Marlon Brando and Eva Marie Saint in On the Waterfront (1954). Photo by Corbis via Getty

Gender and identity

Marlon Brando and Eva Marie Saint in On the Waterfront (1954). Photo by Corbis via Getty

Sex on the brain

Humans, like other mammals, exhibit sex differences in their brains and psychological traits. But what do they signify?

Kevin Mitchell

Marlon Brando and Eva Marie Saint in On the Waterfront (1954). Photo by Corbis via Getty

Kevin Mitchell

is a neurogeneticist. He is associate professor at the Smurfit Institute of Genetics and the Institute of Neuroscience at Trinity College Dublin. His latest book is Innate: How the Wiring of Our Brains Shapes Who We Are (2018). He lives in Portmarnock, Ireland.

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Sex differences really do exist. Men and women, boys and girls, really do behave differently. The question – and difficultly – lies in establishing where these differences come from. Are the sexes wired differently? Or does culture explain observed behavioural differences? The answers are yes and yes; but, unfortunately, biological and cultural explanations are so often seen as mutually exclusive that a middle ground can be hard to discern. Instead, the debate about the origins of sex differences tends to get polarised into extreme positions, with straw persons being erected and toppled by each camp, while epithets of ‘neurosexism’ and ‘sex-difference denier’ are lobbed across the divide.

The debate is especially contentious at the moment, with proponents of nature or nurture each claiming that the latest brain science proves their position. Results from neuroimaging studies are pointed to as ‘proof at last’ that the brains of men and women really are innately different, and that these differences explain the differences we see in behaviour. Yet the very same results are held up as evidence that there really is no such thing as a ‘male brain’ or ‘female brain’, and that any observable differences between the sexes are not innate in origin, but owe to the effects of growing up in a gendered environment. Either way, important implications for social policy are drawn, based on the favoured interpretation of the evidence.

In her recent book, The Gendered Brain (2019), the English neuroscientist Gina Rippon argues against the ‘myth’ of innate biological differences and claims that brain and behavioural differences arise instead from cultural forces. She provides compelling evidence that much of the historical research in this area has been (and, in some cases, continues to be) driven by an overtly or implicitly sexist agenda, intent on finding scientific proof of female inferiority.

In the other corner, the Canadian psychologist Jordan Peterson contends on Twitter that: ‘Sex differences are large and biological/innate. The science is clear. The opposing sociology is delusional.’ In a now-infamous memo, the Google employee James Damore argued in 2017 that innate sex differences in interests and aptitudes partly explain observed differences in occupational choices, especially the relative lack of women in STEM fields (and at Google). Damore was promptly fired for his intemperate comments, and roundly excoriated by many commentators. Yet in other quarters, he was celebrated as a brave proponent of free speech and scientific truth.

Both sides can end up arguing for rather blinkered positions. Peterson, for example, maintains that the pay gap can be explained by women scoring higher, on average, in the personality trait of agreeableness: training women to be less agreeable, he contends, would improve their financial success in the workplace. Meanwhile, the Canadian cognitive scientist Steven Pinker recently decried an article in The New York Times – which looked at why women do more than their share of the housework – for not considering biological sex differences as a possible factor. Even if he had a point, his apparent disregard for entrenched patriarchal norms scotched any sympathy he might have found on social media.

Meanwhile, the American psychologist Michael Reichert argued, also in The New York Times, that ‘violence springs from what boys learn about what it means to be a man’ – as opposed to any inborn tendencies towards physical aggression. This despite the scientific evidence showing that sex differences in physical aggression are universal across human societies, have a cogent evolutionary rationale, manifest in most other mammalian species, and have well-worked-out biological mechanisms.

Much of the cultural discussion around sex differences amounts, in the words often attributed to the American philosopher William James, to people simply ‘rearranging their prejudices’ so as to privilege evidence that supports their position, while focusing the full glare of a skeptical spotlight on contradictory findings. Rippon, for example, rightly critiques shoddy early neuroimaging work that claimed to have found biologically driven brain differences directly accountable for observed sex differences in behaviour. She is far less critical, however, of the equally shaky literature claiming that brain plasticity can drive differences in macroscopic brain structure, which in turn might account for behavioural differences.

