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Gorillas in Our Midst? Human Sexual Dimorphism and Contest Competition in Men

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The literature on human sexual selection has historically focused on the role of female mate choice, but cumulating experimental, correlational, and cross-cultural evidence suggests that male contest competition may have been more influential in shaping men's phenotypes. Cross-species comparison has shown similarities between humans and our closest extant phylogenetic relatives, the Great Apes, in male–male aggression, and archeological evidence also indicates a great antiquity for male–male violence. Compared to women, men possess substantially greater muscle mass, strength, cranial robusticity, physical aggression, pain tolerance, risk-taking, weapons use, and participation in coalitional aggression. Men also exhibit displays of physical prowess and acuity to the formidability of male conspecifics, as well as possessing a suite of traits, such as facial hair and low vocal pitch, that increase perceptions of dominance. These traits are consistent with having been shaped by contest competition over mates: they are sexually dimorphic, appear at sexual maturity, and predict success in male contests as well as success in mating and reproduction. While alternative explanations for some of these sexually dimorphic traits are possible, contest competition among males throughout human evolutionary history is the most parsimonious.
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Chapter 15
Gorillas in Our Midst? Human Sexual
Dimorphism and Contest Competition
in Men
A.K. Hill
1
, D.H. Bailey
2
and D.A. Puts
3
1
University of Washington, Seattle, WA, United States;
2
University of California, Irvine, CA, United States;
3
The Pennsylvania State University,
University Park, PA, United States
INTRODUCTION
Around 500 BCE, the Carthaginian explorer Hanno the
Navigator sailed south with a eet of 60 ships along
the northwest coast of Africa, encountering an island people
he reported were comprised mostly of women. The men
managed to escape, but Hannos expedition captured three
women who fought back so violently that they were
executed by their abductors, their skins carried back to
Carthage. Apparently oblivious to the hypocrisy, Hanno
referred to these people as savages. His interpreters called
them Gorillai,tribe of hairy women(Hanno, 1832). Over
two millennia later, in 1847, Americans Thomas Savage and
Jeffries Wyman borrowed this term when rst describing the
western gorilla (Conniff, 2009). Savage, Wyman, and the
actions of Hannos expedition were products of their times
and easily strike the 21st century reader as racist and vile.
We will revisit the human tendencies to classify others as
in-group versus out-group, potentially regard out-group
members as subhuman, and aggress against them, in our
discussion of coalitional aggression in a later section.
Returning for the moment to gorillas, we note that
research on these intelligent, elusive apes has revealed a
wealth of information about their ecology and behavior.
One of the most conspicuous aspects of gorilla social
structure is that males aggressively defend groups of
females from other males using their massive size, long
canine teeth, and threat displays (Fossey, 1983; Maple and
Hoff, 1982). Such traits are central to the behavioral
repertoire of male gorillas, who experience what Charles
Darwin termed sexual selection, a type of natural selection
engendered by competition over mates (Andersson, 1994).
Since the publication of DarwinsThe Descent of Man
in 1871, a vast literature has accumulated demonstrating the
salience of sexual selection in shaping the phenotypes of
sexually reproducing organisms, including gorillas and
humans. Sometimes referred to as Darwins other idea,
sexual selection favors traits helpful in winning mating
opportunities and is effectuated to varying degrees across
species by a handful of mechanisms (Andersson, 1994).
These mechanisms include mate choice, which favors traits
desired by the opposite sex; sexual coercion, the use of
force or threat against mates; sperm competition, the post-
copulatory struggle for fertilization of ova that favors traits
such as increased sperm production and motility; and
contest competition, favoring traits that help win mates
through same-sex physical contests and/or threatening
displays directed at rivals.
Much research has focused on mate choice, though
recent years have seen a reevaluation of sexual selection in
humans with an increase in attention to contest competition,
whose inuence now seems to have been underestimated
(eg, Carrier and Morgan, 2015; Hill et al., 2013; Puts, 2010,
2016; Puts et al., 2015; Scott et al., 2012). In this chapter, we
consider data bearing on the inuence of contest competition
on mens phenotypes, but rst we evaluate evidence for the
overall intensity of sexual selection among ancestral men.
STRENGTH OF SEXUAL SELECTION IN
MEN
A fundamental reality of reproduction in many sexually
reproducing species is an asymmetry in parental
On Human Nature. http://dx.doi.org/10.1016/B978-0-12-420190-3.00015-6
Copyright ©2017 Elsevier Inc. All rights reserved.
235
investment: one sex, usually females, invests more in pro-
ducing and caring for offspring (Trivers, 1972). Women,
for example, require approximately nine months for
gestation, followed by a long period of lactation and
childrearing, while a contrastinglysmallparentaleffortis
required for men to reproduce (Eibl-Eibesfeldt, 1989;
Geary, 2000). As a result, men have a higher potential
reproductive rate (Clutton-Brock and Vincent, 1991), and
in natural fertility populations, there are at all times more
menavailabletomatethantherearereproductivelyviable
women (Trivers, 1972). This skews the operational sex
ratio (OSR), the number of sexually active males per
fecund female, in a way that tends across species to force
males into competition for the relatively few available
females (Clutton-Brock and Vincent, 1991; Emlen and
Oring, 1977).
The physiological OSR, which includes all individuals
capable of reproducing, can be distinguished from the
behavioral OSR, which includes only those engaged in
mating (Marlowe and Berbesque, 2012). This may be an
important distinction because the amount of time in-
dividuals are typically eligible as mating partners may exert
a greater effect on sexual selection than does the amount of
time individuals are physiologically capable of reproducing
(Kokko and Jennions, 2008; Kokko et al., 2012). The hu-
man behavioral OSR has been estimated to be 8.6, and the
physiological OSR has been estimated to be 11.7 (Marlowe
and Berbesque, 2012). Regardless of which measure is
used, however, the human OSR falls below orangutans
(OSR ¼55.0) and gorillas (OSR ¼83.8), which display
the highest degrees of sexual dimorphism in the primate
order, and above promiscuously mating chimpanzees
(OSR ¼4.5), which also exhibit substantial individual and
coalitional contest competition, as well as a high degree of
sperm competition (Mitani et al., 1996). Hence the human
OSR indicates the opportunity for moderately strong sexual
selection in men.
The intensity of sexual selection is also inuenced by
the spatial distribution of males and females, which affects
whether mates can be monopolized. Across species, female
reproduction is typically limited by access to food, whereas
male reproduction is limited by access to mates. Hence,
females generally distribute themselves according to
resource availability, as well as predation risk, and males
then distribute themselves according to the distribution of
females (Lindenfors et al., 2004). If females are social or
otherwise spatially clustered, then one or a few males
may be capable of monopolizing multiple females, and
competition to be one of the few breeding males will tend
to be intense (Emlen and Oring, 1977). However, if female
group sizes are large, then it may be infeasible for a smaller
number of males to defend them from other males, and
larger multimale/multifemale groups will emerge. Male
mating competition is often intense in such species,
frequently taking the form of male dominance hierarchies
in which dominant males gain more mating opportunities,
especially with estrous females. However, in primates,
canine size dimorphism and body size dimorphism tend to
be less extreme in multimale/multifemale species than in
species with single-male polygyny (Clutton-Brock and
Harvey, 1984), probably in part reecting less intense
sexual selection.
Although humans form large multimale/multifemale
groups, the human mating system is not typical of such
primates, as concealed ovulation prohibits males from
attempting to monopolize estrous females (Gangestad and
Thornhill, 2008; Puts et al., 2013). Perhaps as a conse-
quence, within these larger groups are embedded smaller
groups of individual males mated in relatively stable and
exclusive relationships to one or more females. These
mateships exhibit moderate polygyny. For example, on
average, 21% of married women are married polygynously
across forager societies, although the degree of polygyny
varies substantially across cultures and ecological contexts
(Marlowe and Berbesque, 2012). As in hamadryas baboons
(Schreier and Swedell, 2009), malesproprietariness over
their mates helps maintain these embedded harems(Daly
and Wilson, 1988).
Malesability to monopolize females may also be
contingent on the dimensionality of the mating environ-
ment (Puts, 2010). Males may more effectively monopolize
females in environments that are one-dimensional (eg,
burrows or tunnels) or two-dimensional (eg, land or oors
of bodies of water), as opposed to three-dimensional
environments, such as air, open water, or trees, where the
region that must be defended is larger, and there are many
in-routes for sexually interloping males. As a result,
compared to three-dimensional mating environments,
terrestrial environments may favor greater physical
competition for mates, as in terrestrially breeding seals
(Stirling, 1975) and turtles (Berry and Shine, 1980) relative
to aquatically breeding species. Hominins have evolved in
a two-dimensional environment at least since the
emergence of habitual bipedalism in Australopithecus
anamensis roughly four million years ago (Harcourt-Smith,
2007). This shift in the human paleoenvironment likely
played an inuential role in shaping mating behavior in our
lineage. In particular, the new terrestrial environment may
have enabled ancestral males to restrict sexual access to
mates to a degree that was infeasible in their more arboreal
ancestors.
