The Case for Nature
Many kinds of evidence suggest that biology contributes to human sex differences and to individual differences in masculinity and femininity. Animal experiments show that prenatal hormones create differences in the nervous systems and in the behaviors of males and females. Studies of people with genetic and hormonal abnormalities—CAH females, androgen-insensitive XY individuals, reductase-deficient males, individuals exposed to DES, and Turner syndrome females—suggest that early exposure to hormones, particularly androgens, influences later sex-typed behaviors and abilities.
Numerous studies show that people's testosterone levels are correlated with socially significant behaviors, such as aggression, criminality, sexual activity levels, dominance, occupational success, and spatial ability. Many of the behaviors and abilities linked to testosterone also show substantial sex differences. Animal research shows that hormones play a role in producing sex difference and within-sex variations in maternal behavior. Natural experiments and accidents, such as when genetic males are castrated and reared as females, indicate that prenatal exposure to testosterone often produces male gender identities and male-typical behaviors, even in individuals reared as females.
To show that biological factors contribute to human sex differences, researchers seek four kinds of evidence:
1. Early appearance or sex differences in development
2. Cross-cultural and temporal consistency of sex differences
3. Cross-species consistency of sex differences
4. Empirical links between sex-linked biological factors (e.g., sex hormones and brain structures) and sex-linked behaviors
These kinds of evidence are generally present for sex differences in three behavioral domains: physical aggression, visual-spatial ability, and aspects of sexual behavior, including sexual orientation.
Several kinds of evidence suggest that biological factors contribute to individual differences in masculinity and femininity. Research on both prenatal and adult sex hormones, particularly androgens, shows that hormone levels are related to individual differences in masculinity and femininity. In addition, behavior genetic studies show that individual differences in masculinity and femininity are heritable.
In short, a growing body of evidence supports the conclusion that biological factors contribute, sometimes strongly, to many of the phenomena described by the term gender.