Article
Does sexual selection explain human sex
dierences in aggression?
Archer, John
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Does sexual selection explain human
sex differences in aggression?
John Archer
School of Psychology, University of Central Lancashire, Preston PR1 2HE,
United Kingdom
jarcher@uclan.ac.uk
http://www.uclan.ac.uk/scitech/research/rae2008/psychology/
staff_profiles/jarcher.php
Abstract: I argue that the magnitude and nature of sex differences in aggression, their development, causation, and variability, can be
better explained by sexual selection than by the alternative biosocial version of social role theory. Thus, sex differences in physical
aggression increase with the degree of risk, occur early in life, peak in young adulthood, and are likely to be mediated by greater male
impulsiveness, and greater female fear of physical danger. Male variability in physical aggression is consistent with an alternative life
history perspective, and context-dependent variability with responses to reproductive competition, although some variability follows
the internal and external influences of social roles. Other sex differences, in variance in reproductive output, threat displays, size and
strength, maturation rates, and mortality and conception rates, all indicate that male aggression is part of a sexually selected adaptive
complex. Physical aggression between partners can be explained using different evolutionary principles, arising from the conflicts of
interest between males and females entering a reproductive alliance, combined with variability following differences in societal gender
roles. In this case, social roles are particularly important since they enable both the relatively equality in physical aggression between
partners from Western nations, and the considerable cross-national variability, to be explained.
Keywords: aggression; partner violence; sex differences; sexual selection; social role theory
1. Introduction
Darwin (1859/1911; 1871/1901) regarded the greater
proneness to physical aggression by men than women as
part of a general mammalian pattern, which can be
explained by one aspect of sexual selection, inter-male
competition. Within the social sciences there is a long tra-
dition of explaining sex differences in social behavior,
including aggression, in human-centered terms, as a con-
sequence of social influences. In this article, I argue that
sexual selection provides a better explanation for human
sex differences in aggression than the main contemporary
social science approach, the social role theory.
Aggression is typically defined as behavior intended to
harm another individual, and its study was originally
restricted to direct physical and verbal forms. Psychologists
began systematically documenting human sex differences
in these types of aggression in the 1920s, and studies have
continued to the present time. The forms of aggression
now include indirect (or “relational”
1
) aggression, which
Feshbach (1969) and Lagerspetz et al. (1988) found to be
more typical of girls than boys. They suggested that the
sex difference, hitherto characterized as involving “aggres-
sion,” was more accurately viewed as involving its form.
Contemporary studies include both direct and indirect
aggression. A separate extensive literature documents sex
differences (or similarities) in physical aggression between
partners, although this is not usually considered in discus-
sions of sex differences in aggression. I do consider the evol-
utionary and social forces underlying aggression between
partners in this article, so as to provide a comprehensive
explanation of sex differences in aggression. Nevertheless,
most of the discussion is devoted to within-sex differences,
the main concern of sexual selection and social role
accounts. In assessing the evidence, I address the issues
of ultimate origins, development, mediating variables, and
sources of individual differences, in relation to predictions
derived from the two theories. I argue that sexual selection
provides the more complete explanation of the origins of sex
differences in aggression, and that these differences are
linked to a range of other features indicting the operation
of sexual selection in humans. Although reformulations of
social role theory do encompass some evolved differences,
they are restricted to physical attributes, which form only
part of the sexual selection view.
JOHN ARCHER, Professor of Psychology at the Univer-
sity of Central Lancashire, Preston, United Kingdom, is
the author of more than 100 articles, in a wide range of
journals, in the areas of animal aggression and emotion-
ality, testosterone and behavior, human sex differences,
human aggression, grief and loss, and evolutionary psy-
chology. He is also the author of several books, includ-
ing The Behavioural Biology of Aggression (1988), The
Nature of Grief (1999), and Sex and Gender (2nd
edition, 2002). He is a former President of the Inter-
national Society for Research on Aggression (ISRA),
and is a Fellow of the British Psychological Society.
BEHAVIORAL AND B RAIN SCIENCES (2009) 32, 249–311
Printed in the United States of America
doi:10.1017/S0140525X09990951
# 2009 Cambridge University Press 0140-525X/09 $40.00 249
2. Aggression between same-sex individuals
2.1. Sexual selection as an explanatory framework
for human aggression
2.1.1. Principles of sexual selection
. Sexual selection
involves the choice of members of one sex by the other,
and competition by members of one sex for access to the
other (Darwin 1871/1901). The more competitive and
aggressive sex is usually the male, particularly in
mammals. Sexual selection forms a basis for understanding
sex differences in aggression in animals, although it is not
the only explanation (Ralls 1976; Selander 1972). Trivers
(1972) proposed that the reason why males are typically
the more competitive sex is their lower parental investment,
defined as any contribution by a parent that increases an
offspring’s chances of survival and reproduction, while
reducing the parents’ ability to produce further offspring.
Examples include the energy expended in gamete pro-
duction (Bateman 1948), and feeding or guarding the
young. The initial greater contribution by the female to
the gametes will make desertion to seek access to other
mates a more beneficial option for males than for females,
providing that one parent can care for the offspring.
2
The
sex showing higher parental investment becomes a limiting
resource, the other sex competing for reproductive access.
Competition can take forms other than direct aggression,
for example, sperm competition, or features that aid mate
attraction, or securing a dispersed resource (Andersson
1994, pp. 10–13; Archer 1988, pp. 106–107).
When parental care by both sexes is required, inter-male
competition will be countered by the male’s greater
paternal investment, as occurs in monogamous birds.
Where the female can leave the male to brood the eggs
and care for the young alone, as occurs in polyandrous
wading birds, females will be the more competitive, and
the larger and more aggressive, sex (Jenni 1974). This rever-
sal of the usual sex differences provides crucial support for
Trivers’ theory. In mammals, fertilization is internal and the
female feeds the developing offspring, so that parental
investment is further female-biased, and polygyny is the
usual outcome. In terrestrial mammals, this is typically
associated with inter-male aggression, accompanied by
large size and musculature, and a greater variation in
male than female reproductive success (Andersson 1994).
Differential reproductive rate can be regarded as a more
fundamental principle driving sexual selection than par-
ental investment (Clutton-Brock & Vincent 1991). It
reflects the rates at which males and females are able to
mate again, after producing offspring (and therefore is
closely related to parental investment). Differences in
reproductive rate were associated with sex differences in
mating competition in 29 species where the male shows
some parental care (Clutton-Brock & Vincent 1991), and
females had higher reproductive rates than males when
the usual sex difference was reversed (i.e. when females
were larger and more competitive than males).
The basic principles of sexual selection are complicated
by ecological constraints on the degree to which one sex
can compete for, and mon opolize, access to the other sex
(Emlen & Oring 1977). Where resources that are impor-
tant for reproduction are located in one place, there will
be accentuated competition for access to the sex with
the lowest reproductive rate, usually the female, and
polygyny is more likely. Where resources are widely
distributed, the potential for reproductive competition is
less, and monogamy is more likely. These principles also
apply to variation within a species. For example, the
mating system of the dunnock (Prunella vulgaris) varies
from polygyny to monogamy to polyandry (Davies &
Hartley 1996; Davies & Lundberg 1984), depending on
the food distribution, which affects female range size.
Underlying these associations is the ability of males to
control access to females, or more precisely, the concept
of operational sex ratio (OSR; Emlen & Oring
1977) – the ratio of sexually active males to fertilizable
females at any given time and place. This will be biased
in the female direction as a consequence of males being
excluded from the breeding group by higher mortality,
or being ejected as a result of competition, or maturing
more slowly (Clutton-Brock & Harvey 1977), and in the
male direction if females only gradually become fertile
or available to males (e.g., Smuts 1987b). The OSR is
essentially an index of the degree of male competition
for mates, and is associated with greater variance in male
than female reproductive success.
Pomiankowski and Møller (1995) found greater variabil-
ity in sexually selected than non-sexually selected traits in
males than females from a range of species, and they
attributed this to sexual selection producing traits exagger-
ated beyond those that were optimal for survival. An
alternative explanation in species where there is some
degree of paternal care is that males of many monogamous
species retain traits associated with polygynous mating,
and can therefore be regarded as facultatively polygynous
(Andersson 1994, pp. 157–58; Trivers 1972). The degree
to which this occurs depends both on the context (e.g.,
the availability of alternative mates) and the individual
(e.g., the ability to attract alternative mates). Such individ-
ual differences are said to arise from the extent to which
reproductive effort is concentrated on parental or mating
effort, which is termed a conditional reproductive strategy
(Gross 1996). It would lead to greater variation in sexually
selected traits among males than among females.
2.1.2. Sexual selection applied to human aggression.
Because gestation in female mammals is internal, males
must show a higher potential reproductive rate than
females, and this is associated with being the competitive
sex. The necessity of biparental care in some species will
counter this and reduce the degree of sexual dimorphism.
Theextenttowhichthisappliestohumansisamatterofcon-
tention (Geary 2000), and humans have been regarded as
basically monogamous or polygynous (sect. 2.1). However,
even males of monogamous species are likely to be facul-
tatively polygynous (Trivers 1972), and therefore repro-
ductive competition is likely to be higher in men than
women, irrespective of the basic huma n mating pattern.
Daly and Wilson (1988; 1990) applied the principles of
sexual selection to human homicide, which they regarded
as indicative of the strength of aggressive dispositions in
different individuals and under different circumstances.
They viewed the much higher frequency of male than
female same-sex homicides, and the concentration of
these among men with few resources (sect. 1.8), as conse-
quences of sexual selection. An alter native evolutionary
view (Campbell 1999) explained the lower incidence of
women’s engagement in risky and violent aggression in
terms of their relatively greater parental investment,
Archer: Sex differences in aggression
250
BEHAVIORAL AND BRAIN SCIENCES (2009) 32:3/4
which increases the importance of remaining alive to rear
their offspring. As a consequence, women have evolved a
psychological disposition to be more risk-averse. This
can be viewed as either an alternative to the sexual selec-
tion view, which concentrates on male competition, or as
complementary. It is still derived from unequal parental
investment, and it generates very similar predictions to
sexual selection.
The basic principle underlying all evolutionary expla-
nations of aggression, including sexual selection, is a
cost-benefit analysis, in which the costs and benefits of
behavior are determined by natural selection. For male
aggression, the benefits will be successful reproduction,
and the costs those resulting from injury. For men with
few resources, successful reproduction may only be poss-
ible if they challenge other men and risk injury in escalated
encounters (Daly & Wilson 1988; 1990): consistent with
this, an evolutionary simulation demonstrated the increas-
ing payoffs for risky (dangerous) tactics as the value of
victory increased (Daly & Wilson 1988, pp. 164–65).
For women, access to a mate is less dependent on
within-sex competition, and they typically have more to
lose in terms of reproductive fitness from potentially
damaging confrontations (Campbell 1999). The emphasis
in these explanations is on higher potential reproductive
benefits for males, and higher potential costs for females,
of damaging aggressive encounters. It follows that the sex
difference would be largest for dangerous forms of physical
aggression, and be greater for physical than for direct verbal
aggression. Indirect aggression is less dangerous in terms of
inviting immediate retaliation, and has therefore been
viewed as a lower-cost form of aggression, typically used
more by females (Bjo
¨
rkqvist 1994; Geary 1998; Hess &
Hagen 2006). The cost-benefit analysis locates the source
of the sex difference in the damaging nature of physically
aggressive encounters and would not predict a difference
in features of aggressive motivation, such as the ease with
which the two sexes are aroused to anger.
Typically, evolved dispositions such as those underlying
a sex difference in aggression should not have to rely on
socialization practices that could vary from culture to
culture. How and when they first occur in development
is an issue that is not precisely specified by a functional
explanation, which primarily addresses adaptive signifi-
cance. As Darwin (1871/1901) noted, sex differences
may be small or minimal before reproductive maturity.
The sex difference in size and strength conforms to this
pattern, developing at puberty under the control of testos-
terone (Tanner 1989). Some researchers have suggested
that testosterone controls the greater male physical aggres-
sion in humans (e.g., Book et al. 2001), following the link
found in many mammals and birds (Archer 1988, pp.
135–42). An alternative is that the sex difference in
direct aggression begins early in life (Bjorklund &
Pellegrini 2000), before the cumulative impact of gen-
dered social influences, possibly as a result of a direct or
indirect effect of prenata l androgens (Berenbaum &
Resnick 1997; Pasterski et al. 2007). It would be associated
with other early-developing dispositional sex differences,
such as those in activity (Archer & Lloyd 2002, p. 74;
Campbell & Eaton 1999; Eaton & Enns 1986), and
social preferences for larger competitive groups rather
than smaller, more supportive ones (Geary et al.
2003). Since sex-segregation, and the relationship styles
accompanying this, occur early in life (Archer 1992a;
Maccoby 1988; 1998; Pellegrini 2004), we would expect
sex differences in aggression to occur then too. A further
possibility, not necessarily inconsisten t with an early emer-
gence of sex differences, arises from the finding that males
of many polygynous species delay risky encounters with
older males until they are large enough to compete effec-
tively with them (Andersson 1994). If this applied to
humans, the peak years for high-risk confrontations
would be when young adults become physically mature
and enter the mating arena (Geary 2002). Overall, the
most likely prediction from a sexual selection perspective
is an early emergence of sex differences in aggression com-
bined with a peak in risky competition during young adult-
hood, with a possible influence of pubertal testosterone.
Just as the course of development cannot be precisely
specified from an evolutionary explanation, nor can the
precise mechanisms underlying human sex differences in
aggression. Sexual selection does, however, imply the fol-
lowing: (1) that the sex differences are not wholly the
result of a general-purpose learning mechanism, although
this is likely to be involved; (2) that there are sexually
dimorphic neuroendocrine mechanisms underlying
aggression, accompanying other aspects of sexual dimor-
phism, such as size and strength; (3) that the mechanism
is unlikely to reside in a general sex difference in responses
to frustration or in ease of arousal to anger; (4) that the sex
difference is more likely to involve either greater risk-
taking by males or more fear of physical danger by
females: either or both of these would represent the way
that the motivational system underlying aggression had
responded to evolutionary costs and benefits. These
would represent basic predispositions that could be modi-
fied in development (sect. 2.5), or overridden by environ-
mental contingencies (sect. 2.8).
Based on Trivers’ analysis (sect. 2.1.1), sexual selection
theory can also predict individual differences among
men according to their relative specialization for mating
or parental effort. One prediction is that there should be
a coherent set of individual differences associated with
mating or parental effort; a second is that variability in
sexually selected traits such as physical aggression should
be greater in men than in women, whereas traits that are
not sexually selected, such as anger, should not show sex
differences in variability.
Sexual selection theory also predicts variability in sex
differences in aggression as a consequence of social con-
ditions affecting the cost-benefit contingencies of repro-
ductive competition. For example, inter-male competition
will be accentuated where resources are scarce, and
where there are fewer females than males of reproductive
age; that is, where the OSR is high. Where the OSR is in
the other direction, with fewer males, we would expect
greater female competition and overt aggression.
Table 1 provides a summary of the main points of the
predictions set out in this section, together with the sec-
tions that consider the evidence relating to them.
2.2. Social role theory as an explanatory framework
for human aggression
2.2.1. Principles of social role theory
. The main alterna-
tive explanation of the origins of human sex differences in
social behavior is the “biosocial”
3
reformulation (Wood &
Archer: Sex differences in aggression
BEHAVIORAL AND BRAIN SCIENCES (2009) 32:3/4 251
Eagly 2002) of social role theory (Eagly 1987; Eagly et al.
2000). The basic tenet of social role theory is that sex differ-
ences in behavior arise “from the societal division of labor
between the sexes” (Eagly 1997, p. 1381). Although it is pri-
marily the societal positions of men and women, as bread-
winner and homemaker, and in the workforce of modern
societies, that are important, social role theory acknowl-
edges that the roles of men and women are complex.
Thus, more specific roles, such as those in the family
(e.g., father, grandmother) and in occupations (e.g., police
officer, nurse) also contribute to sex differences in social be-
havior and to within-sex variations.
The biosocial reformulation of socia l role theory (Wood
& Eagly 2002) represents an extension of the earlier
accounts to address issues raised in exchanges with evol-
utionary theorists (e.g ., Archer 1996; Buss 1996; Eagly &
Wood 1999), specifically the origins of sex differences,
and the principles through which men and women are dis-
tributed in societal roles. It is also wider in scope, incor-
porating evidence from social and physical anthropology.
The biosocial account explains how recurrent forms of
the division of labor based on sex arose from an interaction
between the requirements of the social and physical
environment and con straints imposed by the mammalian
method of reproduction and sex differences in size and
strength. Thus, women’s childbearing and nursing of
infants, and men’s greater size and strength, make it
easier for women to perform certain activities and men
to perform others . This explains the cross-cultural con-
sistency in gender roles. The interaction between these
biological differences and particular ecological, social,
economic, and technological forces, explains the variability
in gender roles. The behavior associated with the specific
sex-typed social roles that emerge from these interactions
form the basis of behavioral sex differences, including
those in aggression.
Social roles are therefore viewed as being rooted in
human history, but they originate from phylogenetic
history in the form of reproductive and physical sex differ-
ences. Nevertheless, Wood and Eagly (2002) stated that
Table 1. Summary of predictions about sex differences in aggression from sexual selection and social role theories
Relevant section Sexual selection theory Social role/biosocial theory
Magnitude and nature of the
sex difference
2.4 In accordance with the degree of
physical danger, the largest
differences will be in physical
aggression, followed by direct
verbal, with indirect aggression
showing no difference or more by
females; no difference for anger.
Magnitude will be modest; larger
difference for physical than for
psychological aggression (verbal and
indirect). Presumably, no difference
for anger.
Development 2.5 Early emergence of sex difference in
direct aggression; peak in damaging
and risky competition during young
adulthood.
Sex difference should start small and
increase with age through
childhood, coincident with the
cumulative influence of
socialization.
Mediators 2.6 These will reflect functional
principles, for example, greater
risk-taking by males or greater fear
of physical danger by females, or
both.
Internal mediators should follow from
the characteristics of gender roles
(e.g., empathy, fear of retaliation,
guilt, anxiety associated with
aggression). They arise from general
learning mechanisms associated
with gender roles.
Biosocial model indicates that roles
can vary in response to role-related
costs and benefits.
Individual differences 2.7 There will be a coherent set of
individual differences (including in
aggression) among males,
reflecting greater or lesser
emphasis on mating than parental
effort. A consequence of these
differences will be greater male
than female variability in physical
aggression.
Consistent individual differences
would arise from differences in
internalization of gender roles, and
from the influence of specific roles;
no consideration of possible sex
differences in variability.
Variability in response to
environmental conditions
2.8 Variability across local conditions,
cultures and nations is expected to
reflect resources that are important
for reproduction, principally access
to the mates and the status and
resources important in this process.
There should be variations in women’s
level of aggressiveness
accompanying: (1) changes in role
salience; (2) cross-cultural variations
in women’s relative emancipation;
and (3) changes over historical time
in women’s roles and status.
Archer: Sex differences in aggression
252
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