We are all human, of course: all subject to this kind of confirmation bias. At least a researcher’s position on the primary question of the origins of sex differences is typically made explicit. But people working in different disciplines and reading various literatures will also entertain a host of underlying subsidiary beliefs that are less overt, and that strongly influence how they weigh various types of evidence or argument. They might have strong prior positions on whether individuals have any innate psychological predispositions and if such traits are influenced by genetics; whether findings in animals are relevant to human psychology; whether human minds have been shaped by their recent evolutionary past; if experience can reshape brain structure, or personality traits play a larger role in explaining behaviour.

These deep but usually unstated differences in starting positions leave scientists and commentators talking past each other, and the general public none the wiser. They can even lead to the same data being interpreted in diametrically opposed ways – which raises the question of where the solid scientific ground actually lies. Nowhere is this more apparent than in the interpretation of results from neuroimaging studies.

In a 2015 study that has given rise to the ‘mosaic brain’ hypothesis, the psychologist Daphna Joel at Tel Aviv University and colleagues analysed brain scans from more than 1,400 people, looking for regions of the brain where there was a statistically significant difference in volume between the sexes. They found 10 regions showing such differences, some larger in males, some in females. On the face of it, their findings seemed to support the idea that male and female brains are structurally distinct. However, each of the 10 regions under scrutiny varies in volume across individuals anyway, with the distribution simply shifted slightly higher or lower in the other sex. Joel’s team found that very few individuals showed extreme ‘male’ or ‘female’ values for all 10 regions; instead, most showed a pattern of values falling mainly in the overlapping zones, with only a general trend towards one end or the other.

The authors concluded that the brains of males and females are not categorically distinct. In other words, there is no such thing as a ‘male brain’ or a ‘female brain’. Rather, they suggest that each individual’s brain is a ‘mosaic’ of masculinised and feminised regions, the implication being that we should not expect biologically driven sex differences in behaviour. Yet, within months, multiple other researchers showed that the same data could very reliably be used to categorise individual brains as male or female. While the volume of any individual area is a terrible predictor of sex, a multivariate analysis gives very good discrimination. On this reading, the brains of males and females are not dimorphic, with two completely different forms, like genitalia; instead, they show a correlated set of shifts in the size of various features, similar to what is observed for male and female faces, which are also readily distinguished.

Another neuroimaging study that drew media attention for the contrary readings it spawned was undertaken in 2014 by the neuroscientist Madhura Ingalhalikar and colleagues at the University of Pennsylvania. They measured the connections between brain regions, and found some sex differences in organisation, with females tending to have more connections between the two hemispheres, and males having slightly more running front-to-back within each hemisphere. The data seemed pretty robust, and fit with prior findings of greater cross-hemispheric connectivity in females. Still, the authors were criticised for how they interpreted the findings. They speculated – rather freely – that ‘male brains are structured to facilitate connectivity between perception and coordinated action, whereas female brains are designed to facilitate communication between analytical and intuitive processing modes’. In the press release for their paper, they claimed that the differences could explain why ‘men are more likely better at learning and performing a single task at hand, like cycling or navigating directions, whereas women have superior memory and social cognition skills, making them more equipped for multitasking and creating solutions that work for a group’.

In the absence of any causal link between the observed differences in brain structure and those in behaviour, such claims are purely speculative. Nor were the chosen examples of supposed sex differences in behaviour particularly convincing (are men really psychologically more suited to cycling?). Claims like these rely on unsupported inferences of there being close links between the size of bits of the brain and performance of complex human behaviours.

As it happens, there is good evidence that male and female brains are structurally different at the macroscopic scale. A number of recent, largescale neuroimaging studies have found numerous small but correlated differences that collectively distinguish male and female brains in the samples studied. However, just observing such differences doesn’t prove that they are driven by innate biological factors. Indeed, a prominent argument – advanced by Rippon, among others – is that they are caused by our brains reacting to the differing experiences of males and females in a culture that is pervasively gendered.

The focus on neuroimaging is a bit of a red herring in the sex-difference wars

Our brains are, of course, highly plastic and designed to respond to experience. But most of that plasticity happens on a microscopic scale – changing the weights of connections between neurons. The idea that culturally loaded experience can drive macroscopic differences in the size of bits of the brain is something else entirely. That claim relies on a small number of studies, such as the one from 2000 showing that London taxi drivers have a larger posterior hippocampus, which seems to have acquired an almost mythic lore, despite the collective evidence base being quite limited.

