How has evolution shaped men’s and women’s minds? A new book tells an outdated story

0 September 23, 2026 17:11

Psychologist Steve Stewart-Williams’ third book about evolutionary psychology is A Billion Years of Sex Differences: How Evolution Shaped the Minds of Men and Women.

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Psychologist Steve Stewart-Williams’ third book about evolutionary psychology, a controversial field often drawn on (and distorted) by the manosphere, is A Billion Years of Sex Differences: How Evolution Shaped the Minds of Men and Women. It’s primarily interested in three questions.

What are the “standard-issue” psychological sex differences? He argues there are reliably observed differences in sexuality, our preferences when it comes to choosing a partner, antisocial behaviour and parenting. There are also, he says, robust sex differences in our choices when it comes to the jobs we do and the toys we play with as children.

Review: A Billion Years of Sex Differences: How Evolution Shaped the Minds of Men and Women by Steve Stewart-Williams (Swift Press)

How much of these differences are due to “nature” versus “nurture”? He believes nature is “the dominant player” – or at the very least, just as important as nurture.

Finally, why has nature created these sex differences? Because “the mind is a collection of drives and devices ‘designed’ by natural selection to help us stay alive and have offspring”. And, when it comes to the mission of reproduction, despite the evolution of paternal care and pair bonding, “the best way to have more children sometimes differed for men and women”.

This led to some of those drives and devices being somewhat differently designed in male versus female minds – like a male tendency to incorrectly think women are sexually interested in them, so as to not miss out on any potential sexual opportunities.

Steve Stewart Williams credit Photo Gallery scaled.
Thong Wei Sen White/Swift Press

Stewart-Williams presents his position as the Goldilocks just right account. He contrasts it favourably with mostly unnamed “sex differences minimizers” (who play down the existence of psychological sex differences, and the role of sex-linked biology in creating them), and “sex differences exaggerators” (who do the opposite).

But actually, his view falls at the extreme “nature” end of the scientific spectrum. Stewart-Williams takes care to draw attention to similarities between women and men. He also disagrees with traditionalists who want to coerce everyone into old-fashioned gender roles. However, his book should not be mistaken for a balanced guide.

More than blank slates

I think we all agree our minds are not blank slates, genetically endowed with nothing more than general-purpose learning abilities. But Stewart-Williams assumes much more built-in content than many other scholars interested in the evolution of human minds and social life. His gene-centred understanding of evolutionary theory goes against the scientific trend for a greater emphasis on the role of culture.

Since our spin-off from other apes more than six million years ago, we have evolved culturally created and inherited social categories, skills, beliefs, practices and institutions – together with supercharged capacities for social learning. Some scientists argue this includes innate tendencies to conform, and to copy people of the same sex – an intriguing idea.

According to cultural evolutionary perspectives, these uniquely human evolutionary products enable us to meet the challenges of survival and reproduction through complex feats of cooperation.

One challenge of cooperation societies must solve is power relations. Who has access to the means of production, or instruments of physical force? Who, in the division of labour, takes on the most socially important and valuable tasks? And who occupies positions of authority? Gender is one of many categories used to pattern such arrangements.

This gives us an alternative framework for understanding psychological sex differences, gender roles and hierarchy. I have recently argued such frameworks offer the most promising approach to understanding gender roles.

Stewart-Williams thinks gender roles and hierarchy are, at root, a consequence of psychological sex differences. Men and women naturally tend to slot into different social positions because of inherent psychological differences.

But according to an important idea from psychology called social role theory, it’s the other way round: psychological sex differences are primarily a consequence of gender roles and hierarchy.

According to social role theory, physical and reproductive differences between the sexes make certain kinds of allocations much more efficient than others.

The sex that gets pregnant and breastfeeds does tasks that can readily be combined with those reproductive activities, like laundering, gathering plant food and caring for young children. Members of the faster sex, with much greater upper-body strength, are typically nominated as the tree fellers, the big game hunters and the warriors.

The sex that gets pregnant and breastfeeds does tasks that can readily be combined with those reproductive activities, like cleaning.
Unsplash

But, like other social roles and hierarchies, gender roles are scaffolded by culturally shared beliefs: norms concerning the skills and knowledge men and women should have, how they should behave, and their status, rights and responsibilities.
These norms shape us as individuals, curate our interactions and are embedded in social and legal institutions, like laws, families and organisations.

