The Uncomfortable Science Of Sex Differences - Steve Stewart-Williams - #1120

What It Covers

Evolutionary psychologist Steve Stewart-Williams joins Chris Williamson to examine the biological and evolutionary foundations of human sex differences. Drawing on six distinct lines of evidence, they cover reproductive variance, parental investment theory, mate preferences, aggression patterns, casual sex asymmetries, and the gender equality paradox — separating evolutionary explanation from moral justification throughout.

Key Questions Answered

• •Six-Evidence Framework for Innate Sex Differences: Stewart-Williams outlines six converging lines of evidence that point toward biological contributions to sex differences: early developmental appearance, persistence despite cultural pressure, consistency across time, hormonal correlates (especially prenatal testosterone), cross-cultural universality, and presence in related species. No single line is definitive, but all pointing in the same direction builds a strong cumulative case that socialization alone cannot account for observed differences.
• •Gender Equality Paradox: Counterintuitively, more gender-equal societies tend to show *larger* sex differences in personality and career preferences, not smaller ones — the opposite of what social roles theory predicts. This finding undermines the argument that patriarchy drives sex differences. One rebuttal is that people in segregated societies compare themselves only within their own sex, but this explanation fails to account for the paradox appearing in objective measures like spatial ability and physical traits.
• •Reproductive Variance as the Root Driver: Most sex differences trace back to the difference in maximum potential offspring between males and females. Because females invest more per offspring through gestation, birth, and nursing, their reproductive ceiling is lower. Males, facing higher variance in reproductive success — some producing many offspring, others none — face stronger selection pressure for traits like aggression, risk-taking, status-seeking, and interest in sexual variety.
• •Casual Sex Asymmetry Data: Men score roughly one standard deviation higher than women on sociosexuality (interest in no-strings-attached sex), meaning in any random male-female pairing, the man scores higher approximately 67–70% of the time. The Clark-Hatfield campus study found 75% of men accepted a stranger's offer of sex versus 0% of women. Gay men have more casual sex than straight men; lesbians have less than straight women — revealing each sex's baseline preferences unshaped by the other sex's constraints.
• •Short-Term vs. Long-Term Mate Preferences: Women's preference for physical attractiveness in a male partner equals or exceeds men's preference in short-term mating contexts, even though men weight looks more heavily in long-term partners. The evolutionary logic: in short-term encounters, genes are the only male contribution, so physical indicators of genetic quality dominate. In long-term contexts, women weight resources and status more heavily because these predict investment capacity in biparental care.
• •Aggression Sex Difference Scales with Intensity: The male-female gap in aggression grows larger as aggression severity increases. Verbal aggression shows roughly a 0.5 Cohen's d effect size. Physical aggression is larger. Violent crime is substantially male-dominated. Homicide perpetration exceeds 90% male in every nation with available data. Chimpanzees show virtually identical ratios — 92% male perpetrators, 73% male victims — matching human figures of 95% and 70–80%, suggesting deep evolutionary rather than cultural origins.
• •Naturalistic Fallacy and Moral Neutrality: Evolutionary explanations describe *why* traits exist, not whether they are good or permissible. The same logical error applies whether the cause is evolutionary or sociocultural — identifying a cause does not dissolve individual responsibility. Stewart-Williams argues that holding people accountable remains a useful social institution regardless of causal origins, and that evolutionary findings disprove historical sexist science rather than continuing it, since most psychological sex differences are modest with substantial within-sex variation.