Male mating competition also tends to increase with
reproductive variance among males compared to females
(Bateman, 1948). In humans, male reproductive variance
exceeds that of females but this difference is highly variable
across populations (Brown et al., 2009). For example,
reproductive maxima have been reported as 8 and 14 for
women and 23 and 43 for men among two populations of
lowland South American Indians (Chagnon, 1992; Salzano
et al., 1967), an approximately three-fold increase among
236 PART | I Biological Basis of Human Diversity
males. However, the concentrations of wealth and power
afforded by state-level societies have produced staggering
reproductive variances. According to the Guinness Book of
World Records, the largest number of children known to
have been produced by a single woman is 69, while that for
a man exceeds 1000 (Glenday, 2013). Men who are able to
compete successfully for mates have the potential to pro-
duce a large number of offspring, whereas others may not
reproduce at all. Women, by contrast, accrue less additional
reproductive success from acquiring multiple matesdat
most, they can reproduce approximately once per year, but
interbirth intervals span several years in traditional societies
(Blurton Jones, 1987).
It is thus unsurprising that societies allowing polygy-
nous marriage are far more common in the ethnographic
record than are those with only monogamous marriages
(Murdock, 1967). Yet, even in societies with exclusively
monogamous marriage, the mating system may neverthe-
less tend toward polygyny; men remarry (Buckle et al.,
1996) and reproduce (Jokela et al., 2010) after divorce at
higher rates than women (though see Borgerhoff Mulder,
2009), with reproductive variances comparable to those
seen in societies with polygynous marriage (Brown et al.,
2009). This pattern is not restricted to the West, the
industrialized world, or the Global Northdmen exhibit
greater reproductive variance than do women among some
serially monogamous hunter-gatherers, as well (Hill and
Hurtado, 1996).
The previously mentioned evidence suggests a robust
potential for sexual selection in shaping mens anatomy and
reproductive behavior. Women provide greater levels of
parental investment through producing and rearing
offspring, removing themselves from the mating pool for
longer periods of time with each reproductive event. The
consequent imbalance in the OSR predicts increased
competition among males for mating opportunities. Mens
potential to monopolize mates is reduced by womens ag-
gregation into large groups but promoted by their terres-
triality, which enables some men to defend multiple mates.
As a result, men display greater reproductive variance than
do women. These lines of evidence are consistent with a
strong potential for sexual selection to shape mens phe-
notypes. However, sexual selection also requires
nonrandom mating among men, such that mens pheno-
types affect their mating opportunities. As we will now
discuss, ancestral men appear to have won mates
by developing traits that contributed to success in contest
competition.
SELECTION FOR MALE CONTESTS
When evaluating the inuence of past sexual selection on a
trait, it is useful to consider several types of evidence
(Table 15.1). First, high levels of sexual dimorphism
suggest past sexual selection (Andersson, 1994; Darwin,
1871; Lande, 1980). Second, traits that function in mating
competition are often costly to produce and maintain, and
so often develop or increase in their expression at sexual
maturity when they can begin to compensate for these
costs. Third, if a trait affects success in one or more
mechanisms of sexual selection (eg, by attracting mates or
winning ghts with same-sex competitors), then this
suggests that the trait did so over its evolution. This may be
demonstrated experimentally (eg, Andersson, 1982), or by
providing correlational evidence that the trait inuences
success under a mechanism of sexual selection
(eg, Schwagmeyer and Woontner, 1986). Fourth, if a trait
affects mating success, then this indicates that it did so over
its evolution. Again, experimental manipulations provide
stronger evidence of causation (eg, Andersson, 1982), but
correlational data can also provide evidence of a traits
inuence on mating success and may have the advantage of
increased ecological validity.
None of these lines of evidence is sufcient by itself.
For example, different ecological selection pressures such
as niche partitioning can produce sexual dimorphisms,
although this appears to be relatively rare (Andersson,
1994), and sexual selection may sometimes act similarly in
the two sexes producing low sexual dimorphism (Hooper
and Miller, 2008). In addition, if relevant aspects of the
environment have changed, then a trait may currently
inuence mating success differently than it did ancestrally.
However, in combination, these lines can provide strong
evidence for past sexual selection. If a trait develops at
sexual maturity, is sexually dimorphic, and appears to
inuence mating success through one or more mechanisms
of sexual selection, then it is likely that the trait was shaped
by sexual selection through these mechanisms.
Research exploring the inuence of sexual selection on
mens traits often emphasizes the role of female mate
choice, but this emphasis may be based partly on
misperceptions regarding the freedom with which women
chose mates ancestrally (see, eg, Puts, 2010). Given the
latitude with which women seem to choose mates in
contemporary Western societies, it may surprise some
readers that ancestral women probably experienced far less
autonomy. For example, more than two-thirds of extant
hunter-gatherer groups in a large sample had parentally
arranged marriages (Apostolou, 2007), and while women
may sometimes circumvent kin limitations on partner
TABLE 15.1 Evidence of Past Sexual Selection on a
Trait
lSexually dimorphic
lDevelops or increases in expression at sexual maturity
lAffects success in one or more mechanisms of sexual
selection
lAffects mating success
Gorillas in Our Midst? Human Sexual Dimorphism and Contest Competition in Men Chapter | 15 237
choice (eg, via mock bride theft;Ayres, 1974),
phylogenetic analyses suggest that family members
inuenced mating among the earliest members of our
species (Walker et al., 2011). Moreover, constraints on
female choice are imposed not only by kin but also by
unrelated men via sexual coercion (Puts, 2016; Puts et al.,
2015a), and especially by mens exclusion of male rivals by
force or threat (eg, Hill et al., 2013).
Indeed, the weight of evidence suggests that many
aspects of the human male phenotype are best explained by
an evolutionary history of contest competition. Contests
tend to produce several types of traits that aid in excluding
same-sex competitors by force or threat, including
behaviors such as same-sex aggression and threat displays,
and anatomical traits such as large body size and anatom-
ical weapons (Table 15.2). If any of these traits is present,
then it is a good candidate to examine for evidence of
having been shaped by sexual selection via contests. As we
will see, men tend to exhibit all of the hallmarks of a
species that has experienced an evolutionary history of
male contest competition.
Same-Sex Aggression
Across cultures, men and boys are more physically
aggressive than are girls and women (Archer, 2004; Ellis
et al., 2008), and mens aggression peaks at the age of
greatest mating competition (Archer, 2009). Murder in
particular is overwhelmingly a male phenomenon. Men
greatly outnumber women in both killing and being killed
everywhere that homicide has been studied (Daly and
Wilson, 1988), including both subsistence societies
(Beckerman et al., 2009; Chagnon, 1988; Walker and
Bailey, 2013) as well as industrialized Western nations
(Daly and Wilson, 1990). For example, in the Yanomamo,
horticulturalists living in parts of Brazil and Venezuela,
Chagnon (1988) estimated violence to account for nearly
one-third of male deaths, while Walker and Bailey (2013)
found the mean percentage of violent deaths among 11
traditional lowland South American societies to be 30%.
Males, moreover, accounted for 69% of all such deaths,
with comparable gures seen across the societies in the
sample. From a wide array of human populations, Daly and
Wilson (1988) report 95% of same-sex killings to be
perpetrated by men, even when war-related homicides are
excluded from analysis. This sex difference holds across
populations even as the number of homicides changes
dramatically from one to another (Daly and Wilson, 1990).
As Wright (1995, p. 72) has noted, From an evolutionary
point of view, the leading cause of violence is maleness.
Of course, a male bias in physical aggression and
violence, especially a bias in male-on-male aggression and
violence, is predicted from the hypothesis that ancestral
men competed for mates via contest competition. Physical
aggression may have helped men obtain or defend mates
directly, for example, by killing or injuring a competitor
attempting to win the same mate (Marlowe, 2004), or
attempting to steal ones mate (Chagnon, 1992), but these
may have more frequently been accomplished indirectly
through the threat of physical harm (see later in this
chapter).
