stories. Men prefer graphic
sex. Women prefer relationships and romance. This is
also reflected in the divergent responses of men and
women when asked about what sexual activities they per-
form on the Internet.” (p. 19) Men were much more likely
(37%) than women (6%) to list viewing erotic images and
movies as preferred Internet sex-related activities. Con-
versely, women (21%) were more likely than men (8%) to
report sex-related activities on the Internet involving
attempts to maintain contact with love or sex partners.
Of other patterns in Internet sex searches, younger
women (age 16–18) draw the most interest, consistent
with male orientation toward cues of high reproductive
value and pursuit of a long-term mate.
Summary
A variety of large U.S. and international quantitative
sexuality data are available. These data reveal patterns
with respect to sex (male/female), sexual orientation and
age, among other factors, while also providing a window
into cross-cultural variation. Sex differences in behav-
ioral measures of desire such as masturbation or use of
prostitution are quite robust across these samples, con-
sistent with theory that males have evolved a more pro-
ceptive sexuality. Males engage in higher rates of same-
sex sexual behavior than females in these large U.S. and
international data sets. Age-related declines in various
measures of sexual behavior, such as coital frequency,
also conform to waning selective pressures to maintain
sexual function, a topic we will also turn to in the next
section on human life course variation in sexuality. Most
sexual behavior occurs within the context of long-term
bonds, even when humans today face an unprecedented
array of novel potential mating partners compared to
hunter-gatherer forebears. Yet the international data
also indicate that, at the societal level, slight polygyny is
a better descriptor of human marital patterns than
monogamy. The international data also underscore the
importance of contextual variables such as fertility rate
and OSR. As an example, the societies with higher fertil-
ity rates tend to have more restrictive attitudes toward
female sexuality. The Internet provides a new window
into human sexuality, showing that some of the long-
standing patterns such as sex differences in sexual
desire manifest in this new arena. The collective weight
of this body of evidence argues for the continued rele-
vance of evolutionary theory to understanding human
sexuality today, and presents a standard against which
competing explanations are challenged to explain.
LIFE COURSE PERSPECTIVE ON HUMAN
SEXUALITY
The vast amount of theoretical and empirical attention
to human sexuality from an evolutionary perspective
has focused on prime adult years. A large literature
investigating and adaptively interpreting sex differences
in mating strategies has, for example, tended to empha-
size young adults who do not have children. In this sec-
tion, a life course perspective is applied to human
sexuality. This is important for a number of reasons: sex
differentiation of brains and genitals begins before birth;
juveniles learn and practice sociosexual behaviors (in
addition to economic and social skills emphasized by evo-
lutionary thinkers) that may be suited to their context
and future reproductive success; puberty and adoles-
cence transition individuals into the raised stakes of
potential reproduction; features of human sexuality fluc-
tuate across the prime adult years, including across peri-
partum transitions; and features of human sexuality
change with age in ways that can be understood with
respect to evolutionary-based models of reproductive
senescence.
Life history theory provides a theoretical and empiri-
cal toolkit to make sense of the patterning of human sex-
uality across the life course (Stearns, 1992; Geary, 2002;
Hawkes and Paine, 2006; Muehlenbein, 2010). Individu-
als have limited time and energy to devote to competing
allocations of growth, maintenance, and reproduction.
The specific allocations to these agendas can exhibit age-
, sex-, and context-specific variation in ways that may
make adaptive sense. There may also be tradeoffs across
these allocations, meaning that allocation to one out-
come comes at expense to another. To illustrate, based
on the theory reviewed above, female mammals tend to
108 P.B. GRAY
American Journal of Physical Anthropology
devote more of their reproductive effort to parenting
effort, whereas male mammals tend to devote more of
their reproductive effort to mating effort. The shifts
from growth to reproductive effort at puberty entail
development of secondary sexual characteristics; by
waiting until puberty to develop these characteristics,
organisms delay the potential social and energetic costs
of their maintenance. Peripartum shifts in female socio-
sexuality, which can also impact the sexuality of a long-
term partner, can be viewed from the standpoint of life
history tradeoffs between investment in current and
future reproduction.
The early development of human sexuality
As an unusual anecdotal insight into the development
of human sexuality, an ultrasound taken of a male fetus
appeared to show him engaging in autostimulation (mas-
turbation), an indication that sexuality can precede birth
(Meizner, 1987). Female infants are capable of vaginal
lubrication, and male infants of erections. Infants of
both sexes are capable of orgasm, though without ejacu-
lation. These observations indicate that even young
children have a limited sexual capacity, but a capacity
that can foster engaging in the kinds of sociosexual play
that may eventually attune their sex- and context-
specific reproductive investment. To help capture some
of the key processes and patterns of human sexuality
across the life course, from the perinatal period onwards,
these are summarized in Table 3. I consider some of
these major processes and patterns from early to late in
the life course, touching on some of the most common
changes in sociosexual behavior, underlying physiologi-
cal processes, and with some reference to life history
theory.
Rigorous, quantitative studies of childhood sexuality
are fraught with ethical and methodological challenges,
even more than research on adults. Thus, the available
data are quite limited, and the studies conducted by
Alfred Kinsey and colleagues in the mid-20th century
still stand as pillars in the field. What Kinsey