In his new book A Billion Years of Sex Differences, evolutionary psychologist Steve Stewart-Williams aims to foster a more nuanced, scientifically rigorous public conversation about gender differences. He argues that both traditionalists and progressives often get sex differences wrong—traditionalists exaggerate them, while progressives minimize them and overemphasize nurture.
Key Arguments
Stewart-Williams claims some sex differences are relatively pronounced, such as attraction to men or women, upper body strength, height, likelihood of murder, and occupational interests. Others, like math ability or conscientiousness, are more modest. These differences are best visualized as overlapping bell curves. For example, while the shortest humans are almost all women and the tallest are men, knowing someone is 5ft 8in does not reliably indicate their sex.
Addressing Biases
The author identifies common biases in his field, including the “gamma bias”—minimizing differences that paint men in a better light while highlighting opposite ones. His research found that fictitious studies showing men outperforming women in drawing, honesty, or intelligence were rated as lower quality and more harmful than studies showing the reverse. He also notes a “delta bias,” an aversion to traditional sex differences and a preference for the opposite. However, he does not address bias in the opposite direction, despite historical norms favoring male superiority.
Innate Differences and Free Choice
Stewart-Williams clarifies that arguing for innate differences does not imply one sex is better, nor that differences are fixed or should be enforced. He believes that when people have freedom to choose jobs and lifestyles, men and women tend to gravitate differently—men toward things and status, women toward people and relationships. Studies show sex differences in occupational preference and personality are more pronounced in gender-equal countries, supporting his nature argument.
Evidence for Nature
The book presents evidence that sex differences persist historically and cross-culturally, mirror differences in other species, emerge early in childhood or intensify during puberty, resist socialization, and are influenced by prenatal hormones. Evolutionarily, because humans form pair bonds and men are involved in child-rearing, sex differences are smaller than in many animals. However, different reproductive incentives still drive divergences.
Critique
While the book offers interesting findings, evolutionary psychology is not always the most useful lens. Stewart-Williams suggests domestic violence is better understood as male aggression rather than patriarchy, noting perpetrators often hold anti-patriarchal beliefs. He argues self-control interventions are more effective than targeting ideology. However, domestic violence is higher in patriarchal communities where such behavior is tolerated, indicating culture matters greatly.
Regarding women’s underrepresentation in STEM and leadership, Stewart-Williams attributes it partly to innate differences in ambition and interests. This perspective may be more convenient than examining how institutions were structured when women were excluded. Notably, men’s weaker verbal abilities have not hindered them in literature. Understanding nature's role is important, but it is only a small part of a complex story.
A Billion Years of Sex Differences: How Evolution Shaped the Minds of Men and Women by Steve Stewart-Williams is published by Forum (£22).