Another possible set of behavioral adaptations that may
allow males to remain physically aggressive during contests
relates to pain thresholds and tolerance. Physical pain sig-
nals actual or potential tissue damage and can motivate
withdrawal from a damaging situation and protection of a
damaged body part (Lynn, 1984). However, withdrawing
from injurious behavior imposes costs if the behavior is
potentially tness enhancing. In such situations, organisms
face a tradeoff between avoiding tissue damage and
missing opportunities to increase tness. The relative
weights of the associated costs and benets will vary be-
tween individuals and across contexts. Given stronger
sexual selection and greater reproductive variance among
men compared to women, men are generally expected to
engage in costlier, more injurious behaviors in contexts that
can augment mating success, such as contest competition.
Ancestral men who disengaged from or avoided contests
may have suffered fewer injuries but left fewer offspring.
The experience of less pain for a given stimulus may
represent a proximate mechanism shaped by sexual selec-
tion to facilitate continued engagement in contests despite
injury. If so, then men would be expected to exhibit less
pain sensitivity and greater pain tolerance, perhaps espe-
cially in competitive contexts.
These predictions have been borne out. Relative to
women, men can undergo more intense stimulation such as
physical pressure on the body for longer periods of time
before experiencing pain, they are able to tolerate more
pain, and these effect sizes are moderate to large (Fillingim
et al., 2009; Riley et al., 1998). Moreover, in one study,
men experienced analgesia after competition against a
same-sex competitor regardless of exercise, whereas
women experienced analgesia only after exercise (Sternberg
et al., 2001).
Anatomical Adaptations for Male Contests
While observational studies of aggression among primates
provide the most direct evidence of contest competition,
another relevant line of evidence concerns body size sexual
TABLE 15.2 Traits Favored by Contest Competition
lPhysical aggression
lLarge body size
lStrength
lWeapons
lBehavioral displays of formidability
lAnatomical threat displays
238 PART | I Biological Basis of Human Diversity
dimorphism. In a sample of 18 species of anthropoids
representing 12 genera, Mitani et al. (1996) conrmed the
relationship between body size dimorphism and OSR ex-
pected from sexual selection theory. In the human lineage,
sexual size dimorphism exhibits great antiquity revealed
through a large, albeit fragmentary, hominin fossil record
dating back millions of years (Plavcan, 2012). Yet, research
on levels of sexual dimorphism in earlier hominins is
equivocal. For example, the best-represented fossil homi-
nin, Australopithecus afarensis, which lived roughly 3e
4 million years ago, has been alleged by some researchers
(Gordon et al., 2008; Lockwood et al., 1996; McHenry,
1991), but not others (Reno and Lovejoy, 2015; Reno et al.,
2010; Reno et al., 2003), to have exhibited a level of sexual
dimorphism commensurate with that of extant gorillas and
orangutans.
By the time of Homo erectus, whose existence covers
most of the last 2 million years, sexual dimorphism had
reached the approximate levels of modern Homo sapiens
(Antón, 2003). Among modern humans, skeletal
dimorphism is consistent with a primate species in which
males are 45%e50% larger (Gordon et al., 2008).
Compared to women, men possess 31%e43% more
fat-free body mass (Lassek and Gaulin, 2009; Wells, 2012),
61% more muscle mass overall, and 75% more upper-body
muscle mass (Abe et al., 2003; Lassek and Gaulin, 2009).
As a result, the average man is stronger than 99.9% of
women (Lassek and Gaulin, 2009).
Men with a masculine, muscular body shape have
more sex partners (Frederick and Haselton, 2007; Hill et al.,
2013; Lassek and Gaulin, 2009), particularly in short-term
relationships (Rhodes et al., 2005), and begin having sex
at an earlier age (Hughes and Gallup, 2003) than do less
masculine men. Larger men, both in terms of height
(Frederick and Jenkins, 2015; Mueller and Mazur, 2001)
and body mass (Frederick and Jenkins, 2015), also report
more sex partners. Although mating success does
not necessarily translate into reproductive success among
natural fertility populations, positive relationships have
been observed between mens reproductive success and
both height (Mueller and Mazur, 2001; Pawlowski et al.,
2000) and physical prowess (Chagnon, 1988; Smith
et al., 2003).
Men also appear designed to weather bodily insults,
particularly to the head, which is disproportionately
targeted (Shepherd et al., 1988) and injured (Carrier and
Morgan, 2015)inghts. Brink et al. (1998) examined 2432
bodily injuries in 1156 men and 325 women in Denmark
for a one-year period in the mid-1990s, reporting 69% to
have been craniofacial, with injuries in both sexes tending
to be produced by blunt force at close range. Indeed, sexual
dimorphism in cranial robusticity may partly be attributable
to physical violence among men, as features such as more
robust mandibles and brow ridges may protect against
catastrophic facial fractures (Carrier and Morgan, 2015;
Puts, 2010). Women generally do not experience
commensurate levels of physical aggression (Campbell,
2013; Daly and Wilson, 1988) and display less cranial
robusticity and fewer cranial injuries (Carrier and Morgan,
2015; Shepherd et al., 1988).
Weapons Use
Contest competition often favors the evolution of
anatomical weapons, such as antlers, horns, and, in pri-
mates, large canine teeth. Yet, compared with our closest
living relatives, both men and women possess relatively
small canines, and we lack substantial canine-size sexual
dimorphism (Wood et al., 1991). Diminution in canine size
and a departure from the CP
3
honing complex, a typical ape
feature, begins with one of the earliest hominin candidates
(7 to 6 Ma), Sahelanthropus tchadensis (Brunet et al.,
2002), and continues through the Pliocene genera Ardipi-
thecus (Suwa et al., 2009) and Australopithecus (White
et al., 2000)toHomo after 2.5 Ma (Suwa et al., 2009).
One hypothesis for reduction in canines and other
skeletal features related to biting is that canine weaponry
was supplanted by handheld weapons and forelimbs freed
by bipedal locomotion (Carrier, 2011; Darwin, 1871;
McHenry, 1991). Clubs, spears, and hurled stones may
have obviated biting by keeping enemies at a distance in the
way that antlers appear to have replaced large maxillary
canines in several deer species (Barrette, 1977). In addition,
our shift to habitual bipedalism and its associated
orthograde posture enhanced the injuriousness of physical
blows (Carrier, 2011).
Male chimpanzees make and use tools, including using
branches in dominance displays (but not as offensive
weapons; van Lawick-Goodall, 1968), suggesting that tools
have been used since the last common ancestor of Pan and
Homo. While the earliest evidence of a weapon-inicted
wound has been dated to roughly 100,000 years ago
(Pickering et al., 2000), the emergence of manufactured
stone tools by at least 3.4 million years ago (Harmand et al.,
2015; McPherron et al., 2010) suggests that handheld
weapons were used far earlier. Indeed, it is difcult to
imagine an ancestral species with the mental capacity to
shape stones for use as cutting tools, and with males
ghting over mates, in which males would not also utilize
branches, bones, antlers, and other materials in their
environment as weapons.
The use of projectile weapons such as hurled rocks and
spears may have contributed to the very large male advan-
tage in throwing velocity (3.5 standard deviations by age 12
years; Thomas and French, 1985), as well as mens1.5
standard deviation advantage in targeting and avoiding
projectiles (Watson and Kimura, 1991). This targeting dif-
ference remains large after controlling for experience
(Watson and Kimura, 1991) and appears to depend
developmentally on early androgen exposure (Hines et al.,
Gorillas in Our Midst? Human Sexual Dimorphism and Contest Competition in Men Chapter | 15 239
2003). Across societies, the manufacture and use of weapons
against same-sex rivals is ubiquitous among men and rare
among women (Archer, 2004; Ellis et al., 2008; Smith and
Smith, 1995; Warner et al., 2005).
The development of handheld weapons represented a
watershed moment in hominin evolution, imbuing physical
contests among men with a previously unknown degree of
lethality. Moreover, for the rst time in primate evolution,
individuals were able to aggress from a distance, reducing
the advantage of anatomical weaponry and possibly
spurring the diminution of sexual dimorphism in body, and
especially canine, size. This inference has important
implications: if the use of handheld weapons is responsible
for reduction in the typical trappings of primate intrasexual
selection, then an examination of body size dimorphism
and canine size dimorphism may lead us to underestimate
the intensity of contest competition over the evolution of
our species.
Dominance Displays
Traits such as physical aggression, pain tolerance,
size, strength, facial robusticity, and weapons use may have
served to make men more competitive in physical
contests. However, physical violence is costly energetically
and in terms of risk of injury or death, as well as risk of
retribution (eg, Beckerman et al., 2009; Daly and Wilson,
1988). Across species, male contests frequently involve
displays and mutual assessment of formidability, often
ending when one rival submits before either is injured (see
Smith and Parker, 1976 for a discussion of asymmetric
contests). While the costs of submission in terms of reduced
social status and mating opportunities may be high, the
costs of defeat may be higher. As a result, ancestral
men capable of accurately assessing rivalsphysical
formidability likely obtained a selective advantage (Sell
et al., 2009, 2010; Wolff and Puts, 2010). Because
maleemale aggression is culturally ubiquitous (Daly and
Wilson, 1988) and characterizes all extant apes (Puts,
2010), we can be condent that there was selective pressure
for such acuity ancestrally.
