The Psychology of Gender 4th Edition

144 Chapter 5

reproduce. Consistent with this theory, crime

statistics show same-sex homicide is highest

between the ages of 18 and 30. A large num-

ber of same-sex homicides, in which the vic-

tim and the killer are unrelated, occur in the

context of men trying to establish dominance

or compete for status (Daly & Wilson, 2001).

The question is whether competition over

women is the precipitating factor. Evidence

to support this proposition comes from re-

search that shows married men have the

lowest level of homicide rates, but formerly

married men—that is, divorced and widowed

men—have homicide rates that are similar to

single men (Daly & Wilson, 2001).

Evolutionary theory also can be used to

explain violence in families (Daly & Wilson,

1999). At first glance, familial violence would

seem to violate the basic principles of evolu-

tionary theory. However, a majority of homi-

cides within families occur between spouses

who are genetically unrelated to each other

rather than between blood relatives. Women,

but not men, are at greatest risk for being

murdered when they try to end the relation-

ship. Consistent with evolutionary theory,

the primary motive men have for killing

their spouses is sexual jealousy. Also consis-

tent with evolutionary theory is the fact that

young wives are most likely to be murdered,

perhaps because youth is a sign of fertility,

and fertility would make a woman more at-

tractive to male rivals. Although young men

are the individuals most likely to commit

murder, the wife’s age is a better predictor

than the husband’s age. Evolutionary theory

also has been applied to the study of violence

toward children. Among parents who abuse

or kill their children, the incidence is much

higher among stepparents than biologi-

cal parents (Daly & Wilson, 1999). In sum,

some patterns of violence are consistent with

evolutionary principles.

This theory can explain some differ-

ences in sexual behavior, for example, why

men have more favorable attitudes toward

casual sex and a lower threshold for interpret-

ing an ambiguous behavior by a female in sex-

ual terms. The theory conflicts, however, with

the finding that the sex difference in number

of sexual partners is small and that the vast

majority of both men and women prefer a

long-term relationship over a short-term

sexual relationship (Pedersen et al., 2002).

Cultural factors may have overrid-

den the influence of evolutionary theory on

sexual behavior. Due to the introduction

of effective contraceptives, sexual behavior

does not always lead to reproduction. The

fact that contraceptives are so commonly

used suggests that reproduction is often not

the intention of sex. The sociobiological view

of sex differences in sexual behavior assumes

that sexual intercourse will lead—or is in-

tended to lead—to reproduction. Today, I

doubt that the majority of men are thinking

about establishing paternity and the major-

ity of women are thinking about their part-

ners’ ability to support a child when deciding

whether or not to engage in sex.

Aggression

Evolutionary theory has been suggested as

an explanation of sex differences in aggres-

sion, in part because sex differences emerge

early in life (Archer, 2009). A meta-analysis

of five studies of toddlers showed that sex dif-

ferences in aggression are already substantial

(d=+.44; Archer & Cote, 2005). In addition,

sex differences in aggression are consistent

over childhood (ages 6–11;d=+.56) and pu-

berty (ages 12–17;d=+.46; Archer, 2004).

Aggression peaks in young adulthood, at a

time when men are in competition with each

other for women and for the opportunity to

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