Understanding patterns of lethal violence among humans requires understanding some important sex differences between males and females. Globally, men are 95 percent of homicide offenders and 79 percent of victims. Sex differences in lethal violence tend to be remarkably consistent, on every continent, across every type of society, from hunter-gatherers to large-scale nation states. In their 2013 study on lethal violence among hunter-gatherers, Douglas Fry and Patrik Söderberg’s data showed that males committed about 96 percent of homicides and were victims 84 percent of the time. In her study on violence in non-state societies, criminologist Amy Nivette shows that, across a number of small-scale pastoralist and agriculturalist societies, males make up 91-98 percent of killers. To illustrate the consistency of this relationship even further: we see the same pattern among chimpanzees, where males make up 92 percent of killers and 73 percent of victims.
To be sure, there is some cross-cultural variation. While I can find no well-studied population where women are known to commit more lethal violence than men, there are some societies where women make up an equal number, or even the majority, of homicide victims. These societies generally seem to have low rates of homicide overall, as the United Nations Office on Drugs and Crime mentions in their 2013 study on global homicide:
Available data suggest that in countries with very low (and decreasing) homicide rates (less than 1 per 100,000 population), female victims constitute an increasing share of total victims and, in some of those countries, the share of male and female victims appears to be reaching parity.
Hong Kong, with a low homicide rate overall, has a comparatively smaller sex difference in homicide offending, and women make up a majority of homicide victims at 52 percent. Yet even in Hong Kong, males commit 78 percent of reported homicides. The world over, the majority of homicide offenders and victims tend to be reproductive-age males, between their late teens and early 40s.
To understand why this pattern is so consistent across a wide variety of culturally and geographically diverse societies, we need to start by looking at sex differences in reproductive biology.
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Predictably, among humans, males engage in more direct, violent competition for mates than females do, and females provide more caregiving than males do. However, humans are unique in that some male participation in caregiving is ubiquitous across cultures. Human infants are particularly helpless during early development, requiring extensive provisioning and caregiving. Human males face the same tradeoff between securing mating opportunities and providing parental care that males of other species face, and the extent to which males utilize either of these strategies can vary significantly due to social and ecological factors.
Noting these sex differences in reproductive biology and parental investment is important because they help explain why males tend to engage in more violence than females. Aggressively engaging in violent conflict is more likely to reduce a female’s fitness, as it may bring unnecessary danger to her offspring, or cause an injury that may prevent her from reproducing in the future. For a male, however, violent conflict can potentially increase his reproductive success through increases in status, or by aggressively monopolizing access to key resources. Among the Yanomami of the Amazon, and the Nyangatom of East Africa, for example, males who participate in more violence and warfare have increased reproductive success. Even in the contemporary United States, there is evidence that more violent males have more sexual partners.
