The Primate Diaries

The Primate Diaries

Notes on science, politics, and history from a primate in the human zoo.

The Science of Sexism: Primate Behavior and the Culture of Sexual Coercion


Warning: content may be triggering for survivors of sexual assault/abuse.

Chimp Riveter by Nathaniel Gold

"Chimp Riveter" by Nathaniel Gold

Elizabeth Wilde’s mouth was stuffed with cloth and her hands were tied behind her back. Hogs rummaged in the yard outside as her master, John Lumbrozo, forced himself on her repeatedly and threatened her with death if she resisted. When the 22-year old indentured servant girl later showed signs of being pregnant, witnesses reported that this respected doctor of Charles County, Maryland gave her a chemical “Phisick” that induced abortion. On a hot day in June, 1663 Elizabeth Wilde gave birth to "a Clod of blood," while her rapist stood over her and performed the delivery of her dead fetus.

“With the fetus and afterbirth in the chamber pot," wrote historian Amanda Lea Miracle in her dissertation on the incident, "the doctor threw the contents into the street. And, as neighbors pointed out to her, a further indignity was that any roaming pig could devour it.”

Lumbrozo had previously used sexual coercion against two other women, both of whom were his indentured servants. One, known only as Mrs. Hammond, he admitted to having sex with during his failed bid to challenge a lawsuit from her husband, Mr. John Hammond. The suit was filed, not for sexual assault, but because gossip over the incident was “severely hurting Hammond’s reputation so that his livelihood was in jeopardy.” In the other documented case, Lumbrozo attempted to force his servant, Margery Gold, to have sex with him because her husband owed him money. According to court testimony, Lumbrozo “took her and threw her upon the bed and would have forced her [but] she cried out and thereupon the doctor let her go.” The case was dismissed because there was no penile penetration.

Our society has made significant advances for women's rights and sexual equality during the last century, but this case study from nearly 350 years ago would not be out of place in the headlines of today. Just this year Dominique Strauss-Kahn, Managing Director of the International Monetary Fund (an institution that claims to provide assistance for poor countries), was arrested for the alleged sexual assault on his maid, a refugee and rape survivor from African Guinea. According to statistics compiled by the US Equal Employment Opportunity Commission there were 12,772 workplace sexual harassment cases in 2010 (including all forms of sexual coercion, and representing a fraction of the number that actually occurred) and 84% of these cases were brought by women. Employers have gotten increasingly serious about cracking down on such abuses but during the last decade they were still held liable to the tune of $540 million. What is going on here? Could this kind of gender inequality be an intrinsic feature of human nature that we're stuck with or is it simply a failure to create an environment that prevents such behaviors from reoccurring?

Primatologists and evolutionary biologists have taken this question seriously and have developed some surprising conclusions that could inform our approach to this issue. Unlike Randy Thornhill and Craig Palmer's book A Natural History of Rape, a thesis that was criticized by scholars both in biology and gender studies, other evolutionary researchers have developed a much more balanced analysis. One example is from the recent edited volume Sexual Coercion in Primates and Humans by Martin Muller and Richard Wrangham. As they wrote in their introduction:

[M]ales in a number of primate species appear to use force, or the threat of force, to coerce unwilling females to mate with them....Although the utility of this distinction has been disputed, there is no doubt that sexual coercion is a potentially important mechanism of mating bias within the broad framework of sexual conflict theory.

There are three forms of sexual coercion that researchers have documented in both human and nonhuman primates: harassment, intimidation, and forced copulation. Harassment is the most common and results when males make repeated attempts to mate that imposes costs on females, intimidation is the use of physical violence inflicted on females who refuse to mate with a given male, and forced copulation (or "rape" in the human literature) is the least common form that involves violent restraint for immediate mating. The researchers found convincing evidence that the first two forms of sexual coercion (but not the third) increased the long-term reproductive success among males in Japanese macaques, baboons, and our closest evolutionary relatives the chimpanzees. This suggests that, at least for these three species, sexual coercion has been selected as an adaptive strategy in male sexual behavior.