Men are therefore expected to attend closely to
the formidability and volatility of their same-sex competi-
tors, to exercise caution accordingly, and to use nonviolent
means such as threats and negotiation to obtain status
and valued resources. Men appear to utilize cues such
as facial appearance (Carrier and Morgan, 2015; Sell et al.,
2014; Sell et al., 2009; Zilioli et al., 2014), muscularity
(Hill et al., 2013), and height (Stulp et al., 2015)to
assess one anothers formidability. However, some traits
that inuence dominance perceptions may have been
shaped by selection specically to signal formidability, as
we now discuss.
Behavioral Displays of Formidability
Men may avoid potentially deadly conict by displaying
their formidability to rivals in diverse ways, including
greater risk-taking behavior when peers are present
(Ginsburg and Miller, 1982; Morrongiello and Dawber,
2004) and when those peers are male (Ermer et al., 2008).
In support of this, research has associated dangerous
risk-taking with perceptions of physical formidability
(Fessler et al., 2014), suggesting that risk-taking and acuity
to risk-taking may have evolved for success in contests.
Among the Yanomamo, contests often involve shout-
ing matches, chest pounding duels, side slapping duels,
club ghts, ghts with axes and machetes, and shooting
with bows and arrows with the intent to kill(Chagnon,
1988, p. 986). Among the Meriam of Australia, the hunting
of large sea turtles is physically demanding, potentially
injurious, and may function to signal formidability to other
men (Bliege Bird et al., 2001; Smith et al., 2003). Optimal
foraging and reciprocal altruism are unlikely to explain
turtle hunting behavior. Turtle hunting is inefcient as a
means of procuring food, hunters typically give away the
meat at feasts, and this altruism tends not to be recipro-
cated. In addition, women do not report greater attraction to
turtle hunters, so turtle hunting does not appear to function
in mate attraction either (Smith et al., 2003). However,
turtle hunting is respected by men, and turtle hunters report
earlier onset of sexual behavior and larger numbers of sex
partners than nonhunters, as well as 2.4 times greater life-
time reproductive success, with an even larger difference
for hunt leaders (Smith et al., 2003).
Mens greater average interest in playing and observing
sports, both in the contemporary United States (Deaner
et al., 2012) and across traditional societies (Deaner and
Smith, 2013), may also reect selection for displays of
formidabilitydas well as for physical aggression, interest
in competition, and the predisposition to engage
in activities that build strength and hone skills useful in
contests. The male bias in sports participation is striking:
in a sample of 50 societies taken from the Human Relations
Area Files, males participated in 95% of all sports, females
in only 20%, with men predominating especially in
combat-related sports (Deaner and Smith, 2013).
Anatomical Threat Displays
Humans are among the most visually sexually dimorphic
primates (Dixson et al., 2005), and recent research suggests
that at least some conspicuous traits may function to
increase mens appearance of formidability. For example,
beards and eyebrow hair grow at puberty in males and may
signal formidability through associations with physical
maturity and testosterone levels and by increasing the
apparent size of the jaw and brow (Guthrie, 1970;
240 PART | I Biological Basis of Human Diversity
Muscarella and Cunningham, 1996; Neave and Shields,
2008). Male faces with beards are rated as more dominant
but not more attractive than the same faces clean-shaven
(Dixson and Vasey, 2012; Muscarella and Cunningham,
1996; Neave and Shields, 2008).
Likewise, both correlational (Hodges-Simeon et al.,
2010) and experimental (Feinberg et al., 2005; Puts et al.,
2006, 2007; Wolff and Puts, 2010) research shows positive
relationships between vocal masculinity, such as low pitch
and vocal timbre, and perceptions of mens dominance.
Mens vocal tracts are 15% longer, and their vocal folds
60% longer, than womens(Fant, 1960; Titze, 2000),
several times the 7%e8% expected from the sex difference
in stature (Gaulin and Boster, 1985). Elevated testosterone
levels at puberty cause malesvocal folds to grow longer
and thicker than those of females, both absolutely and
relative to overall body growth (Harries et al., 1997;
Hollien et al., 1994). Mens larger vocal folds consequently
vibrate at a fundamental frequency approximately half that
of females during phonation, which we perceive as a lower
pitch. Similarly, maleslarynges descend a full vertebra
lower than femalesat puberty (Fitch and Giedd, 1999),
producing a longer vocal tract and resulting in lower, more
closely spaced formant frequencies and a deeper, richer-
sounding timbre.
In a cross-cultural sample of voice recordings, men
accurately assessed physical strength from the voice even
when listening to unfamiliar languages (Sell et al., 2010).
Although pitch and timbre track body size within-sex only
modestly (González, 2004; Lass and Brown, 1978; Pisanski
et al., 2014; Rendall et al., 2005), masculine voices have
also been associated with physical aggressiveness, testos-
terone levels, and peer evaluations of ghting ability (Hill
et al., 2013; Puts et al., 2012a). It may be the case that
masculine voices are reliable signals of dominance even
while masculine voices are only modestly associated with
any particular correlate of dominance.
Dominance, Mating, and Reproductive
Success
Displays of formidability, whether behavioral or anatom-
ical, may have contributed to mating success among
ancestral men by increasing dominance (coerced social
status) and prestige (freely conferred deference; Henrich
and Gil-White, 2001). Displays of formidability may in-
uence prestige because dominant men can make strong
leaders and powerful allies and may also possess skills
worthy of emulation. Indeed, experimental evidence sug-
gests that social status is conferred upon dominant men in
proportion to their being viewed as likely to generate
benets for the group via within-group enforcement and
between-group representation (Lukaszewski et al., 2015).
Both dominance and prestige can thus aid in social
competition over all contested resources, including food
and territory, as well as mates (West-Eberhard, 1983).
Indeed, success in competition with other males has been
shown to increase mens preferences for feminine female
mates (Welling et al., 2013), suggesting that success in
maleemale competition increases access to desirable sex
partners.
In non-Western samples, both dominance and prestige
have been associated with increased mating and reproduc-
tive success in men (Chagnon, 1988; Smith et al., 2003;
von Rueden et al., 2011). However, this does not imply that
sexual selection has favored high levels of unrestrained
male belligerence. As noted previously, physical aggres-
sion is costly and should be dependent upon context,
including the likelihood of defeat and the threat of retri-
bution. For example, among the extremely bellicose
Waorani of Ecuador, men who participated in the most
raids of other villages did not have more wives or offspring
(Beckerman et al., 2009). Although failure to avenge ho-
micides may be perceived as a sign of weakness among the
Waorani, raiding also brought immediate retribution
against the raiders village. It is thus possible that a mod-
erate level of raiding represented the optimal balance be-
tween the costs of retribution and the costs of appearing
weak.
Among Western undergraduate students, a component
of mating successdnumber of sex partners in the past
yeardwas positively related to self-rated ghting ability in
two samples (Wolff and Puts, 2010), and male acquain-
tancesratings of ghting ability, as well as size and
muscularity, in another sample (Hill et al., 2013). Displays
of physical competitive ability such as sports performance
(Faurie et al., 2004; Honekopp et al., 2007) and gang
membership (Palmer and Tilley, 1995) have also been
positively related to mating success.
In addition, traits that inuence perceptions of domi-
nance predict mating and reproductive success. For
example, a masculine, dominant-sounding voice has been
associated with greater mating success in samples of US
undergraduates (Hill et al., 2013; Hodges-Simeon et al.,
2011; Puts, 2005), as well as with greater reproductive
success in a sample of Tanzanian foragers (Apicella et al.,
2007). Likewise, dominant facial appearance has been
found to predict eventual military rank and
reproductive success among military cadets (Mueller and
Mazur, 1997).
Coalitional Aggression
Males are more likely than females to kill and be killed by
conspecics among our closest living relatives, chimpan-
zees (Wrangham et al., 2006), who, like humans, engage in
Gorillas in Our Midst? Human Sexual Dimorphism and Contest Competition in Men Chapter | 15 241
coalitional aggression. Wrangham and Glowacki (2012,
p. 20) argue that humans generally conform to the pattern
seen in chimps: consistent intergroup hostility, safe
killing, and benets from intergroup dominance.Thus,
aggressive behavior among allied groups of males, which
presents early via boyhood competition (Geary et al.,
2003), has likely not only long been a feature of human life
(eg, Bamforth, 1994; Frayer, 1997), but may also be a
more primitive feature of our primate heritage. There are,
however, important distinctions that set humans apart.
