Why Men Are More Aggressive
The nature, nurture, and evolution of sex differences in aggression
The capacity for violence is one of the oldest and most troubling features of human nature. Why, though, are some people so much more prone to it than others? And why, across cultures and throughout history, have men committed the overwhelming majority of violent acts?
In this extended excerpt from my book, A Billion Years of Sex Differences, I look at what decades of research reveal about one of the largest and most consequential sex differences known to science. Among other things, we’ll see that:
Men are not only the main perpetrators of violence but also the main victims
Five lines of evidence point to an innate contribution to sex differences in aggression
Contrary to popular opinion, the most violent members of society aren’t young men
Greater male aggression often appears despite culture, not because of it
Greater male aggression has the same evolutionary underpinnings as the stag’s antlers and male gorilla’s canines
If you enjoy this excerpt, you can explore the full collection of excerpts here, or pick up a copy of A Billion Years of Sex Differences here.
The Strong, Violent Type
Let’s start our tour of the dark side with the antisocial trait that looms above all others: aggression. As mentioned, men are more aggressive than women, at least when it comes to face-to-face or direct aggression. Men shout more, fight more, and kill more. In experiments, they’re more likely to give people electric shocks, and in the real world, they’re less likely to back down from a trivial altercation. Even men tamed by modern civilization – men who’d never throw a punch in real life – watch more violent movies, play more violent video games, and have more fantasies about killing people.
To be fair, when it comes to indirect aggression – spreading rumours about people, excluding them from the group – we find either no sex difference or that women engage in slightly more. For everything else, though, men are in pole position.
As with most sex differences, the sex difference in aggression gets progressively larger for rarer, more extreme acts: a good example of the tail-of-the-distribution effect. Men are somewhat more likely to engage in verbal aggression; much more likely to engage in physical aggression; much, much more likely to engage in violent crime; and much, much, much more likely to kill someone – or, for that matter, to kill themselves.
It’s not that men are vastly more anger-prone than women; the sexes don’t particularly differ in how angry they get or how often. Instead, men tend to be less self-controlled, and less concerned about the negative consequences of overt aggression, from ruptured friendships to injuries and imprisonment.
Few would deny that men are the main perpetrators of violence. Who are its main victims? Many assume that it’s women – and when it comes to sexual violence, they’re right. For every other kind of violence, however, men are more often found in the victim column. This pattern is particularly stark when we look at homicide data. In almost every society on Earth, men are more likely than women to be murdered. Most of the time, the killer is a man as well. Women kill much less often, but when they do, the victim is usually a husband or a boyfriend. Apparently, both sexes prefer to kill men.
Now for the usual disclaimer: The fact that, on average, men are more aggressive than women doesn’t mean that men are the aggressive sex whereas women are the gentle sex. That would be like saying that, because more men than women have a same-sex sexual orientation, men are the gay sex whereas women are the straight sex. Some members of both sexes are highly aggressive, and most members of both are not, at least in the modern world. You know what I’m going to say next, though: Among the minority who are highly aggressive, more are XY than XX.
An Unfortunate Inheritance
Where does the aggression sex difference come from? For some the answer is obvious: culture. A standard cultural explanation would be that, in various ways, society encourages male aggression but discourages aggression in females. Parents, teachers, and peers reward boys for aggressive behaviour, at least in its milder forms. And even when they don’t reward it, they often turn a blind eye to it in boys in a way they never would with girls: Boys will be boys, after all. Meanwhile, movies and other media constantly model male aggression for impressionable young male minds. The subtext is clear: ‘This is the way you should be.’
As always, no one should deny that social factors help shape the sex difference in aggression. But is the difference entirely due to social factors?
Some would say yes. Writing in The New York Times, for instance, psychologist Michael Reichert argued that, although males are more violent than females, this tendency is not innate. However, at least five lines of evidence suggest that Reichert was wrong, and that there’s also an innate contribution.
1. Stubbornness of the Sex Difference in Aggression
One line of evidence for an innate contribution is the fact that the aggression sex difference persists even in the face of countervailing social pressure. Contrary to the idea that we encourage male aggression, parents and teachers spend a lot more time telling boys not to be aggressive than they do girls. This isn’t because they approve more of girls’ aggression; it’s because boys are more often aggressive. But notice the implication: Rather than being created by socialization, the sex difference survives even in the face of socialization in the other direction.
To pre-empt a common misunderstanding, this isn’t to say that socialization has no effect on the sex difference. Clearly, it does; the size of the difference varies considerably from culture to culture and time to time. The point is that socialization couldn’t be the entire explanation, because the difference persists even when socialization pushes harder against male than female aggression. Indeed, socialization may often make the difference smaller than it would otherwise be, not larger.
2. The Developmental Trajectory of Male Aggression
A second line of evidence concerns the unfolding of the aggression sex difference over the lifespan. As psychologist John Archer observed, if socialization alone were pulling the strings, we’d expect to find no difference in aggression in infants, followed by a steady increase in the size of the difference across childhood, as socialization makes its mark on our minds. But that’s not what we find at all.
First, the sex difference in aggression is apparent from the moment kids are mobile enough to make aggression an option: Boys fight more and lash out more, and engage in more rough-and-tumble play. Importantly, these differences appear long before boys and girls start selectively imitating same-sex role models, and long before they even understand that they’re boys or girls.
Second, rather than starting at zero then escalating in boys as the effects of socialization compound, aggression starts out high for both sexes. The most violent category of human being is the toddler, and the only reason we don’t worry more about the epidemic of toddler violence is that toddlers are small and cute, and can’t do much damage with their tiny fists. From this early summit of violence, both sexes become progressively less violent as the years go by. This is a major stumbling block for the theory that children are socialized to be aggressive. To my eye, it suggests instead that they’re socialized not to be. Throughout the process, however, boys are always more aggressive. Is this because anti-aggression socialization is stronger for girls than for boys? No; as we’ve already seen, it’s usually the opposite. But even if it were stronger for girls, the data don’t readily fit with a socialization explanation. Rather than the sex difference getting bigger as children absorb the message that female aggression is less acceptable, the gap remains roughly static throughout childhood.
So far so bad for the socialization hypothesis – and it’s only downhill from here. The next point where the facts come unstuck from the theory is in the decade or so following the onset of puberty. During this biologically critical window, the aggression sex difference suddenly explodes as men ramp up their aggressive antics. The evolutionary psychologists Margo Wilson and Martin Daly dubbed this rapid-onset increase in male misbehaviour the young-male syndrome. The most reliable evidence for the syndrome comes from homicide records. In childhood, neither sex is particularly homicidal. In the years after puberty, however, homicide rates suddenly skyrocket for males. Is this because society starts socializing males to be more aggressive? On the contrary, young male aggression is a serious problem facing every society, and thus most try to restrain rather than inflame it. Nevertheless, it persists.
Finally, as young adulthood gives way to middle age, the sex difference in aggression shrinks, largely because men begin mellowing. The socialization explanation gives us no reason to expect this. As we’ll see soon, though, an evolutionary explanation does.