Biology and Human Sex Differences
The Case for Nature
So far we have presented strong circumstantial evidence that biology plays a role in many behavioral phenomena related to gender. Animal research shows that early sex hormones lead to differences in the nervous systems of males and females, which in turn influence the behaviors of males and females. Data from humans with hormonal abnormalities and evidence from natural experiments suggest that early hormonal events in people are related to later gender-linked behaviors. And studies show that normal variations in sex hormones—particularly testosterone—are related to a number of socially significant human behaviors, many of which are sex-linked. Still, none of these studies directly confronts the question: Do biological factors cause human sex differences?
How can we assess the role of biology in human sex differences? There are four kinds of relevant evidence (Maccoby & Jacklin, 1974):
1. The age at which sex differences emerge
2. The consistency of sex differences across cultures and over historical time
3. The consistency of sex differences across species
4. The relation of physiological factors (e.g., sex hormones and brain structures) to behaviors that show sex differences (e.g., aggression, visual-spatial ability)
Why are these four kinds of evidence relevant? Let us consider each in turn. The earlier a sex difference occurs in life, the less likely it is to be learned and the more plausible it is that biology plays a role in producing the difference. The most clear-cut case would be if a sex difference appears immediately after birth. Some human sex differences do in fact appear at a very early age. For example, male infants are somewhat more irritable and active than female infants are (Eaton & Enns, 1986; Phillips, King, & DuBois, 1978). Unfortunately, newborn infants do not show many of the behaviors that gender researchers are most interested in studying, behaviors such as aggression, visual-spatial performance, and mating practices. If a sex difference does not occur until late in development, then the likelihood increases that social learning and cultural factors play a role. For example, boys begin to exceed girls in math performance in their teenage years, and this gender difference many reflect adolescent girls' acquired views about which behaviors are seen as feminine and which are not (Eccles & Jacobs, 1986).
Evidence on the developmental timetables of sex differences is at best suggestive about the roles of biology and socialization. You may recall from Chapter 1 that sex differences in aggression are largest in children, moderate in adolescents, and smallest in adults (Hyde, 1986). One interpretation of this pattern is that, although boys are biologically predisposed to be more aggressive than girls, with increasing age this sex difference is tempered bv socialization, which often works to reduce everyone's aggressiveness. However, even when a sex difference does not emerge until late in development, it still may be strongly influenced by biological factors. Sex differences in many sexual behaviors do not emerge until after puberty. However, these differences cannot occur until boys and girls physically (i.e., biologically) mature into men and women. Baldness is a largely genetic trait that does not show a sex difference until later in life. However, this does not imply that male pattern baldness is learned.
The consistency of sex differences across cultures constitutes a second kind of evidence about the contribution of biology to sex differences. The more consistent a sex difference is across cultures, the more likely it is influenced by biological factors. Conversely, the more a sex difference varies across cultures, the more likely it is that it is culturally caused. If a sex difference occurs consistently, despite all the variations in learning and socialization practices that occur across cultures, then a biological signal—an innate predisposition—is probably showing through all the cultural noise. If men are more physically aggressive than women in virtually all cultures, for example, there is probably a biological predisposition toward higher aggressiveness in men, which shows itself regardless of cultural learning. On the other hand, if sex differences come in all degrees and in all directions across cultures (men sometimes show the behavior more than women, men and women show the behavior equally, and women sometimes show the behavior more than men), then it would seem that there is no innate predisposition underlying the cultural variations. If men are more likely to be doctors in some countries but the reverse is true in other countries, then becoming a doctor would seem to be culturally, not biologically determined.
Given the changes that have occurred in gender roles in the past few decades, another way to probe biological contributions to sex differences is to examine whether the sex differences have changed over time. Some sex differences—for example, in SAT math scores—seem to have narrowed somewhat (Feingold, 1988). Other sex differences—such as those in mental rotation test performance—have not (Masters & Sanders, 1993). If sex differences have decreased as gender roles have become less extreme, this would suggest the influence of cultural factors. On the other hand, if sex differences remain constant, despite changes in men and women's roles, then it becomes more plausible that biological factors underlie the differences.
By a similar sort of reasoning, the more consistency a sex difference shows across species, particularly species closely related to human beings, the more plausible it is that there are biological factors contributing to the sex difference. If young males engage in more rough and-tumble play than young females—regardless of whether they are rhesus monkeys, orangutans, gorillas, chimpanzees, or human beings—then the case is strengthened that these sex differences are due in part to biological factors. Sex differences in rough-and-tumble play in nonhuman primates cannot be explained in terms of cultural learning. And similarities between primates and humans suggest some degree of evolutionary continuity between the two (e.g., chimps and humans share some 98% of their genes).
Perhaps the most direct evidence that biological factors contribute to sex differences is evidence that biological factors that differ between the sexes, such as sex hormones, are related to behaviors showing sex differences, such as aggression, (Recall the evidence discussed earlier on links between hormones and behavior.) Most studies that investigate the relationship between sex-linked biological factors and human behaviors are correlational. This means that they observe variables (e.g.,testosterone levels, aggression) as they naturally occur in some population and investigate whether they are related to one another. Unfortunately, correlational studies cannot provide clear-cut information about cause-effect relationships. For example, as described earlier, research shows that testosterone levels are correlated with aggressiveness. Does this mean that high testosterone causes increased aggression? Not necessarily. why not? The cause-effect relationship could be in the opposite direction: aggressiveness could cause testosterone levels to increase. Or a third variable, such as having aggressive parents, could lead both to elevated testosterone levels and to increased aggressiveness.