Notably, hunter-gatherer groups display an ability for
peacemaking involving protracted periods of nonviolence
toward rival groups that is uncharacteristic of chimpanzees.
As Wrangham (1999, p. 18) writes, Peace is the normal
human condition, in the sense that most human groups, for
most of the time, are not at war.
A further difference is that chimpanzees engage in far
more overt aggression overall, but human aggression is
more often lethal, so that chimps and humans living in
subsistence societies exhibit similar levels of lethal
aggression (Wrangham et al., 2006). For example, the
Arnhem Land people of Australia are characterized by an
unusually high rate of physical aggression among human
populations (Wrangham et al., 2006) but nevertheless
display a rate of physical attack two orders of magnitude
below that of chimpanzees. In light of this, a comparison
with chimps seems apt only to a point in informing
our understanding of the possible evolutionary history of
human violence. Perhaps the lethality of human weapons
elevates the importance of threats, deference, and peace-
making in relation to physical attacks when negotiating
intragroup dominance hierarchies and intergroup conict.
In addition, the substantial death tolls attributable to
violence recorded among traditional human societies occur
at a level of social complexity greater than physical contests
between two males. Of importance is the presence of
organized and sanctioned group violence that involves
armed conict, including confrontations that combatants
recognize may result in deliberate killing,as Webster
(1998, pp. 313e314) has dened warfare. Conicts,
according to Webster, are perpetrated with the intent of
maintaining the status quo or bringing about a shift of
power relations, usually the latter.This denition is
similar to understandings of coalitional violence in
chimpanzees, which Wranghams (1999) imbalance-
of-power hypothesis argues is contingent on (1) hostility
and (2) power asymmetries among groups. The psycho-
logical traits that might be favored in the service of
coalitional aggression include, as Wrangham (1999, p. 23)
suggests, a tendency to classify others as in-group or out-
group, to regard members of out-groups as potential prey,
to be alert to (or search for) power asymmetries between in-
group and out-group parties, and to be ruthless in attacking
out-group parties when the perceived power asymmetry is
sufciently great.
While there are no doubt myriad proximate motivations
for organized group violence in our species, such as a
desire for slaves, territory, political control, revenge,
resolution of economic disagreements, and more fruitful
environments (eg, Keeley, 1996), selection ultimately
favors traits that contribute to reproduction. Hence, it is at
least parsimonious to hypothesize that reproduction lies at
the root of coalitional violence, as well. Even when a desire
for resources or political control is the immediate cause,
these desires may themselves have been forged in the res
of mating competition.
By way of raiding, men are able to forcibly procure
female mates through bride theft,which appears to be a
species-typical behavioral trait (Ayres, 1974). Across 10
traditional Amazonian societies, women were captured
during 26% of raids occurring within a language family and
54% of raids occurring across language families (Walker
and Bailey, 2013). Intergroup aggression among both
chimpanzees (Mitani et al., 2010) and humans (Bollig,
1990; Mathew and Boyd, 2014) may additionally enable
males to obtain territory and resources that contribute to
their mating success. Among human subsistence societies,
men appear well aware of what is in the reproductive
balance. As Chagnon (1988) reports of the Yanomamo, a
desire for women is the main impetus for engaging in
warfare, and, importantly, this is the top reason given by
Yanomamo, a nding not unique to that particular society.
While reasons for warfare and lesser forms of coalitional
violence are undoubtedly complex, the desire for mates is
acknowledged as a nearly ubiquitous motivation for
preindustrial warfare, even among scholars generally
unsympathetic to sociobiological theories of behavioral
evolution (eg, Keeley, 1996).
Male coalitional violence, regardless of its most
immediate cause, has produced an archaeological record
riddled with evidence of violent, often lethal, physical
aggression among men stretching back beyond the advent
of agriculture (Lahr et al., 2016). In one North American
paleoindian burial site, roughly 16% of skeletal remains
indicate violent death, 5% showing evidence of having
been scalped, and 4% decapitated (Milner et al., 1991).
Moreover, males account for a higher percentage of victims
of violence among all individuals of known sex. This is far
from an aberrant nding, with other burial sites yielding
similar results. Andrushko et al. (2005) estimated from a
burial site of 59 males and 86 females that at least 20% of
males, but only 2% of females, experienced a violent death,
likely the result of warfare, as evinced by perimortem
amputation. The men, furthermore, tended to be young
adults, which is the age range of ercest competition for
mates.
242 PART | I Biological Basis of Human Diversity
Of course, the relationship between male coalitional
violence and reproductive success is not always linear and
positive. As noted earlier, among the Waorani of Ecuador,
Beckerman et al. (2009) report poorer reproductive success
as well as exceptionally high mortality rates among the
most ardent warriors. Among humans everywhere, there are
great costs associated with aggression (Chagnon, 1988).
This may result in a curvilinear relationship between
aggression and reproductive success, with a maximum that
is likely contingent on numerous aspects of the social
environment. There would have thus been great benet
ancestrally associated with correctly assessing the potential
costs and likelihood of success in a raid, just as there would
have been great benet in correctly assessing the physical
formidability of a single male rival. Just as there would be
costs to stealing another mans mate, there are costs to
participating in a raid to steal the mates of many men. The
costs and benets associated with intragroup aggression
may have selected for high levels of intragroup cooperation
specically in the context of warfare. Indeed, experimental
research has shown males to exhibit greater group-level
contribution in the face of competition from other groups
(Van Vugt et al., 2007).
ALTERNATIVES TO MALE CONTESTS
We have reviewed evidence that mens phenotypes are
partly products of ancestral contest competition for mates.
Men exhibit each of the traits typical of species with male
contests, and these traits appear to have been shaped by
sexual selection; they are sexually dimorphic and predict
mens mating success as well as success in contest
competition. Many of these traits (eg, deep voices, beards,
muscularity) also emerge at sexual maturity. The excep-
tions are behaviors that require years of practice to hone
relevant skills: ghting, weapons use, behavioral displays
of formidability, and coalition formation (eg, Pellis and
Pellis, 2007; Thomas and French, 1985)dall of which
exhibit prepubertal sex differences, although the sexes may
further diverge at puberty. However, we have not yet
considered alternative hypotheses: whether some of the
previously mentioned traits were shaped by other selective
pressures, or arose as byproducts of selection on develop-
mentally correlated traits.
For example, some human sexual dimorphisms may
partly be products of a sexual division of labor that is
essentially ubiquitous across forager societies: men spend
more time hunting, especially larger game, and women
spend more time gathering or hunting smaller game
(Murdock, 1967). Thus, ecological selection may have
contributed to mens greater size, strength, and weapons
prociency to the extent that these contributed to hunting
success ancestrally (Kaplan et al., 2000). However, other of
mens traits, such as beards, deep voices, more robust faces,
and high levels of same-sex aggression, are not easily
understood as adaptations for hunting. In addition, given
that male contests and sexual size dimorphism probably
characterized the common ancestor of the great apes,
contest competition likely predates specialized hunting and
the human sexual division of labor by several million years.
Hence, it is more likely that the sexual division of labor is
partly a consequence rather than the initial cause of these
anatomical dimorphisms, although hunting likely imposed
additional selection pressures on these male traits.
Some aspects of mens phenotypes may also have been
produced via female choice, or through a combination of
contests and female choice. Because traits that evolve in
contest competition are often costly to produce and main-
tain and are constantly tested by competitors, such traits
may represent honest indicators of heritable tness, and
females may consequently evolve preferences for them
(Berglund et al., 1996). On the one hand, masculine bodies,
faces, and voices in men have indeed been found to
increase attractiveness to women (Frederick and Haselton,
2007; Puts et al., 2012b). On the other hand, the inuence
of facial and vocal masculinity on ratings of dominance
is considerably larger and more consistently positive than
the effects on attractiveness (Puts et al., 2012b). Recent
work suggests that male facial masculinity may not be
universally preferred by women across human societies,
whereas it much more consistently conveys the impression
of aggressiveness (Scott et al., 2014). Likewise, beards
reliably increase perceptions of age, aggression, dominance,
and social status across societies, but generally decrease
attractiveness to women (Dixson and Vasey, 2012;
Muscarella and Cunningham, 1996; Neave and Shields,
2008). Furthermore, in samples spanning Western (Hill
et al., 2013), traditional agricultural (Llaurens et al., 2009),
and preindustrial (Smith et al., 2003) societies, mensmating
success has been found to relate more strongly to dominance
among men and the traits that contribute to dominance than
to attractiveness to women. Across the suite of male
secondary sexual characteristics, then, selection for success
in physical contests may have been either attenuated or
augmented by selection for attractiveness to females. In
general, however, mens traits function far more effectively
in the context of male contests than in mate attraction, and
thus they do not appear to have evolved primarily as sexual
ornaments to attract women.
Another possibility is that some of mens traits
represent developmental byproducts of male body size or
testosterone levels. Strength increases with body mass and
height (Balogun et al., 1991), for example, although other
male traits are not known to relate to body size (eg, beards)
or relate only weakly (eg, voice pitch; Pisanski et al., 2014).
However, even traits that are correlated with size are far
more sexually dimorphic than would be predicted from sex
differences in size alone (Puts et al., 2012b). Similarly,
Gorillas in Our Midst? Human Sexual Dimorphism and Contest Competition in Men Chapter | 15 243
androgens such as testosterone play important roles in the
development of male-typical traits, so one might conjecture
that these traits are merely developmental side-effects of
androgens. This viewpoint confuses proximate and ultimate
explanation, leaving unresolved the question of why
humans have evolved to respond to testosterone by
growing facial hair and longer-thicker vocal folds, for
example. Why instead do we not respond to testosterone by
growing antlers, as in red deer (Suttie et al., 1995), or
canines, as in many other primates (Van Wagenen and
Hurme, 1950)? Why does testosterone not increase paternal
investment, as in the California mouse (Peromyscus
californicus)(Gleason and Marler, 2013), rather than
having the opposite effect, as it does in many vertebrates,
apparently including humans (Kuzawa et al., 2009; Puts
et al., 2015b)? Clearly, different species, even closely
related ones, can evolve quite different responses to the
same hormones. An evolutionary history of male contests
parsimoniously explains why, in humans, a particular
constellation of sexually dimorphic traits including large
size may be developmentally linked to testosterone and to
each other.
Anal alternative to contest competition in our
hominin ancestors is phylogenetic inertiadtheideathat
we have inherited our traits from ancestral species rather
than experiencing selection for these traits in our own
species. Fossil and comparative evidence indicate that we
did indeed inherit traits such as greater male size and
aggression from an ancient hominin ancestor. However,
for other traits such as deep voices (Puts et al., 2016),
beards, and the use of handheld weapons, this appears
not to have been the case. Even for traits such as greater
male size and aggression that were likely sexually
dimorphic in our common ancestor with chimpanzees, we
would expect considerable reduction in modern humans
if these traits were not functional over recent hominin
evolution, given their substantial costs. And yet, as dis-
cussed earlier, mens physical aggression is equally lethal
to that of male chimpanzees, and we are more sexually
dimorphic than chimpanzees in both skeletal size and fat-
free mass.
Despite the comparatively strong overall evidence for
the importance of contest competition over mens evolu-
tion, each of these alternative factors may have played a
role. These are not mutually exclusive alternatives; any
aspect of the phenotype can experience multiple selection
pressures, as well as responding to selection on other traits
with which it is developmentally correlated.
CONCLUSION
We have reviewed multiple converging lines of evidence
supporting a role for contest competition in shaping the
human male phenotype. Sex differences in parental in-
vestment, reproductive rates, and reproductive variance;
the OSR; and patterns of mating and marriagedall indicate
a history of moderately strong sexual selection among our
male ancestors.
Contest competition in particular tends to favor size,
strength, aggression, weapons, and threat displays, and
men display all of these features. Traits that point to an
evolutionary past in which our male ancestors competed
for mates through force and threat include a proclivity
for same-sex violence including coalitional aggression,
higher pain threshold and tolerance compared to women,
increased body size and strength, facial robusticity,
fashioning and use of weapons, beards, deep voices, and
behavioral displays of formidability. This evidence is
taken from research across elds ranging from human
anatomy and physiology to psychology, ethnography,
paleoanthropology, animal behavior, and archaeology.
The alternative hypotheses that mens traits were shaped
by selection for hunting ability, female mate choice, or
selection operating on developmentally correlated traitsd
or that mens traits are consequences of phylogenetic
inertiadcan help account for some of the above aspects of
mens phenotypes, but not others. The success of any
hypothesis is contingent on a parsimonious explanation of
the totality of evidence, and only contest competition
accomplishes this. In some ways, we may be more
gorilla-like, or chimp-like, than we prefer to suppose.
It is important to bear in mind, however, that while
human nature includes a propensity for violence in both
individual and coalitional forms, we are also capable of
negotiation, compromise, and restraint. For a species
currently numbering in the billions that now possesses
weapons capable of bringing about its own annihilation, the
importance of understanding our capacity for violence is
more than academicdit can potentially illuminate and
suggest solutions to problems of pressing societal concern.
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Gorillas in Our Midst? Human Sexual Dimorphism and Contest Competition in Men Chapter | 15 249
... Secondly, under the male-male competition hypothesis, authors have argued that formidable (i.e. physically strong and imposing) men are better equipped to compete with other men for resources, status, and partners (Hill et al., 2016;Puts, 2016), through e.g. direct physical contests or by deterring rivals indirectly (Hill et al., 2016;Sell et al., 2012). ...
... physically strong and imposing) men are better equipped to compete with other men for resources, status, and partners (Hill et al., 2016;Puts, 2016), through e.g. direct physical contests or by deterring rivals indirectly (Hill et al., 2016;Sell et al., 2012). For instance, increased musculature may intimidate competitors by signaling fighting prowess (Sell et al., 2009) and strength (Durkee et al., 2018), while facial masculinity and voice pitch may also have an indirect relationship with perceived formidability (Butovskaya et al., 2018;Haselhuhn et al., 2015;Raine et al., 2018;Little et al., 2015;Puts and Aung, 2019;Scott et al., 2014). ...
... In species with male intrasexual competition, males tend to evolve to become larger, stronger, and more formidable than females, as they are in humans. Some authors argue that malemale violence has influenced human evolution (Hill et al., 2016;Gat, 2015), and male intergroup aggression increases mating/reproductive success in both non-industrialized human societies and in non-human primates (Glowacki and Wrangham, 2015;Manson et al., 1991). (And indeed the non-human evidence might suggest this form of dimorphism has been under selection since prehominid ancestors, although the strength of such selection pressures have likely fluctuated over this time [172].) ...
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Humans are sexually dimorphic: men and women differ in body build and composition, craniofacial structure, and voice pitch, likely mediated in part by developmental testosterone. Sexual selection hypotheses posit that, ancestrally, more 'masculine' men may have acquired more mates and/or sired more viable offspring. Thus far, however, evidence for either association is unclear. Here, we meta-analyze the relationships between six masculine traits and mating/reproductive outcomes (96 studies, 474 effects, N = 177,044). Voice pitch, height, and testosterone all predicted mating; however, strength/muscularity was the strongest and only consistent predictor of both mating and reproduction. Facial masculinity and digit ratios did not significantly predict either. There was no clear evidence for any effects of masculinity on offspring viability. Our findings support arguments that strength/muscularity may be sexually selected in humans, but cast doubt regarding selection for other forms of masculinity and highlight the need to increase tests of evolutionary hypotheses outside of industrialized populations.
... But total body mass provides only a crude picture of the forces shaping these sex differences because, on average, men and women allocate that mass much differently (Plavcan, 2012b). Women have considerably more fat and men have more lean (and muscle) mass (Pond, 1998;Kyle et al., 2005;Wells, 2007Wells, , 2012aLassek and Gaulin, 2009;Hill et al., 2017;Puts et al., in press). Because these sex differences in body composition are present but significantly less pronounced in infants and children and increase dramatically with puberty (Wells, 2007;Kirchengast, 2010;Taylor et al., 2010), this is an ontogenetic sign that they are related to the different reproductive strategies of the two sexes, as explored below. ...
... Of course, in addition to competition between individual men within groups, there is also substantial evidence for persistent violent competition between groups in the form of warfare or raiding, which is the most common form of killing seen in chimpanzees (Bowles, 2009;Puts, 2010;Allen and Jones, 2014;Puts et al., 2015;Puts, 2016;Hill et al., 2017;Mann, 2018). Competition between patrilineal groups has been suggested as the cause of a post-Neolithic Y-chromosome bottleneck (Zeng et al., 2018). ...