But what about humans? This is a difficult question to answer since, for reasons of privacy, researchers can't very well study human sexuality by stationing a field researcher in our bedrooms. However, reported statistics on extreme forms of intimidation can perhaps give an indication of how common sexual coercion is in modern societies. Therefore, in the same edited volume, Evolutionary Psychologist Martin Daly republished the findings that he and his late wife, Margo Wilson, reported in their 1996 paper in the journal Current Directions in Psychological Science (pdf here). To address this question they analyzed statistics for uxoricide (killings of wives) in England, Canada, and the United States between the years 1965 and 1990. Just like those researchers who studied nonhuman primates, Wilson and Daly operated under the assumption that the use of sexual coercion would be highest against those women with the highest "reproductive value." In other words, men would be most likely to use threats or even violence against younger women who had the majority of their childbearing years ahead of them.

Uxoricide Statistics from Wilson and Daly (1996)

Figure 1. Uxoricide statistics for England, Canada, and the US (Chicago) show that the extremes of sexual coercion are highest among younger women. Reproduced from Wilson and Daly (1996).

Nonlethal sexual intimidation data from Wilson and Daly (1998)

Figure 2. Nonlethal sexual intimidation statistics in married and unmarried women. Reproduced from Wilson and Daly (1998).

As the graphs indicate, their results strongly supported this hypothesis. The highest number of uxoricides occurred in women from puberty to 24-years-old followed by those who were between 25 and 34 (see Figure 1). The lowest rate of uxoricide occurred in those women who were either approaching menopause or were already post-menopausal (50-years-old and older). The researchers also found identical trends for cases of sexual assault committed against both married and unmarried women, indicating that the murders likely had the same motivations as other cases of sexual violence (see Figure 2).

Of course, one objection to these trends could be that younger men are simply more violence prone and would therefore be more likely to assault their partners. However, what Wilson and Daly discovered was that older men who were with younger women actually had a higher rate of intimidation than did younger men. These conclusions fit right in with those of our nonhuman male counterparts showing sexual coercion as a reoccurring feature of human behavior. These findings still need to be compared with results from non-Western societies to eliminate any potential bias produced as the result of culture or environment. But the uncomfortable implication is just what feminist scholars have been arguing all along: the patriarchy is real and it will require committed focus to reduce or eradicate sexual coercion in modern societies.

However, an important thing to remember whenever reading about the implications of human evolution is that biology is not destiny. Fortunately, just as research with nonhuman primates has allowed us to better identify the problem in our own species, it can also provide us with some of the solutions. An important aspect of primate sexual coercion that shouldn't be ignored is where it doesn't exist and why. Bonobos are as closely related to us as chimpanzees are since we shared a common ancestor with both between four and six million years ago and the two species later diverged from each other only about a million years ago. There has never been an observed case of male sexual coercion in this species despite the fact that males are still somewhat larger than females.

A unique aspect of bonobo society is that they are a female-dominated species thanks to the network of support that exists between bonobo females. Chimpanzee females are largely isolated from one another, but bonobo females come to one another's aid. While there may be genetic differences that account for the lack of sexual coercion in bonobos, one important factor is the different environment that promotes these cooperative networks and limits the usefulness of male coercion (see my interview with Frans de Waal for more on this topic). Male bonobos mate more frequently by gaining support from these female networks rather than using sexual coercion as can be found in chimpanzees. Males grow up with this "culture" and observe the older males in their troop emphasize grooming over aggression and then adapt their own behavior in order to maximize their reproductive success.

But bonobos aren't the only ones. To illustrate how powerful the influence of culture can be for primate societies consider the most extreme example of a sexually coercive species: savanna baboons. Male baboons have been known to viciously maul a female that has rejected their advances and the level of male aggression is strongly correlated with their mating success. However, in a unique natural experiment Stanford primatologist Robert Sapolsky observed what developed when the largest and most aggressive males died out in a group known as Forest Troop (because they were feeding at the contaminated dump site of a Western safari lodge). In the intervening years Forest Group developed a culture in which kindness was rewarded more than aggression and adolescent males who migrated into the troop adopted this culture themselves. As Sapolsky related in his essay "A Natural History of Peace" for the journal Foreign Affairs (pdf here):