The idea that brain areas might grow with use, or that levels of neural activity might change in regionally specific ways as a result of the quality of experience is both vague and speculative. Despite myths to the contrary, we are effectively using all of our brain all of the time, while awake at least. If brain tissue were really like muscle, our brains would be busting out of our skulls. And if growth of one area occurred at the expense of neighbouring regions (which would seem like a design flaw), then you’d expect a complementary pattern of brain differences – each bit that is relatively bigger in males would be adjacent to a bit that is relatively smaller – which is not observed.

Given that neuroanatomical sex differences are consistently observed in children – even reported in infants as young as one month old – and are ubiquitous across other animal species (with well-worked-out developmental mechanisms in many cases), it seems likely that observed neuroanatomical sex differences in humans are the result of conserved programmes of masculinisation or feminisation of brain development. But here’s the thing – we don’t know what these differences mean. Really, we have no clue. This is not unique to sex differences: we don’t know what any differences in the size of little bits of the brain mean. And this is despite countless efforts to link variation in size of this or that brain region or this or that nerve tract to a corresponding variation in psychological or behavioural traits, and no shortage of reports of such correlations in the literature.

The relationship between bits of the brain and cognitive functions or behaviours is simply not so modular. This is only a modern version of phrenology, where the size and shape of depressions and bumps on the skull was supposed to reveal the size of underlying brain areas and the consequent psychology of individuals. The complexity of the cellular circuitry and connectivity of any given region is too great for its function to be straightforwardly mapped to the amount of neural real estate it occupies.

What we do know is that most of the known sex differences in the brains of other animals are found in small but important populations of cells, themselves located in tiny brain regions with exotic names such as the ‘interstitial nucleus of the hypothalamus’ or the ‘bed nucleus of the stria terminalis’. These structures mainly control the subconscious organisation of behaviour and physiology, with important roles in mating, reproductive physiology, social behaviours, threat monitoring, aggression, fear, energy balance, and the like. By contrast, while the cerebral cortex is easy to assay with neuroimaging, it’s not necessarily the business end of the brain when it comes to the kind of behavioural differences we are interested in.

The focus on neuroimaging is thus a bit of a red herring in the sex-difference wars. The technology is simply not able to detect all the differences that might exist in neural circuitry between men and women, nor are scientists able to interpret those differences it can detect, let alone resolve the issue of whether any purportedly associated differences we observe in male and female behaviour are due to biological or cultural factors.

An equally contested area in investigating the basis of behavioural sex differences is whether differences in psychological traits, including personality traits such as conscientiousness, aggressiveness, impulsivity, risk-taking, nurturance and so on, might drive observable differences in behaviour. If such traits – thought to reflect some basic brain processes – differ consistently between males and females, then that would seem to favour a biological explanation for differences in behaviour. But, as with neuroanatomical differences, merely observing differences in such traits is not sufficient to settle the debate as to their origins or effects. What is observed is a spectrum – from traits where sex differences have a clear, conserved biological basis and strongly drive behaviours, to traits whose origins are murkier and the link to behaviour far more tenuous. The traits with strongest evidence of biological origins are, not surprisingly, the ones most closely linked to reproduction and mating strategies.

Sexual preference is the most obvious. So obvious that it is often overlooked, as if it just happens by default that some human beings are attracted to males and some to females. Those states don’t just happen. They are the outcome of a programme of masculinisation or feminisation of neural circuits that mediates sexual attraction, with principles and mechanisms well-worked-out in other mammals. Physical aggression is also closely tied to mating strategies, and shows strong sex differences. Human males are far more physically violent than females, across all cultures, committing the vast majority of serious assaults and homicides, and making up the vast majority of the victims. A similar sex difference is observed in many mammals, including most primates, in accord with the ecological pressures of competition for mates.

These differences in sexuality and aggression relate closely to reproductive strategies and behaviours; they are expected from an evolutionary perspective, have direct correlates in other species, and are associated with specific neural mechanisms that are beginning to be well-elucidated in model organisms. There is no good reason why a biological origin for these differences should be controversial.

But then such differences are also not really the things that much of the debate hangs on. Of much more relevance are possible differences in cognitive abilities, personality traits, aptitudes and interests.

Much has been written, over the centuries, about women’s supposedly inferior cognitive abilities. In fact, modern IQ tests show no difference in mean scores between men and women (though men show higher variance), and in many countries girls now regularly outperform boys in academic exams. There are, however, measurable differences in very specific cognitive abilities, such as a male advantage in mental rotation of three-dimensional objects, and a female advantage in verbal fluency. The difference in mental rotation shows up early, by age four or five, is moderate in size, and universally observed across cultures. Much is made of these differences. An OECD report in 2017 reviewed evidence that ‘students with higher scores on tests of spatial ability were substantially more likely to enter careers in science and mathematics’, yet the same report summarised data showing that spatial ability was fundamentally malleable, and could be improved by training and experience, suggesting an interplay of nature and nurture.

Even in the most individualistic societies, there are limits on the extent to which we independently create ourselves

There are other consistent sex differences in personality traits. In particular, females average slightly higher on the broad traits of neuroticism, agreeableness and conscientiousness. More specifically, males tend to score higher on traits such as assertiveness, sensation-seeking and dominance, while females average higher on gregariousness, sociability and nurturance. In psychometric analyses of interests, females consistently show a greater interest in people, on average, while males show a greater interest in things. Unlike sexual behaviours and aggression, most of these cognitive and personality traits are not so convincingly linked to reproductive success or to ecological roles. And, since they don’t have direct correlates in other species, we know much less about their biological underpinnings. They might have biological origins (since genetic differences influence these traits in a general sense), but there is also plenty of scope for cultural effects to have an important influence.

If the origins of these differences remain unclear, so too do their consequences. And yet arguing about the kinds of effects that these small average differences in psychological traits have on patterns of real-world behaviour and societal outcomes are the real flashpoints in this debate: are women suited to careers in STEM areas or not? Is the pay gap due to differences in traits such as agreeableness? Generally speaking, correlations between personality traits and a variety of consequential social outcomes – happiness, educational attainment, job performance, health, longevity – are weak, and the predictive power for individuals is very low. And that’s when we look at the full range of trait values across the whole population. But the sex differences discussed here are tiny relative to that range, meaning that any predictive value for outcomes will be correspondingly reduced.

When scientific findings are interpreted for media consumption or popular debate, the complexity and dynamism underlying the relationship between personality traits is typically underplayed. Our behaviour is not simply determined on a moment-to-moment basis by the tuning of these parameters. Innate predispositions provide a baseline – some initial tendencies to behave in one general way or another. And these initial tendencies influence how we interact with the world and subjectively experience it, as well as the kinds of environments we select and build. They can have a cumulative effect on how our individual habits and characters emerge, how we adapt to our environments, and the expectations we set of ourselves. But the idea that this just happens without any outside influence is naive.

Even in the most individualistic societies, there are limits on the extent to which we independently create ourselves. Societal outcomes are not simply an expression of the free choices of individuals, as some commentators seem to imply. With regard to sex differences, we have to consider the wider factors in play, including group dynamics, gender affiliation, the presence or absence of role models, societal norms and expectations, outright sexual discrimination and other systematic effects of culture.

For some behaviours, these forces can collectively act to amplify small group-average differences in psychology and habit-formation by setting expectations that become self-reinforcing. For example, aggressiveness (of a nonviolent nature) might be rewarded in males, while being discouraged in females. For other differences, such as choice of professions, culture might impose arbitrary norms and expectations that don’t reflect innate biological differences at all.

Given how little we know about how all these factors interact, it seems wildly premature and more than a little arrogant to assert that the small differences observed on lab-based measures of psychological traits are a sufficient explanation of observed differences in societal outcomes. We don’t have a ‘get out of evolution free’ card, but we are also not meat robots whose behaviour is determined by the positions of a few knobs and switches, independent of any societal forces. One thing is clear: we’ll never get to grips with the complexity of the interactive mechanisms in play if the debate remains polarised. We need a synthesis of findings and perspectives from genetics, neuroscience, psychology and sociology, not a war between them.

Kevin Mitchell

is a neurogeneticist. He is associate professor at the Smurfit Institute of Genetics and the Institute of Neuroscience at Trinity College Dublin. His latest book is Innate: How the Wiring of Our Brains Shapes Who We Are (2018). He lives in Portmarnock, Ireland.
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