Men and women are different. But saying so has become taboo, according to the evolutionary psychologist Steve Stewart-Williams. A vocal minority, he states, sees positing the existence of sex differences as “a moral transgression”, to which the typical response is to “cancel, punish and shun”. This is particularly pronounced in academia, where biases distort discussion, he argues. In the Western world, there’s a predisposition towards minimising sex differences. There are other biases. “Studies finding anti-female bias are shouted from the rooftops,” while “those finding no bias or bias against men are reported in a whisper or simply ignored”, Stewart-Williams writes in his new book, A Billion Years of Sex Differences.
This lack of honesty matters. “If we want to make the world a better place, we need good science,” he maintains. That’s not possible “if scientists are afraid to diverge from the pre-specified conclusions of an academic political monoculture”. He argues that not only are there clear differences between the sexes, but the basis for them – in large part – is our “million-year journey of human evolutionary history”. This goes for everything from dating and toy preference to mental and physical health.
“In large part” is key. Stewart-Williams is no gender traditionalist. He rejects those who “exaggerate” differences between men and women. So too, the sex difference-minimisers who argue that if differences exist they’re “a product of discrimination and socialisation” – and that equality requires their abolition. Both views lack balance and fall foul of the science, he says.
Stewart-Williams advocates for a third way: one that acknowledges both nature and nurture, and which neither overstates nor underplays sex differences. Readers are taken through a long list of “standard-issue” differences, all of which, it’s argued, are hugely influenced by nature. These go from the obvious and uncontroversial (men are larger than women), to the uncomfortable but accepted: men, on average, are more violent and take more risks. Nature helps explain differences in both our physical and mental health – women are more prone to mental health disorders, men to substance abuse – and our desired occupations and playthings. Men tend to be more interested in “things” and things-related jobs (like engineering or computer programming); for women it’s people and people-related jobs. We’re even predisposed to like certain toys: cars, weapons and building blocks for boys; dolls, homemaking toys and fashion items for girls.
These are, Stewart-Williams acknowledges, “basically a compilation of antiquated stereotypes”. But that doesn’t mean they don’t exist or have a solely cultural cause. Nor does arguing that their root lies in evolution mean that these differences are fixed forever or inherently good or bad.
While some sex differences are “genuinely large”, these are almost always physical. Most human sex differences fall into the “fuzzy or statistical” category. Imagine a standard bell curve. The horizontal axis represents scores on a given trait. The vertical axis represents the number of people with a given score. When we say men are, for instance, more aggressive than women, we are talking about two bell curves that overlap. The one representing men is slightly further to the right. As Stewart-Williams explains, men and women can be found at almost every level of aggression. On an individual basis, some women are more aggressive than some men. But the average level of aggression is somewhat higher for men.
For each trait, Stewart-Williams argues that the sex difference observed has a part-evolutionary explanation. Many appear “remarkably early in life”, long before children are even properly aware of their own sex, or during puberty. That’s not proof of an innate contribution, he writes, but it certainly increases the plausibility of evolutionary factors. These differences have persisted over time: if they were solely down to culture “why would so many [traits] be so uniquely resistant to efforts to reform them?” These sex differences have been observed across different cultures, and many are found in animals.
The ultimate explanation is what Stewart-Williams calls “reproductive variability”. Male animals – including humans – can have more children than females. For both sexes, any trait that helps an individual edge towards the maximum possible number of children for their sex has a good chance of being selected, and subsequently passed on. Depending on the animal, this might be traits that enhance males’ ability to attract females, or beat up rival males, or even coerce unwilling females into sex.
But, importantly, human sex differences are far smaller than they are for other species. Why so? It’s because of how complicated we are. We fall in love and form long-lasting relationships. With humans, both sexes are choosy about their mates and care for their children. This has lowered the maximum number of children men can produce, bringing it closer to the number for women. “As that happened, it ushered in a corresponding reduction in the magnitude of sex differences in our species,” writes Stewart-Williams.
These arguments are – in large part – persuasive. Stewart-Williams’s casual, Ted-talk-esque style succeeds in making complex science understandable. He’s often witty, too. But after a few chapters, what initially seemed charming and enjoyable starts to grate. The same points are made over and over again, and the prose begins to feel formulaic. And while the arguments for an evolutionary influence are persuasive for many traits where sex differences observed, some fail to convince.
“Men are overwhelmingly the perpetrators, and women overwhelmingly the victims” of sexual assault and rape, Stewart-Williams claims. But with intimate partner abuse, he continues, there is only a difference when it comes to “extreme” physical aggression. (“Less extreme” aggression is not defined.) “In the modern West, men and women engage in similar levels of physical aggression against partners, and women, if anything, engage in more verbal aggression,” he says. The 2006 paper cited for the claim argues firmly that the basis of this is strongly cultural (going against Stewart-Williams’s argument). “The relative difference between men’s and women’s partner aggression, and the level of men’s physical aggression against their partners, appears to be strongly influenced by the predominant beliefs of each society,” it concludes.
For the rest of the chapter, it’s claimed – not all that convincingly – that both sexual violence and intimate abuse are best explained by evolution. Rape is not primarily about power or misogyny. “The main contributors are men’s greater interest in casual sex and sexual variety, coupled with women’s greater sexual choosiness.” When combined with “men’s greater size, strength, and willingness to use aggression… you’ve got a recipe for occasional disaster”. Occasional? According to the most recent figures for England and Wales, one in 250 women aged 16 or over were the victim of either rape or assault by penetration in the year ending March 2025. Close to one in 33 female (739,000) experienced a form of sexual assault.
The discussion on sexual misconduct aside, this is an important book. In A Billion Years of Sex Differences, Stewart-Williams is making a more profound argument: magnifying or minimising sex differences doesn’t just ignore science, it leads to harm. By failing to treat us as individuals, we end up discriminating against men and women. In medicine, both exaggerating and denying sex differences can be dangerous: doctors overlook gender atypical problems or presentations, for example missing eating disorders in men and heart attacks in women.
Gender-role individualism is Stewart-Williams’s proffered solution. We should “establish a fair and level playing field”, and let people make informed choices. We can work towards eliminating sex differences “created or amplified by unjust bias or barriers”. But we must acknowledge that “the sexes do differ on average, and we can’t just lie about that”, he says. Instead, we should “tell the truth carefully”.
This is the book’s ultimate message. Truth matters. If we deny differences that “everyone can see with their own two eyes”, the public simply won’t trust scientists on “politically charged topics”. Perhaps that ship has already sailed. Trust in “experts” has been in freefall for years. But an environment in which scientists, politicians, academics and others feel able to tell the truth – however uncomfortable – and not to be punished for doing so, is worth fighting for.
A Billion Years of Sex Differences
Steve Stewart-Williams
Forum, 358pp, £22
[Further reading: This is the end of Russia]