Sex and Violence: The Evolution of Male Aggression

Very little in life makes sense except in the light of sex.

It is the source of whole disciplines within my field, evolutionary biology. Within the animal kingdom, sex roles are among the most ubiquitous observed behavioural phenomena. With few exceptions, most species conform to the usual sex roles of choosy, docile females and aggressive, promiscuous males.

Humans are not amongst the exceptional species. Across time and cultures, certain elements of male and female sex roles consistently manifest themselves, due to their deep biological origins.

The sexual revolution was based, in part, on the idea that mate choice preferences are equal between the sexes. But, as it turns out, that’s not true—even when women are able to technologically suppress their fertility to avoid the burdens of childbearing. To be sure, some commonly observed sex differences are socially constructed rather than biologically based. Still, there’s a limit to how much can we technologically de-construct and re-construct sex roles to fit our desires.

As discontent with the fruit of the sexual revolution grows, many thinkers have turned to evolutionary biology as a way to understand the relationship between men and women. This approach can be enlightening, but it also has its limits. In order to develop normative conclusions about how the sexes should relate, you need some kind of ethical framework. Often, what masquerades as objective science smuggles in deeply flawed philosophical commitments, such as a Nietzschean will-to-power, or repeats morally abhorrent and scientifically disproven bunkum like eugenics or “race realism.”

Nonetheless, if we keep in mind the limits of the discipline, evolutionary biology can still teach us a great deal about ourselves, our behaviour, and how that shapes the societies in which we live. Nowhere is the potential contribution of evolutionary biology clearer than the differences between the sexes.

Anisogamy and Aggression

Consider, for example, one of the most enduring male traits, which persists in spite of widespread attempts to squash it: aggression.

The origin of this sexual difference is anisogamy. Anisogamy is the difference in the size of gametes between the sexes, with males being defined as those individuals whose bodies produce many small gametes, and females being defined as those who produce a smaller number of large gametes.

From the start, the female sex has to bear a higher cost than the male sex in sexual reproduction, purely in terms of the energy required to produce gametes. Throughout the process of reproduction, there is a cascade of further costs borne by the female sex. This divergence becomes more intense with each further step of evolutionary complexity.

In less complex organisms, the greater cost for the female is slight, leading to only minor behavioural differences. Once organisms evolve to incubate offspring within the maternal body, this adds to female cost, leading to further behavioural differences. Once parental care evolves, it predominantly falls upon the female. For mammals, in particular, lactation adds another layer of female-specific care after birth. At each step, the number of offspring a female can have is reduced, while the number a male can have stays roughly the same. This leads to a sex ratio imbalance: there are always slightly more sexually receptive males than females.

From an evolutionary perspective, is in the best interest of females to be choosy in who they mate with, to ensure that the children into which they invest so much energy receive the best genetic inheritance. Wise mate choice can also insure that they have access to the resources they need during the vulnerable time before and after gestation. Males have no such evolutionary incentive, and try to mate with as many individuals as possible.

These reproductive matters may seem unrelated to masculine aggression and risk taking. Yet, in in my field, they are viewed as the ultimate source of all biological differences between male and female behaviour.

Males Can’t Afford to Be Average

Females are an easy bet, evolutionarily speaking. A female who reaches reproductive maturity will probably find a willing male to mate with. Males, on the other hand, are a risky proposition. Most males will struggle to find a mate, and will be competing strongly with each other for females. The limiting factor in sex is, therefore, almost always the female of the species.  

A good male can have much higher fitness (i.e., many more offspring) than the highest quality female, but most males will end up being evolutionary dead ends. Even in humans, one of the most egalitarian species whose sex roles aren’t completely reversed, it is estimated that over 60 percent of males who have ever reached reproductive maturity have never reproduced. The other 40 percent have a valuable resource, which they have to protect from other, sexually frustrated males: access to females. Thus, evolution has selected for males to be, on average, stronger and taller than females, both to be able to hunt (and thus to provide for females, who are taking care of babies), and to be able to fight off other males.

This difference in reproductive output between males and females has evolutionary consequences in terms of behaviour other than aggression as well. Males cannot afford to be average. The average male has no kids. Thus, male behaviour traits skew towards the more extreme: playing conservatively is a guaranteed loss in males, so evolution gambles in men, in hopes of striking it lucky. These extremes can run in multiple directions. Compared to females, males are both more likely to be geniuses and more likely to be intellectually disabled. Men are more likely to be billionaires and more likely to be homeless. But one area where the extreme tends to only run in one direction is in aggression.

Almost all men are, on average, more aggressive than the average woman. Yet men also tend to be more cooperative—better at forming shallow social bonds to work together with other individuals. While these might seem contradictory, they are actually two sides of the same coin. Males act aggressively to protect their mates, and they act cooperatively to be better able to defend those mates. This leads to some aspects of human history to become almost the exclusive domain of men, most notably that supreme combination of violence and cooperation: war.

More prosaically, it manifest itself in differences in how men and women interact with their friends. Men tend to focus more on activities with a structure, while women tend to focus on conversation and emotional intimacy. This difference in how men and women interact with their same-sex peers also influences how they interact with the wider world. It’s more difficult to be cognizant of the full personhood of a larger number of people with whom you have shallow interactions, compared to a smaller number of people with whom you have emotionally intense relationships.

It’s unsurprising, then that systems thinking, with regular, logical rules is more common in men than in women. At its most extreme, this is reflected by higher rates of autism in men than in women. The dominance of systems thinking among men is likely also a significant driver for sex differences in the scientific professions.

Female Traits

For much of the history of evolutionary biology, the study of females was neglected in favour of males. Since the 1970s, however, there has been an explosion of the study of females. This includes the role of cryptic female choice in sexual selection, the evolution of cheating by females, and even greater focus on the evolution of the sexual role of female reproductive anatomy. In species with parental care, females tend to be the ones who engage in it. Even in species where biparental care is the norm, such as humans, females tend to engage in more care for offspring than males.

This leads to women being evolved towards less physical aggression and overt competition, as their main evolutionary task is to raise a child rather than to get a partner. Females are incentivized to focus on fewer, but more intense, social bonds, since the most costly element of their fitness is child rearing, which is time consuming in a way which the male’s role isn’t. This creates an opportunity cost, as females have to spend time rearing offspring rather than socialising with the wider society. Childrearing also tends to leave women more vulnerable, and necessitates a degree of reliance on other individuals. Stronger social bonds, which take more time to develop and thus are more limited in number, are more likely to lead to help than the plentiful shallow bonds favored by men.

Many of the most important areas of modern life, such as business and politics, reward those who are both aggressive and able to form shallow interpersonal bonds to cooperate on large projects. Over a century after the first female MP, the House of Commons is still dominated by men, and the further up the greasy pole you go, the more dominated by men it is. Indeed, it would be even more male-dominated, if it weren’t for the positive discrimination placed in favour of women by both major parties in candidate selection over the last twenty years.

Female politicians have often complained that politics is a man’s world. Indeed, the female politicians who have gone furthest, such as Thatcher and May, have had to learn to play by men’s rules, rather than reinvent a kindler, more female, style of politics. Boardrooms, which are open to less public scrutiny, have also seen an increase of female presence over the last fifty years, yet they too remain male dominated. Of course, the human male is overrepresented in other, less desirable arenas, too. Criminals and soldiers are more likely to be men, and there are no calls to employ the Equality Act on gangs to ensure that they include sufficient women. Yet current legislation in Britain presupposes that these differences are due solely to prejudice and discrimination, ignoring the evidence that they are just as biological as the differences in gametes between males and females.

All of these differences may make the reader think that the old canard, that men are from Mars and women are from Venus, has some truth to it. This couldn’t be further from the truth; for all of the differences between men and women, for most things men and women overlap substantially. Where there are differences, they are usually of degree, and of population level averages. But there are differences, on average, in the behaviour of the sexes. These differences between the sexes in behaviour can often have biological origins, and those behavioural differences can have population-level consequences. Current legislation, and indeed most of polite society’s opinions about sex roles, ignores this fundamental fact.

There are limits to absolute equality between the sexes precisely because there are biological roots for the behavioural differences between them. To deny this is to deny reality.

Yet biology is not destiny, and science is not ethics. Information about the reality of sex differences does not tell us what to do about them. But we can only have a serious discussion of what to do about these differences if we know what they are, how ingrained they are, and what causes them.


Each week, you will receive an email informing you of new essays, podcasts, events and books.