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Human sexual dimorphism has been widely misunderstood. A large literature has underestimated the effect of differences in body composition and the role of male contest competition for mates. It is often assumed that sexually dimorphic traits reflect a history of sexual selection, but natural selection frequently builds different phenotypes in males and females. The relatively small sex difference in stature (∼7%) and its decrease during human evolution have been widely presumed to indicate decreased male contest competition for mates. However, females likely increased in stature relative to males in order to successfully deliver large-brained neonates through a bipedally-adapted pelvis. Despite the relatively small differences in stature and body mass (∼16%), there are marked sex differences in body composition. Across multiple samples from groups with different nutrition, males typically have 36% more lean body mass, 65% more muscle mass, and 72% more arm muscle than women, yielding parallel sex differences in strength. These sex differences in muscle and strength are comparable to those seen in primates where sexual selection, arising from aggressive male mating competition, has produced high levels of dimorphism. Body fat percentage shows a reverse pattern, with females having ∼1.6 times more than males and depositing that fat in different body regions than males. We argue that these sex differences in adipose arise mainly from natural selection on women to accumulate neurodevelopmental resources.
... Both hypotheses predict any relationships should be absent among women, as their formidability is far lower than that of males (Lassek & Gaulin, 2009) and they have not been sexually selected for physical contest competition (Hill, Bailey, & Puts, 2017;Puts, 2010). Studies have generally shown support for both hypotheses using a variety of measures of formidability and egalitarianism. ...
... The most obvious difference is that the present study tested height, whereas previous studies almost all tested upper body strength and muscularity. Both height and muscularity are thought to be key components of formidability (Blaker & van Vugt, 2014), and were both intrasexually selected to be higher in males throughout our evolutionary history (Hill et al., 2017;Puts, 2010). However upper body strength is far more sexually dimorphic than height is, with some studies estimating that male strength is on average around 3 standard deviations higher than women's (Lassek & Gaulin, 2009). ...
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People vary widely in their attitudes towards how much their government should redistribute wealth. Evolutionary theory may shed light on why this variation occurs. Numerous studies have established an association between upper body strength and attitudes towards equality and wealth redistribution in males, showing that physically stronger men are more likely to hold self-serving beliefs on these issues. This effect is typically weaker or absent in women. A question that has received little attention is whether there are similar associations between other aspects of formidability and attitudes towards wealth redistribution. One such aspect is height. I tested this prediction using data from the European Social Survey, in a sample of 27031 people from 20 European countries. Results show that taller people are more likely to have self-serving attitudes towards government redistribution of wealth. The result was robust to numerous control variables and alternative model specifications, but the direct effects of height were small. Taller individuals were less supportive of government wealth redistribution overall, but were especially averse if they were also wealthier. Post-hoc analyses suggested that for lower income deciles, the association was reversed. For these people, there was a positive association between height and support for wealth redistribution. However, effects were equally strong in males and females, and so are not fully consistent with current evolutionary psychological theories of resource distribution.
... Several recent authors have promoted the idea of male-male competition as the major driver in human evolution due to apparent male adaptations for combat, aggression, and violence (Hill, Bailey, & Puts, 2017;Puts, 2010). However, whilst it is true that, relative to women, men are more physically robust and behaviourally aggressive, the suggestion that present sexual dimorphisms are sufficient evidence to infer the predominance of male competition in human evolution is undermined by recent reductions in human skeletal size and robusticity (Frayer, 1980;Frayer & Wolpoff, 1985;Hill et al., 2017;Ruff, 2002;Ryan & Shaw, 2015), as well as diminished craniofacial masculinity and prognathism (Cieri et al., 2014), and increases in cooperative capacity and social tolerance (Hare, 2016;Sterelny, 2011). ...
... Several recent authors have promoted the idea of male-male competition as the major driver in human evolution due to apparent male adaptations for combat, aggression, and violence (Hill, Bailey, & Puts, 2017;Puts, 2010). However, whilst it is true that, relative to women, men are more physically robust and behaviourally aggressive, the suggestion that present sexual dimorphisms are sufficient evidence to infer the predominance of male competition in human evolution is undermined by recent reductions in human skeletal size and robusticity (Frayer, 1980;Frayer & Wolpoff, 1985;Hill et al., 2017;Ruff, 2002;Ryan & Shaw, 2015), as well as diminished craniofacial masculinity and prognathism (Cieri et al., 2014), and increases in cooperative capacity and social tolerance (Hare, 2016;Sterelny, 2011). ...
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Objectives Pre-historic decline in human craniofacial masculinity has been proposed as evidence of selection for elevated sociability and a process of ‘human self-domestication’ thought to have promoted complex capacities including language, culture, and cumulative technological development. This follows experimental observation of similar changes in non-human animals under selection for reduced aggression. Two distinct domestication hypotheses posit developmental explanations, involving hypoplasia of embryonic neural crest cells (NCCs), and declining androgen influence, respectively. Here, I assess the operation and potential interactions between these two mechanisms and consider their role in enhanced human adaptation to a cooperative sociocultural niche. Methods I provide a review and synthesis of related literature with a focus on physiological mechanisms effecting domesticated reductions in masculinity and sexual dimorphism. Further, I examine pre-historic modes of socio-sexual selection likely to drive human self-domestication via reduced aggression and masculinity. Results I find pluripotent NCCs provide progenitors for a wide range of vertebrate masculine features, acting as regular targets for sexually driven evolutionary change; suggesting domesticated hypoplasia of NCC-derived tissues would be sufficient to explain declines in masculine traits and features. However, lineage specific androgen receptor variability likely moderates these NCC-based effects. Conclusions These findings extend theorised mechanisms driving noted physiological, morphological, and behavioural changes thought to indicate enhanced sociability and human and self-domestication. Multiple current explanations for human sociability are consistent with physiological domestication under socio-sexual selection favouring dampened masculine physiology and behaviour as adaptations to an enhanced sociocultural niche. The analysis highlights multiple avenues for further investigation.
... But, what role did PA patterns have in mating in hunter-gatherer societies? Regular PA, such as walking, running, and climbing, was likely an important part of our ancestors' everyday lives to deal with adaptive problems, allowing them to gain resources, status, and compete with other sexual rivals (Hill et al., 2017;Puts, 2016). Given the importance of physical strength in overcoming ancestral problems, women may have considered body movement skills as a sign of a desirable mate who can support them and their children (McCarty et al., 2013). ...
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Evolutionary psychology has provided a groundbreaking approach to understanding human cognitive architecture during the last few decades, yet this approach has been overlooked when investigating and designing cognitive enhancement therapies. In this article, we posit that understanding why integrated cognitive, emotional, and motor (CEM) systems evolved in the first place can help researchers develop more effective cognitive enhancement strategies. From an evolutionary perspective, a wide range of CEM systems was designed by natural selection to deal with adaptive problems and maximize reproductive success during the ancestral hunter-gatherer era. However, adaptive CEM systems lie largely dormant in modern life due to the environments and challenges in which they evolved no longer being relevant. In exploring this perspective, we present a theoretical model to explain why interconnected CEM systems evolved in ancestral environments. From this viewpoint, and considered in light of current cognitive enhancement strategies and successes involving cognitive training, neurostimulation, and physical exercise, we establish a novel framework for evolutionary cognitive enhancement (ECE) therapy that aims to reawaken adaptive CEM systems. Keywords: Adaptations; Ancestral Environments; Body Movements; Cognitive and Physical Training; Evolutionary Perspective
... 1016/j.paid.2020.110496. pared to women in addition to their heightened proclivity for physical aggression and weapon use (Hill et al., 2017;Lassek & Gaulin, 2009). Formidable men possess an adaptive advantage in intrasexual competition that would subsequently connote their heritable fitness to prospective mates (Puts, 2010), though adaptively beneficial beyond direct reproduction as well, such as in facilitating coalitional exploitation. ...
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Selecting formidable male coalitions to navigate intergroup threats and resource acquisition evolved to enhance survival through group living, given men's enhanced ability to extract and protect resources through physical aggression. Though advantageous in certain contexts, formidable men can nonetheless inflict intragroup costs, suggesting preferences for this trait varies with resource availability in local ecologies. This study tasked participants (477 women, 140 men; MAge = 19.98, SD = 4.22) with building coalitions from arrays of physically strong and weak men to acquire resources in hopeful and desperate ecologies before assessing endorsement of several aspects of conservatism. Individuals high in social dominance orientation reported greater aversion to physically strong men in desperate ecologies, although strength was generally preferred independent of ideological differences. Results suggest a tradeoffs framework in coalition-building based on the inferred costs and benefits of physically strong allies.
... For example, the association between mating and reproductive success has been shown to differ by sex (with a stronger correlation in men, e.g., Jokela et al., 2010) and cross-culturally depending on fertility rates, access to contraception, and polygyny (e.g., von Rueden & Jaeggi, 2016). As a further example on sex differences, in both mate attraction and intrasexual selection, men typically engage in more overt competition (e.g., Hill et al., 2017;Kordsmeyer et al., 2018), whereas women use epigamic displays (enhancing their appearance) and indirect aggression (e.g., gossiping, excluding rivals, Arnocky & Vaillancourt, 2017). Hence, in line with extant knowledge social status and dominance in men and physical attractiveness in women should be weighted more strongly (see also Kanazawa & Savage, 2009). ...
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According to evolutionary theory, human cognition and behaviour are based on adaptations selected for their contribution to reproduction in the past, which in the present may result in differential reproductive success and inclusive fitness. Because this depiction is broad and human behaviour often separated from this ultimate outcome (e.g., increasing childlessness), evolutionary theory can only incompletely account for human everyday behaviour. Moreover, effects of most studied traits and characteristics on mating and reproductive success turned out not to be robust. In this article, an abstract descriptive level for evaluating human characteristics, behaviour, and outcomes is proposed, as a predictor of long-term reproductive success and fitness. Characteristics, behaviour, and outcomes are assessed in terms of attained and maintained capital, defined by more concrete (e.g., mating success, personality traits) and abstract (e.g., influence, received attention) facets, thus extending constructs like embodied capital and social capital theory, which focuses on resources embedded in social relationships. Situations are framed as opportunities to gain capital, and situational factors function as elicitors for gaining and evaluating capital. Combined capital facets should more robustly predict reproductive success and (theoretically) fitness than individual fitness predictors. Different ways of defining and testing these associations are outlined, including a method for empirically examining the psychometric utility of introducing a capital concept. Further theorising and empirical research should more precisely define capital and its facets, and test associations with (correlates of) reproductive success and fitness.
... In general, our findings are consistent with the broader perspective that men's anatomy, behavior, and psychology have been shaped by an evolutionary history of contest competition 27,65 , the use of force or threat of force to exclude same-sex competitors from mates 59 . Contest competition should favor psychological mechanisms to attend to and assess the formidability and threat potential of competitors 19 . ...
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Human voice pitch is highly sexually dimorphic and eminently quantifiable, making it an ideal phenotype for studying the influence of sexual selection. In both traditional and industrial populations, lower pitch in men predicts mating success, reproductive success, and social status and shapes social perceptions, especially those related to physical formidability. Due to practical and ethical constraints however, scant evidence tests the central question of whether male voice pitch and other acoustic measures indicate actual fighting ability in humans. To address this, we examined pitch, pitch variability, and formant position of 475 mixed martial arts (MMA) fighters from an elite fighting league, with each fighter’s acoustic measures assessed from multiple voice recordings extracted from audio or video interviews available online (YouTube, Google Video, podcasts), totaling 1312 voice recording samples. In four regression models each predicting a separate measure of fighting ability (win percentages, number of fights, Elo ratings, and retirement status), no acoustic measure significantly predicted fighting ability above and beyond covariates. However, after fight statistics, fight history, height, weight, and age were used to extract underlying dimensions of fighting ability via factor analysis, pitch and formant position negatively predicted “Fighting Experience” and “Size” factor scores in a multivariate regression model, explaining 3–8% of the variance. Our findings suggest that lower male pitch and formants may be valid cues of some components of fighting ability in men.
... The coevolution of physical conflict with the sexual dimorphism in formidability has led to conflict becoming sexually asymmetric, with men engaging more frequently in physical confrontation (Sell et al., 2012). Although the concomitant size asymmetry imposed by human sexual dimorphism is not as large as it is for other primates (Plavcan, 2012), human males nonetheless possess greater muscle mass and cranial robusticity compared to women in addition to a heightened proclivity to engage in physical aggression and weapon use (Hill, Bailey, & Puts, 2017;Lassek & Gaulin, 2009). Formidable men possess an adaptive advantage in intrasexual competition that would subsequently connote their heritable fitness to prospective mates, making formidability sexually selected (Puts, 2010), though adaptively beneficial beyond direct reproduction as well, such as in facilitating coalitional exploitation. ...
Article
Selecting formidable male coalitions to navigate intergroup threats and resource acquisition evolved to enhance survival through group living, given men's enhanced ability to extract and protect resources through physical aggression. Though advantageous in certain contexts, formidable men can nonetheless inflict intragroup costs, suggesting preferences for this trait varies with resource availability in local ecologies. This study tasked participants (477 women, 140 men; MAge = 19.98, SD = 4.22) with building coalitions from arrays of physically strong and weak men to acquire resources in hopeful and desperate ecologies before assessing endorsement of several aspects of conservatism. Women high in social dominance orientation built more formidable coalitions in resource-abundant ecologies. Men's coalitional interests were unaffected by these factors. We frame results through evolved sex differences in coalition-building based on men and women's different formidability valuation thresholds while considering ancestral logic behind political ideology related to resource acquisition.
Article
We tried to understand individual differences in two super-categories of cues to vulnerability. In a qualitative, act-nomination study (N = 79), we found several underpowered patterns in that more physical cues of vulnerability were listed than psychological ones, no sex difference were observed for number of psychological vulnerabilities, but men listed more physical vulnerabilities than women, however; these effects are descriptive only. We then surveyed participants (N = 262) on how much a curated list of cues from Study 1 made men and women vulnerable. A composite of the Dark Tetrad traits (i.e., narcissism, psychopathy, sadism, and Machiavellianism) that we called “antagonism” was associated with seeing targets as more vulnerable whereas those who were empathetic perceived targets as less vulnerable. Physical vulnerability was associated with higher ratings of male targets' vulnerability. For psychological vulnerability, antagonism was associated with lowered perceptions of vulnerability of female targets. Women rated others—regardless of their sex—as more vulnerable than men did, but this effect was strongest for physical cues. And last, women rated other women as more vulnerable—regardless of cue type—than other men, but men rated both sexes as equally vulnerable. Our results are discussed within an evolutionary framework.
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The nature of inter-group relations among prehistoric hunter-gatherers remains disputed, with arguments in favour and against the existence of warfare before the development of sedentary societies. Here we report on a case of inter-group violence towards a group of hunter-gatherers from Nataruk, west of Lake Turkana, which during the late Pleistocene/early Holocene period extended about 30 km beyond its present-day shore. Ten of the twelve articulated skeletons found at Nataruk show evidence of having died violently at the edge of a lagoon, into which some of the bodies fell. The remains from Nataruk are unique, preserved by the particular conditions of the lagoon with no evidence of deliberate burial. They offer a rare glimpse into the life and death of past foraging people, and evidence that warfare was part of the repertoire of inter-group relations among prehistoric hunter-gatherers.
Article
Why are physically formidable men willingly allocated higher social status by others in cooperative groups? Ancestrally, physically formidable males would have been differentially equipped to generate benefits for groups by providing leadership services of within-group enforcement (e.g., implementing punishment of free riders) and between-group representation (e.g., negotiating with other coalitions). Therefore, we hypothesize that adaptations for social status allocation are designed to interpret men's physical formidability as a cue to these leadership abilities, and to allocate greater status to formidable men on this basis. These hypotheses were supported in 4 empirical studies wherein young adults rated standardized photos of subjects (targets) who were described as being part of a white-collar business consultancy. In Studies 1 and 2, male targets' physical strength positively predicted ratings of their projected status within the organization, and this effect was mediated by perceptions that stronger men possessed greater leadership abilities of within-group enforcement and between-group representation. Moreover, (a) these same patterns held whether status was conceptualized as overall ascendancy, prestige-based status, or dominance-based status, and (b) strong men who were perceived as aggressively self-interested were not allocated greater status. Finally, 2 experiments established the causality of physical formidability's effects on status-related perceptions by manipulating targets' relative strength (Study 3) and height (Study 4). In interpreting our findings, we argue that adaptations for formidability-based status allocation may have facilitated the evolution of group cooperation in humans and other primates. (PsycINFO Database Record
Chapter
In this chapter, we explore how men’s phenotypes, including their psychologies, have been shaped by contest competition, a form of sexual selection involving the use of force or threat of force to exclude same-sex competitors from mates. We first evaluate the intensity of sexual selection in men, finding evidence of a positive and moderately strong relationship between men’s mating success and reproductive success, and between each of these and putative sexually selected traits. Next, we use a comparative and functional approach to evaluate how men’s phenotypes were shaped specifically by contest competition. Contrary to inferences made from our negligible canine size and modest body mass dimorphism, we find that human beings are sexually differentiated along dimensions expected of a species with an evolutionary history of male contests, and that men’s phenotypes show evidence of design for contest competition. Finally, we explore how contests may have contributed to male mating and reproductive success over human evolution.