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A young bank teller is shot dead during a robbery. The robber flees in a stolen van and is chased down the motorway by a convoy of police cars. Careening through traffic, the robber runs several cars off the road and clips several more. Eventually, the robber pulls off the motorway and attempts to escape into the hills on foot, the police in hot pursuit. After several tense minutes, the robber pulls a gun on the cops and is promptly killed in a hail of gunfire. It is later revealed the robber is a career criminal with a history of violent crime stretching all the way back to high school.

Now tell me: Are you picturing a male or a female robber? If you look back at the last paragraph, you’ll notice that I didn’t actually specify the robber’s sex. Nonetheless, I’d be willing to bet that you were picturing a man. Don’t worry—you weren’t being sexist; you were simply playing the odds. Most men are not especially violent, but most people who are especially violent are men. And rare though they might be, men such as our fictitious robber are the extreme of a more general trend, namely that men are more violent than women, more in-your-face aggressive, and more prone to taking risks.

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In most cultures, boys are taught not to be aggressive, but in all cultures, boys and men are more aggressive anyway.Photograph by Refat / Shutterstock

Why? Where do these all-too-familiar sex differences come from? A recent New York Times opinion piece weighed in on this difficult question, and came to a fairly common conclusion. The headline captured the gist: “It’s Dangerous to Be a Boy: They smoke more, fight more and are far more likely to die young than girls. But their tendency to violence isn’t innate.” (Emphasis added.) In other words, sex differences in aggression come entirely from the environment: from culture rather than biology, nurture rather than nature. Let’s call this the Nurture Only position.

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Now, there’s no real doubt that social forces help to shape violence and aggression. Decades of research have shown that people’s behavior—aggression included—is responsive to incentives and training. The question, then, is not whether social forces matter, but whether social forces are the whole story. And the answer, in a nutshell, is “almost certainly not.” Biology matters as well. Here’s how we know…

An initial line of evidence is that it’s not only in the West that we find sex differences in aggression. Wherever in the world we look, men are more violent and aggressive than women, especially with other men. The clearest and most persuasive evidence for this comes from homicide statistics: In every country, without fail, men commit the vast majority of homicides (and are more likely to be the victims of homicide as well). If the sex difference in aggression is just an arbitrary product of culture, why does it rear its ugly head in every human group?

That was a rhetorical question, of course, but I should mention that some sociocultural theorists think they have an answer to it. The eminent psychologists Alice Eagly and Wendy Wood, for instance, argue that, although men clearly do engage in higher rates of violence, this isn’t a result of evolved differences in men and women’s minds. Instead, it’s an indirect effect of evolved differences in men and women’s bodies: namely that men are larger, stronger, and faster than women, and that women get pregnant and produce milk for the young. Because of these non-negotiable physical differences, men in every culture are funneled into social roles involving aggression and physical strength, whereas women are funneled into roles involving childcare. Like the ex-soldier who keeps on polishing his boots every day for the rest of his life, the roles we play in the world have enduring effects on our behavior and personalities. Over time, men actually become more aggressive and women become more caring.

In short, Eagly and Wood provide a non-evolutionary-psychological explanation for the cross-cultural trend: The psychological sex differences are found in every culture because the physical sex differences are found in every culture. The physical differences are direct products of evolution; the psychological differences are not. If we re-engineered society’s gender roles, argue Eagly and Wood, the psychological differences would quickly fall away.

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It’s a clever argument, and one worth taking seriously. On balance, though, I don’t think it flies. To begin with, the Eagly–Wood theory raises some awkward questions. Why wouldn’t natural selection create psychological sex differences as well as physical ones? The mere existence of the physical differences tells us that human males have been subject to stronger selection for aggression and violence than females. Why would this selection pressure shape our muscles, our skeletons, and our overall body size, but draw the line at our brains? And why would natural selection give men the physical equipment needed for violence but not the psychological machinery to operate it? This would make about as much sense as giving us teeth and a digestive system, but not a desire to eat.

Why wouldn’t natural selection create psychological sex differences as well as physical ones?

On top of that, if sex differences in aggression were all down to gender roles, the differences would be larger in cultures with stricter gender roles and greater gender inequality. That’s not what we find, though. On the contrary, it seems to be the other way round. A recent large-scale, multinational study revealed, for instance, that sex differences in adolescent physical aggression are smaller, rather than larger, in less gender-equal nations. Culture clearly matters when it comes to sex differences in aggression—but the effect of culture is apparently very different than the social role theory would lead us to expect.

Not only does culture affect us in unexpected ways, many of the social forces invoked by the cultural theorists have an unfortunate habit of not existing. Consider the claim that society encourages males to be aggressive. This is probably true in some ways; we do sometimes give boys the message that they ought to be tough and not cry. Overall, though, we spend a lot more time discouraging male aggression than female aggression. Why? Because males are more aggressive! Or consider the claim that we tell girls to be quiet and passive. Again, we probably do this sometimes. More often, though, we tell boys to be quiet and passive. Why? Same reason: Boys are louder and more disruptive!

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In the first piece of research I did as a graduate student, I found that people judged an aggressive act performed by a man to be less acceptable than the same act performed by a woman. This perception apparently translates into real-world behavior. In childhood, boys are punished more often and more severely for aggression. Similarly, in adulthood, male defendants get harsher sentences for the same crimes, even controlling for criminal history. Males, it seems, are more aggressive despite culture, not because of it. And this isn’t just the case in the West. In most cultures, boys are taught not to be aggressive, but in all cultures, boys and men are more aggressive anyway.