Gender ideologies are another piece of these social systems. They justify whatever arrangements are in place as fair and desirable: ordained by natural laws or a higher power.

Asking different questions

This alternative scientific framework draws attention to important, difficult questions. Why are gender systems so diverse, yet with clear patterns? How are they stable, but changeable? Why do sex differences vary so much across individuals, time periods, populations and contexts? How do sexed biology, experience and culture interact to construct minds?

A Billion Years of Sex Differences is largely uninterested in such questions.

Stewart-Williams considers whether the nurture-focused social role theory can do a good job of explaining sex differences in the mind. He declares it a scientific failure on two broad grounds.

The first is that evidence for nurture’s influence is weak, he argues.

A key part of his case is the so-called gender-equality paradox. “For many traits, sex differences are larger, rather than smaller, in more gender-equal nations – the exact opposite of what we’d expect if the differences were due to rigid sex roles, gender inequality, or patriarchy,” he writes.

For instance, he considers a study of sex differences in frequent physical same-sex fighting in adolescence, in low-to-middle income countries. Overall, teenage boys more often got into fights with each other than girls did. But this difference between the sexes was larger in more gender-equal countries.

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Stewart-Williams cites a study that teenage boys fight with each other physically more than girls – and more so in countries where the genders are more equal.

Results like these pose a problem for social role theory, which proposes that psychological sex differences stem from gender roles and hierarchy. Those roles are more similar in more gender-equal countries, and the hierarchy is less steep. Sex differences in physical aggression should be smaller in those countries – in this study, they were bigger.

But testing for associations between gender gaps in behavioural patterns and country-level differences on gender equality indices is challenging. Academics are still arguing about how to do this properly.

Some researchers have reported that gender gaps relating to science, technology, engineering and mathematics (STEM) are bigger in more gender-equal countries. However, a recent review of this research concluded evidence for the gender-equality paradox in STEM “is generally inconsistent and weak”.

And an article published last year concluded statements that more gender-equal countries have larger gender gaps in personality and behaviour are based on a statistical illusion.

Extreme explanations

However these debates shake out, studies showcase considerable cross-cultural variation in sex differences.

The study of same-sex fighting among teenagers showed sex differences range from being eight times more common in boys than girls (in Tunisia and Suriname) to virtually undifferentiated by sex (in Zambia, Tonga, Benin and Ghana). Such findings show “culture matters enormously when it comes to sex differences”, as Stewart-Williams himself puts it.

This seems to undermine one of his core claims: that nurture’s influence on sex differences is weak.

Stewart-Williams’ “best guess” about what is going on in such cases is that “in enriched and relatively unconstrained environments, natural differences between individuals, and average differences between the sexes, have more opportunity to flower and grow”. So, average psychological sex differences become larger because “people have more scope to follow their hearts and nurture their own individuality”.

But this downgrades nurture to little more than a background resource: an extreme position on how complex human traits develop.

The second reason social role theory is wrong, Stewart-Williams argues, is strong evidence for nature’s powerful role in creating sex difference. Stewart-Williams concedes each of his sources can be criticised individually. But taken together, he says, they all point to the same conclusion – that there is a “significant innate contribution” to psychological sex differences.

But do they?

One source of evidence comes from studies of girls and women with congenital adrenal hyperplasia, a genetic condition that causes excess production of androgens (including testosterone) during foetal development. Girls with the condition may be born with masculinised genitalia. Many studies have explored whether this also masculinises their minds.

Discussing this research, Stewart-Williams says links between foetal “hormone levels and sex-typical behaviour are often fairly strong”. This isn’t right. For instance, like other men, most boys exposed to low levels of prenatal androgens become heterosexual. And studies have failed to reliably show greater aggression in girls with congenital adrenal hyperplasia, compared to girls without the condition.

But these girls do show more interest in “boy toys” and masculine careers than other girls. Stewart-Williams attributes this to the masculinising effects of surplus androgens on their minds. But why would evolution recruit prenatal hormones to make boys predisposed to be more interested in toy cars and farming?

