# How evolutionary psychology's latest book rehashes old assumptions about sex differences
Psychologist Steve Stewart-Williams has released a new book examining evolutionary explanations for mental differences between men and women. Titled "A Billion Years of Sex Differences: How Evolution Shaped the Minds of Men and Women," the work explores three central questions about how evolutionary processes produced divergent cognitive traits across sexes.
The book arrives amid growing skepticism toward evolutionary psychology itself. The field has faced sustained criticism for relying on just-so stories, circular logic, and unfalsifiable hypotheses that often reinforce existing cultural biases rather than reveal universal truths about human nature. Stewart-Williams, a psychologist at the University of Swansea, has written extensively on evolutionary approaches to human behavior, but the new work enters contested intellectual territory.
Evolutionary psychology gained prominence in the 1990s through popular books like "The Adapted Mind" and "Sperm Wars," which proposed that modern psychology reflects adaptations to ancestral environments. The field offers explanations for phenomena ranging from mate selection to parental investment to aggression. However, critics argue the discipline frequently confuses proximate mechanisms (how something works now) with ultimate explanations (why it evolved), and it routinely cherry-picks evidence while ignoring alternative explanations rooted in culture, economics, or recent social history.
The manosphere, an online ecosystem of men's rights activists and incels, has weaponized certain evolutionary psychology claims to justify male dominance and female submissiveness as biologically inevitable. This appropriation has tarnished the field's reputation and prompted legitimate researchers to distance themselves from pop science interpretations. Stewart-Williams' previous work has engaged with these debates, though critics contend that careful academic framing cannot entirely prevent misuse by ideologically motivated audiences.
The central challenge with sex differences research lies in disentangling nature from nurture. Human development involves continuous feedback between genes and environment. A cognitive trait observed in contemporary populations could reflect evolutionary history, current social structures, educational access, economic incentives, or some inseparable mixture of all. Studies in Western university populations cannot simply extend to human universals without acknowledging publication bias, sampling limitations, and cultural variation.
Modern neuroscience complicates older evolutionary narratives. Brain plasticity persists throughout adulthood. Experience reshapes neural architecture. Women's participation in fields like mathematics, science, and medicine has demonstrated that apparent "natural" differences in ability often dissolve when opportunity expands. Historical data show that supposed sex-typical traits shift dramatically across generations and cultures, suggesting learned behaviors rather than fixed evolutionary outcomes.
Stewart-Williams' book enters this fraught landscape at a moment when evolutionary psychology itself requires substantial methodological reform. Replication crises have exposed problematic practices in behavioral science. Pre-registration, effect size reporting, and resistance to post-hoc theorizing remain rare in the field.
The book's reception will likely depend on whether readers approach it as rigorous science or popular speculation. For those seeking nuanced evolutionary biology combined with honest acknowledgment of uncertainty and cultural variation, the work may disappoint. For those already convinced that evolution explains modern psychology comprehensively, it will likely confirm existing beliefs. The field's fundamental problem persists: extraordinary claims about universal human nature require extraordinary evidence, and evolutionary psychology has not yet provided it convincingly.