A new book by psychologist Steve Stewart-Williams examines the evolutionary origins of psychological differences between men and women, sparking debate over the roles of nature and nurture.

In his third book, 'A Billion Years of Sex Differences: How Evolution Shaped the Minds of Men and Women,' psychologist Steve Stewart-Williams argues that nature plays a dominant role in creating psychological sex differences. He posits that evolutionary pressures led to distinct male and female mental traits, including differences in sexuality, parenting, and career preferences. The book presents this perspective as a middle ground between those who minimize sex differences and those who exaggerate them.

The Nature-Nurture Debate

Stewart-Williams contends that human minds are not blank slates and that evolutionary drives designed by natural selection explain observed sex differences. He argues that the 'gender-equality paradox'—where sex differences appear larger in more gender-equal nations—supports his view that innate differences flourish when social constraints are removed. He suggests that in enriched environments, individuals have more freedom to follow their natural inclinations.

Alternative Scientific Perspectives

Critics argue that Stewart-Williams' focus on genetics overlooks the significant role of culture and social learning. Social role theory suggests that psychological differences are primarily a consequence of gender roles and hierarchies, rather than their cause. This theory posits that reproductive and physical differences, such as upper-body strength, lead societies to allocate tasks differently, which are then reinforced by cultural norms and institutions.

Evidence and Interpretation

The book cites studies of girls with congenital adrenal hyperplasia, a condition involving excess prenatal androgen exposure, to support the role of biology in shaping behavior. However, critics note that evidence linking prenatal hormones to aggression is inconsistent. Other research suggests that children often conform to gender-labeled items regardless of innate predispositions, highlighting the influence of social cues on behavior.