Stewart-Williams later concedes the large sex differences in preferences for modern toys and occupations are a bit puzzling, given the evolutionary novelty of both. And there is another explanation of what is going on with these girls.

We learn from cultural cues

Consider a study where primary-school-aged children were shown various paired gender-neutral items (like a green and silver balloon, or a toy horse and cow).

For each pair, one was randomly labelled as “for girls”, the other “for boys” – or was shown being chosen by a woman or man. Boys, and girls without congenital adrenal hyperplasia, “obeyed” those gender cues. They were much more likely to select whichever item was arbitrarily signalled as being for their sex.

But, for reasons we don’t understand (such as direct effects of testosterone, a less secure gender identity, or an awareness that they are “not like other girls”), girls with congenital adrenal hyperplasia were much less likely to conform to gender norms. Maybe this partly or wholly explains their greater open-mindedness about gendered toys and jobs.

A study showed girls with a condition giving them more prenatal testosterone were less likely to conform to gender norms. Understanding why is complex.
Allan Mas/Pexels

This makes sense from a cultural evolutionary perspective. The idea here is that a crucial inherited adaptation is our flexible ability to learn from culture, including picking up gender norms. This is adaptive because genes can’t predict which behaviours will be advantageous in diverse and changing gender systems.

None of this is to deny the sex-differentiated biological influences on male and female minds that surely exist. But stable non-genetic inputs into development can help ensure adaptive traits are reliably inherited from one generation to the next.

We have an important channel of inheritance that can help this happen: culture. It has the bonus of offering greater flexibility in adapting to the diverse and rapidly changing environments in which we evolved.

For any trait, sensitivity to gender norms – paired with a stable gender system that offers a rich channel of inherited information about how to be a man or a woman – may make genetic inheritance of that information unnecessary.

But what about animals?

As part of his case for nature’s role, Stewart-Williams argues that “many of the standard-issue sex differences” are also found “in other animals”, such as sex differences in parental care seen across many mammals.

But different species can evolve different balances of genetic versus non-genetic inheritance to solve adaptive challenges. And humans use culturally inherited social categories, skills, beliefs, practices and institutions to work out arrangements for caring for kids.

For example, gender role beliefs about fatherhood lay out the cultural ground rules about dads’ roles and responsibilities towards their offspring. This shapes day-to-day activities, such as how much time they spend with their infants. In turn, this modifies hormones and brains. Why assume any biologically based tilts will topple this immense and intricate gender scaffolding?

In other words, even a biological category, like father, is also a cultural one. This helps explain the immense diversity in fathering across time and place.

For example, a study of the Aka Pygmy fathers of Central Africa found they spent nearly half their day holding or within arm’s reach of their babies. Fathers were also more likely than mothers to hug and kiss their infants. In contrast, their US counterparts at the time of the research were mostly the breadwinners and rough-and-tumblers.

Even a biological category, like father, is also a cultural one.
Toa Heftiba/Unsplash

What to do about sex differences?

Stewart-Williams advocates for a “gender-role individualism” approach, which he argues upholds the “fundamental values” of “well-being and freedom”. Of course, this sounds much more promising than promoting misery and oppression. But his mantra – to “let people be themselves!” (so long as they don’t cause harm) – reflects the extreme perspective that socialisation plays a minimal role in people’s self-understanding.

In the toy department, for example, the gender-role individualist can simply “let kids know all the options, but ultimately […] accept their choices”. The problem is obvious. As the study with the balloons and farm animals labelled as “for boys” or “for girls” shows, those options are culturally loaded, shaping our choices.

Stewart-Williams is right that our evolutionary history, including sex-linked biology, must play some role in any satisfying explanation of sex differences in our minds. But we are also deeply cultural – and no other animal is anything like us.

Evolution has not “shaped” the minds of men and women in the biologically deep-rooted way Stewart-Williams suggests – that shaping is an ongoing process. Any scientific account that neglects this is likely just wrong.
Cordelia Fine is the recipient of funding from the Australian Research Council, and has previously received funding from the Women’s Leadership Institute Australia and the The Victorian Government Commission for Gender Equality in The Public Sector.

Tags: Academic Journalism, The Conversation, The Conversation Academic News