(To preempt a common misunderstanding, this isn’t to deny that culture has any effect; we’ve already seen that it does. The point is simply that culture couldn’t be the entire explanation for the sex difference in aggression, because the sex difference in aggression appears even when culture pushes harder against male aggression than female.)

Some argue that, even if culture doesn’t create the aggression sex difference out of nothing, it does still amplify a relatively trivial inborn difference. Often, though, culture may do the reverse: By clamping down on male aggression, culture may make the sex difference in aggression smaller than it would otherwise have been.

Other evidence pushes us toward the same conclusion. The evolutionary psychologist John Archer points out that if the gender gap in aggression were due solely to socialization, it would presumably be smallest in the very young, then grow steadily as the years went by. After all, the longer we live, the more time the forces of socialization have to sink their claws into our minds and behavior, and thus the bigger any gender gaps should be.

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That’s the theory; the reality, however, is quite different.

First, the sex difference in aggression appears very early in life—usually before children take their first bite of their first birthday cake. From the moment they can move around under their own their steam, boys engage in more rough-and-tumble play than girls. The same sex difference is found in other juvenile primates, and appears to be related to testosterone exposure in the womb. In humans, the sex difference shows up long before kids understand that they’re boys or girls, so it can’t just be that they’re conforming to social expectations about how boys and girls ought to act. In any case, children are terrible at conforming to social expectations, as any parent who’s tried to persuade their progeny to sit nicely and quietly in a restaurant will readily confirm. And not only does the sex difference in aggression emerge early, it remains static until puberty. Absolute levels of aggression trend downward for both sexes; however, the gap between the sexes barely budges. If socialization creates the sex difference, why doesn’t continued socialization before puberty pry the sexes apart?

If we could cryogenically freeze all the males in this age bracket, we would instantly eliminate most of the crime and violence that plagues human societies.

Second, as with many sex differences, the sex difference in aggression suddenly swells at puberty, and is larger among adolescents and young adults than among any other group. Like bull elephants in musth, human males often go a little crazy at this stage in the life cycle. The evolutionary psychologists Margo Wilson and Martin Daly dubbed this the young male syndrome: Males in the grip of this syndrome are more likely than any other demographic to be imprisoned, to kill someone, or to be killed by someone else—most often another young male. The behavioral geneticist David Lykken summed up the situation well when he observed that, if we could cryogenically freeze all the males in this age bracket, we would instantly eliminate most of the crime and violence that plagues human societies.

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How would the Nurture Only approach explain the violence gap that opens up between the sexes at puberty? Is there a sudden surge in gender socialization—a surge which, for some unknown reason, happens at exactly the same stage of life in every culture and in many sexually dimorphic species? Is it just a coincidence that this alleged surge in socialization comes at the same time as the massive surge in circulating testosterone that accompanies puberty in males?

Third, after the violence and mayhem of early adulthood, male aggression steadily nosedives through the remainder of the lifespan. The socialization hypothesis offers no particular reason to expect this. But the decline in violence coincides almost perfectly with the decline in testosterone found in men throughout the adult years, and mirrors the decline found in males of other species. Once again, this is much easier to explain in evolutionary than in sociocultural terms.

A final line of evidence that sex differences in aggression have biological underpinnings is that these differences are not unique to human beings. Indeed, in some cases, the parallels across species are striking. Consider humans and chimpanzees. Among humans, males commit around 95 percent of homicides, and are around 79 percent of homicide victims. Among chimps, on the other hand, males commit around 92 percent of “chimpicides,” and are around 73 percent of chimpicide victims. In short, the sex difference in lethal aggression in the two species is remarkably similar in size.

And chimps are just the beginning. Sex differences in aggressive behavior are found in a great diversity of species, including most mammals. Why would differences that clearly have an evolutionary origin in other species have an entirely different genesis in our own? In the absence of a convincing answer to this question, the default assumption should be that they wouldn’t have a different genesis, and thus that the sex differences we see in our species have the same root cause as those in our nonhuman kin.

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None of this implies, by the way, that we’re necessarily stuck with male aggression, or stuck with aggression in general. As the psychologist Steven Pinker demonstrated in The Better Angels of Our Nature, levels of violence and warfare have fallen steadily over the decades, centuries, and millennia, despite the fact that aggression is part of human nature. In various ways, from policing and government to trade and moral norms, we’ve managed to pull ourselves, to a significant extent, out of the vortex of violence and bloodshed that characterized our species for the bulk of its tenure on Earth.

If we want to continue on this trajectory, however, or ideally to hasten our progress, our best bet is presumably not to delude ourselves about the true causes of our behavior. As policy wonks like to say: Wrong diagnosis; wrong cure. Let’s get the diagnosis right so that we can maximize our chances of curing the scourge of human violence.

Steve Stewart-Williams is an associate professor of psychology at Nottingham University Malaysia Campus and the author of The Ape That Understood the Universe: How the Mind and Culture Evolve (Cambridge University Press, 2018), excerpts from which formed the basis of this article. Follow him on Twitter @SteveStuWill.

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