A Billion Years of Sex Differences: Innate or Learned?

A review of A Billion Years of Sex Differences: How Evolution Shaped the Minds of Men and Women by Steve Stewart-Williams; 362 pages; Forum, London (June 2026)

“The truth will set you free, but first it will piss you off!”
—Gloria Steinem, 2019

In January 2005, at a private conference hosted by the National Bureau of Economic Research, Harvard President Lawrence Summers violated a central taboo of liberal discourse. In a discussion of why men were overrepresented in tenured science and engineering positions at top universities, he suggested that one possible cause was genetic differences between the sexes.

Summers offered this as one hypothesis among others, including bias against women, the different socialisation of the sexes, and women’s lesser willingness to trade off parental care for time-consuming jobs. His point was that we must understand the causes of unequal representation before trying to change it.

But Summers’s even-handed approach was ignored in the feeding frenzy that followed. The mere suggestion that sex differences in aptitude might be innate—based on genes—was a red line crossed: Summers had raised the spectre of “genetic determinism”. Innate sex differences in aptitude or preference, according to liberal logic, will inevitably produce bias, bigotry, and discrimination—in this case against women—finally yielding inequity: unequal representation of the sexes in valued jobs.

An academic firestorm ensued. As Steven Pinker wrote:

Nancy Hopkins, the eminent MIT biologist and advocate for women in science, stormed out of the room to avoid, she said, passing out from shock. An engineering dean called [Summers’s] remarks “an intellectual tsunami,” and, with equal tastelessness, a Boston Globe columnist compared him to people who utter racial epithets or wear swastikas. Alumnae threatened to withhold donations, and the National Organization of Women called for his resignation. Summers was raked in a letter signed by more than 100 Harvard faculty members and shamed into issuing serial apologies.

Summers resigned as President of Harvard the following year, a prominent casualty in the culture wars surrounding sex differences. And there was a lesson in this tale for all of us: if you embrace a women-are-different-from-men perspective, be careful—maybe to the point of silence—about what you say. Liberal convention is sacrosanct.

But the questions he raised remain, and there are several. First, are there real differences in behaviour and aptitude between the sexes? Although the sexes have obvious physical differences, many “blank-slaters” think that things are roughly equal from the neck up, with no average sex differences in thoughts, preferences, or behaviours. Second, if there are such differences, where did they come from? Are they due to sociocultural influences, like patriarchy and socialisation, or are they largely innate—that is, based on genes? Finally, if some sex differences are inherited, did the genes involved arise by natural selection in our ancestors, with the differences enhancing reproduction? Answering “no” to all of these questions makes you a full-blown blank-slater, while answering “yes” makes you a genetic determinist—perhaps the most damaging label that can be stuck on a liberal thinker.

In A Billion Years of Sex Differences, Steve Stewart-Williams argues that it’s time to engage these questions with data. Summarising existing research, he comes down closer to genetic determinism than to blank-slatism. That is, he sees significant sex differences in thinking, behaviour, and preferences as having a substantial genetic basis, but he recognises too a role for sociocultural influences.

Stewart-Williams handles his hot potato gently. He is careful to admit when we lack good evidence for a purported sex difference; when we lack information about the genetic basis of a difference we can document; and when, as with homosexuality, we have that genetic information but remain ignorant about how the difference evolved. (Homosexuality is one such puzzle, since it has a partial genetic basis, but, under simple Darwinian logic, genes that reduce reproduction should be eliminated from populations.)

And he addresses at length—and counters—the view that inherited sex differences will lead to bigotry. Instead, he recommends that both sexes should have equal opportunities, with everyone free to choose their own path. In the end, it’s obvious that the more we understand the basis of sex differences, the better equipped we are to address them. This measured, timely, and informative book is essential reading at a time when many scholars not only insist that male–female differences are culturally generated but go further, denying that there are just two sexes.

The first notion Stewart-Williams dispels is the blank-slatist claim that there are no average differences between the sexes. In a useful appendix, he summarises research on the behaviours and personality traits that have shown large and repeated differences between males and females. Some of these are obvious but not often recognised as innate sex differences. One is the strong difference in sexual attraction: most—but not all—men seek women as sexual partners, and most women seek men.

Psychological research indicates that men and women also differ strongly in how they seek mates. On average, women prefer older partners with high status, while men are more drawn to youth and physical attractiveness. Men have a higher sex drive and are more willing to seek casual sex. And once in a relationship, men show far more sexual jealousy than women, being preoccupied with a partner’s sexual infidelity, while women’s jealousy is more emotional than sexual, focusing on a partner’s commitment to the relationship.

Another difference we know well: in every country surveyed, men commit vastly more crimes of violence than women, including roughly 90 percent of all homicides in America.

One of the largest and most repeatable differences between the sexes is the toy preferences of children. On average, young boys prefer male-typical toys like trucks, blocks, and toy soldiers, while young girls prefer dolls and kitchen equipment. This difference shows up very early in childhood and holds across societies worldwide.

The list goes on, but I’ll end by mentioning differences in preference that seem to translate into professional inequities. On average, women have more interest in people than things, which partly explains why occupations like computer science and engineering are dominated by men while the health and social sciences harbour far more women. Related to this are observations that men are better than women at “systematising”, defined as “understanding and reasoning about systems, including natural systems like atoms and galaxies, abstract systems like logic and maths, and human-made system like machines and computer programs”. Women, in contrast, are better at “empathising”: putting themselves in other people’s shoes and understanding their feelings and motivations.

You can see this last difference in a compilation of sex differences among young American doctors in different branches of medicine. Fields involving less contact with patients regularly show higher proportions of men: anaesthesiology (62 percent), radiology (71 percent), neurological surgery (73 percent), and orthopaedic surgery (76 percent). Fields demanding more contact with patients, and presumably requiring more empathy, show a preponderance of women: hospice and palliative care (63 percent), dermatology (66 percent), and paediatrics (75 percent). Yet across all fields of medicine, the proportion of women among young doctors is precisely 50 percent, suggesting little bias against women entering medicine.

Stewart-Williams notes that such differences could reflect sociocultural pressures like socialisation or bias rather than innate preferences, but we have some data against purely social explanations. Two big meta-analyses, for instance, show little evidence for the differential socialisation of boys and girls—and, anecdotally, we all know of despairing parents who tried to give their children gender-neutral upbringings, only to see them revert to behaviours characteristic of their sex.

In some areas there is also evidence against systemic bigotry. A recent study of sex bias in STEM fields—science, technology, engineering, and mathematics—examined six metrics of professional success: grant funding, papers accepted, strength of letters of recommendation, hiring, teaching ratings, and salaries. The data showed parity between the sexes in the first three areas, a preference for hiring women, and an advantage for men in teaching ratings and salaries. (The men’s salary advantage, at about four percent, was smaller than I would have predicted.) It is this kind of information, rather than assertions about what’s “obvious”, that we need when dealing with inequities.