Sex Differences in Aggression

The earliest known homicide victim was a Neanderthal man who died 50,000 years ago (Trinkaus & Zimmerman, 1982). He was stabbed in the left front of his chest, indicating a right-handed attacker . As paleontological detective work has become increasingly sophisticated, evidence of prehistoric violence among our forebears has mushroomed (Daly & Wilson, 1988). Ancient skeletal remains contain cranial and rib fractures that appear inexplicable except by the force of clubs and weapons that stab. Weapon fragments are occasionally found lodged in skeletal rib cages. Humans apparently have a long evolutionary history of violence (Buss, 2005).

In a sample of homicides committed in Chicago from 1965 through 1980, 86 percent were committed by men (Daly & Wilson, 1988). Of these, 80 percent of the victims were also men. Although the exact percentages vary from culture to culture, cross-cultural homicide statistics reveal strikingly similar findings. In all cultures stud ied to date, men are overwhelmingly more often the killers, and most of their victims are other men. Any reasonably complete theory of aggression must provide an explanation for both facts—why men engage in violent forms of aggression so much more often than women do, and why men comprise the majority of their victims.

An evolutionary model of intrasexual competition provides the foundation for such an explanation. It starts with the theory of parental investment and sexual selection (Trivers, 1972). In species in which females invest more heavily in of fspring than males do, females become the valuable limiting resource on reproduction for males. Males become constrained in their reproduction not so much by their own ability to survive but, rather, by their ability to gain sexual access to the high-investing females. In other words, in a species in which females can bear only a small number of of f-spring, such as the human species, females will express great care in their choice of mates, and males will be forced to compete for access.

Because female mammals bear the physical burden of gestation and lactation, there is a considerable sex difference in minimum obligatory parental investment. Therefore, males can have many more of fspring than females can. Stated dif ferently, the ceiling on reproduction is much higher for males than for females. This difference leads to dif ferences in the variances in reproduction between the sexes. The dif ferences between the haves and have-nots, therefore, become greater for males than for females: most females will have some of fspring. Among males, however , a few males will sire many offspring, whereas some will have none at all. This is known as effective polygyny.

As a general rule, the greater the variance in reproduction, the more ferocious the competition within the sex that shows higher variance. In an extreme case, such as the elephant seals of f the coast of northern California, 5 percent of the males sire 85 percent of all offspring produced in a given breeding season (Le Boeuf & Reiter, 1988). Species that show high variance in reproduction within one sex tend to be highly sexually dimorphic, highly different in size and structure. The more intense the effective polygyny, the more dimorphic the sexes are in size and form (Trivers, 1985). Elephant seals are highly size dimorphic: males are four times lar ger than females (Le Boeuf & Reiter , 1988). Chimpanzees are less sexually dimorphic: males are roughly twice as lar ge as females. Humans are mildly dimorphic, with males roughly 12 percent larger than females. Within primate species, the greater the effective polygyny, the more the sexual dimorphism, and the greater the reproductive variance between the sexes (Alexander et al., 1979).

Effective polygyny means that some males gain more than their fair share of copulations, whereas other males are shut out entirely , banished from contributing to the ancestry of future generations. Such a system leads to ferocious competition within the high-variance sex. In essence, polygyny selects for risky strategies, including those that lead to violent combat with rivals and those that lead to increased risk taking to acquire the resources needed to attract members of the high-investing sex.

Violence can occur at the top as well as the bottom of the hierarchy . Given an equal sex ratio, for each man who monopolizes two women, another man is forced to be a bachelor (Daly & Wilson, 1996). For those facing reproductive oblivion, a risky, aggressive strategy may represent a last resort. The homicide data reveal that men who are poor and unmarried are more likely to kill, compared with their more affluent and married counterparts ( ilson & Daly, 1985). This finding is correlational of course, so we cannot know with certainty that being poor and unmarried is a cause of violence (a third variable, such as the personality trait of aggressiveness, might be responsible for being poor , unmarried, and violent).

This account provides an explanation for both facts revealed in the cross-cultural homicide record. Males are more often the perpetrators of violence because they are the products of a long history of ef fective polygyny. Throughout human evolution,

Men tend to engage in riskier tactics of competition, such as aggression and violence.

male sexual strategies have been characterized by risky intrasexual competition for females, or for the social status and resources that attract females. The fact that men die, on average, seven years earlier than women is but one of the many markers of this aggressive and risk-taking intrasexual strategy (Promislow , 2003).

Men are the victims of aggression far more than women because men are in competition primarily with other men. It is other men who block any given man' s access to women. With increased aggression comes a greater likelihood of injury and early death. The patterns of aggression, in summary , are well predicted by the evolutionary theory of intrasexual competition (Buss & Duntley , in press). Even psychologists who argue that most psychological and behavioral sex dif ferences are due to social roles concede that sex dif ferences in aggression are most likely caused by a long evolutionary history in which women and men have confronted dif ferent adaptive problems.

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