The results were that Forest Troop was left with males who were less aggressive and more social than average and the troop now had double its previous female-to-male ratio. The social consequences of these changes were dramatic. There remained a hierarchy among the Forest Troop males, but it was far looser than before: compared with other, more typical savanna baboon groups, high-ranking males rarely harassed subordinates and occasionally even relinquished contested resources to them. Aggression was less frequent, particularly against third parties. And rates of affiliative behaviors, such as males and females grooming each other or sitting together, soared. There were even instances, now and then, of adult males grooming each other - a behavior nearly as unprecedented as baboons sprouting wings. . . By the early 1990s, none of the original low aggression/high affiliation males of Forest Troop's tuberculosis period was still alive; all of the group's adult males had joined after the epidemic. Despite this, the troop's unique social milieu persisted - as it does to this day, some 20 years after the selective bottleneck.

In other words, adolescent males that enter Forest Troop after having grown up elsewhere wind up adopting the unique behavioral style of the resident males. As defined by both anthropologists and animal behaviorists, "culture" consists of local behavioral variations, occurring for nongenetic and nonecological reasons, that last beyond the time of their originators. Forest Troop's low aggression/high affiliation society constitutes nothing less than a multigenerational benign culture.

These nonhuman primate examples suggest that an increased focus on women's rights might not only reduce the current levels of sexual coercion but could benefit society as a whole. If women have increased social power (both politically and economically) they would be better able to resist male sexual coercion due to stronger networks of social support. At the same time this increased social power would be expected to help create a change in male culture that would influence how young men interact with women when trying to gain sexual access.

While specific policies that protect women from coercion and exploitation remain important, what we're ultimately after is social change. This is something that applies to men and women alike since many women actively participate in upholding sexual double standards. But while we work on promoting gender parity both politically and economically we should also follow the example of our baboon cousins and model the way that men interact with women. This means that more men should take issues of women's rights seriously so that younger men who look up to them will follow in turn. By doing so perhaps we can create an environment where sexual coercion remains a thing of the past.

This was the ultimate tragedy for Elizabeth Wilde and her own case of sexual coercion. The culture of colonial Maryland held that if a woman became pregnant she must have consented to sex, even if there was overwhelming evidence that she had been raped. As a result, it was the victim who ended up going on trial to face felony charges for "illicit sex" and the murder of her child. In the end her only recourse was to marry the man who raped her, a resolution that was morally acceptable to this deeply Christian community, and which cleared her of all charges.

However, despite the limited options available, Elizabeth Wilde was not completely helpless and turns out to have had a reproductive strategy of her own. Once she realized that she'd been impregnated by her employer, Wilde confided in her fellow servant Joseph Dorrosell that she'd ingested "Ratsbeane" (or ratsbane, arsenic trioxide, a common poison at the time) in order to end the pregnancy. She was in terrible pain and feared death. It was only after Lumbrozo returned home and discovered her writhing in agony that he administered his own abortifacient. But, as Dorrosell testified, "it had done her no wrong" because it was the self-ingesting of rat poison that "did kill the child." With little hope of finding justice in a patriarchal society, Wilde made the choice to deny her victimizer his reproductive success.

We are fortunate today, at least in the Western world, that women have expanded the choices available to them after many years of successive, slight modifications to the status quo. Evolution may provide the background for some of our species' recurrent behavioral strategies, but we have the ability to transform our physical and cultural environments in order to influence which strategies are ultimately the most successful. Many of our primate relatives and our own species' history offers one approach. Perhaps we can choose another.


Muller, M.N., Wrangham, R.W. (2009). Sexual Coercion in Primates and Humans: An Evolutionary Perspective on Male Aggression Against Females. Harvard University Press.

Miracle, A.L. (2008). Rape and Infanticide in Maryland, 1634-1689: Gender and Class in the Courtroom Contestation of Patriarchy on the Edge of the English Atlantic. Ph.D. Dissertation, Bowling Green State University.

The views expressed are those of the author and are not necessarily those of Scientific American.

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