The Gendered Ape, Essay 4: Is Rape in our Genes?

Editor’s Note: Frans de Waal’s new book, Different: Gender Through the Eyes of a Primatologist, has generated some controversy and misunderstanding. He will address these issues in a series of short essays which will be published at 3QD and can all be seen in one place here. More comments on these essays can also be seen at Frans de Waal’s Facebook page.

by Frans de Waal

Sexual dimorphism is the difference in size and appearance between the sexes. An adult male chimpanzee (left) next to an adult female. Whereas males are hairier and heavier than females, the average size dimorphism in chimpanzees and bonobos is only slightly larger than in our species. Photograph by Frans de Waal.

The violence of men against women is one of the most blatant and dangerous aspects of gender inequality, an issue often ignored by men but of obvious concern to women.

In many primates, males are bigger and stronger than females. The same applies to humans, in which the two genders show little overlap in upper body strength. In one German study, highly trained women athletes reached only the average physical strength of untrained men.

The evolution of sexual dimorphism in size and strength is thought to be driven mostly by male-male competition. The main purpose of greater male size is not dominance over females, but competition with rivals. In humans, this competition is reflected in the homicide statistics of most countries, including the US, in which male-on-male murders prevail.

Nevertheless, male violence against females is common. In our societies, spousal abuse, rape, and femicide are either on the rise or more frequently reported. It is a domain in which the human species stands out by its exceptionally high incidence. Since it often occurs between individuals who are close, one contributing factor is the habit of human families to live in relative isolation in huts and houses. These arrangements, which are unique among the primates, facilitate male control. During the Covid crisis and its lock-down policies, domestic abuse increased worldwide.

We don’t use the term “rape” in relation to other animals, and instead speak of “forced copulation.” This behavior is wholly absent in bonobos for the simple reason that the females collectively dominate the males. But also in chimpanzees, which are male-dominated, it is exceedingly rare. Even though I must have witnessed at least one thousand chimpanzee copulations, I have never seen this behavior.

Male chimpanzees do intimidate fertile females, sometimes quite violently. This is more common in East African chimpanzees, which are the ones we hear most about, while rare or absent in West Africa. As documented by Swiss primatologist Christophe Boesch, Western chimpanzee communities are more cohesive. Since they spend more time together, the gender power balance has shifted towards the females. When females travel and groom together, rather than being spread out over the forest, they form a block of shared interests. They call on each other for help. This puts a halt to brutal male tactics. According to Boesch, violent sexual harassment and coerced matings are absent in Taï Forest in Ivory Coast.

In sum, forced copulation is highly exceptional in our closest ape relatives whereas sexual violence does occur but is subject to species and cultural differences. If females are around to support each other, they develop a #MeToo movement like the bonobos. Bonobos have effectively curbed male sexual violence. But also in chimpanzees, we can recognize this potential for female solidarity.

In captive chimpanzee colonies, for example, where females are together all the time, social life is tightly regulated, and males can’t get away with obnoxious behavior. I have seen males bluff with all hair on end at females who are reluctant to mate, but there always comes a point when other females jump in to save the screaming victim. They go after the unrelenting male and teach him to behave.

In contrast, in a primate without any social networking, such as the largely solitary orang-outan, females are on their own. Orang-outans are exceptional in that forced copulation by adolescent and young adult males is not uncommon. The females prefer sex with fully grown males.

So, if you ask me if our ancestors were rapists, I’d say that chances are low given the behavior of our two closest ape relatives, and given that our forebears likely lived in tight-knit communities marked by female networking.

We don’t know this for sure, though, and alternative views do exist, most prominently the one of Randy Thornhill and Craig Palmer in their 2000 book “A Natural History of Rape,” which proposed rape as an evolved adaptive strategy, also in humans. They see rape as a fertilization strategy for certain males, who spread their genes this way.

Their argument relies on the rather small number of species marked by forced copulation (e.g. ducks, scorpion flies, orang-outans, humans), whereas if rape really were such a great strategy, it should be widespread in the animal kingdom. It isn’t. I criticized their book in The New York Times (see below), and some anthropologists have moreover noted that men who rape in small-scale human societies risk being expelled or killed by the woman’s relatives. Rape may have been a rather maladaptive strategy in ancestral communities.

Apart from focusing on how to protect women against male violence (such as through sisterly networking), society has the task to educate boys in a way that discourages such behavior. Call me old-fashioned, but I am no big believer in gender-neutral upbringing: sons are no daughters, and vice versa. Boys need an education that recognizes future gender differences in physique. Respect for women ought to be drilled into them.

Quoting from my book: “If descriptions of primate and human behavior teach us anything, it’s that sons will grow up to be more prone to violence. They will also acquire considerably greater body strength than daughters. Every society needs to come to grips with this dual potential for trouble and find ways to civilize its young men and steer their aggressive drive into a constructive direction. To make sure they become sources of strength rather than abuse, boys need to acquire emotional skills and attitudes geared specifically to their gender. They need to learn that strength comes with responsibility.”


My NYT review of “A Natural History of Rape,”

Anthropologist/primatologist Barbara Smuts has written interesting reviews on the issue of intersexual violence: Smuts, B. B. (1992). Male aggression against women: An evolutionary perspective. Human Nature 3: 1-44.

For further details and references to the literature, read “Different: Gender Through the Eyes of a Primatologist” (Norton, 2022). A video about the book can be seen here: