Evolution and Inequality: Why Some Men Rape

by James S Chisholm and Victoria K Burbank

The first casualty in war is always truth, so it's no surprise that evolutionary theory is under fire again in the culture wars reignited by Randy Thornhill and Craig Palmer's A Natural History of Rape: Biological Bases of Sexual Coercion (MIT Press, 2000). This is a troubling and troubled book, for in its use of evolutionary theory to understand rape, it gives our only scientific theory of life a bad name. Writing against Rape in The Weekend Australian (4-5/3/00, p. 3), for example, Emma Tom attacks evolutionary theory's "frequent appearance as a defence witness for racism and sexism." With friends like Thornhill and Palmer evolutionary theory doesn't need enemies! While understandable in the context of Rape, however, attacks like Tom's are foolish, for they throw out the evolutionary baby with the Thornhill-Palmer bathwater. This is a precious baby that in some important ways is just beginning to grow. Modern evolutionary theory aspires to a rational version of truth that many believe has important implications for human flourishing. As Peter Singer argues in his new book, A Darwinian Left: Politics, Evolution, and Cooperation (Weidenfeld and Nicolson 1999), "evolutionary theory can help us to identify the means by which we may achieve some of our social and political goals, including various ideas of equality..."

In an attempt to make the world safe for evolutionary theory, therefore, we outline below the reasons why we believe that A Natural History of Rape is flawed. But these happen to be the very reasons why evolutionary theory is far from the racist, sexist brute that Tom and too many others still fear. We should mention first, however, that, like Tom, we have not yet read A Natural History of Rape. Our comments are based on long familiarity with Thornhill's work and, like Tom, an adaptation of A Natural History of Rape that appeared in The Sciences (Jan/Feb, 2000). All quotes attributed to Thornhill and Palmer are from this article.

A Natural History of Rape goes back to Thornhill's research in the '80s on mating in scorpionflies. In many scorpionfly species the males exhibit two reproductive strategies. They either offer the female a "nuptial gift" (eg, a dead insect) and then mate with her or "they chase a female and take her by force." The first strategy is preferred, but when males lack the resources to bestow nuptial gifts, "rape" is available as an alternative strategy. "Giftless males" tend to be "the wimpier ones that failed in male-male competition." Thornhill and Palmer's basic argument is that, like scorpionfly rape, human rape is "the result of aeons of past Darwinian selection" and that "there is no doubt that rape has evolutionary - and hence genetic - origins." They purport to demonstrate that from an evolutionary perspective "rape is, in its very essence, a sexual act." Pitting nature starkly against culture, they reject the view that power differentials between and among men and women are important for understanding rape, arguing instead that it is a natural (if deplorable) biological phenomenon that enabled ancestral males to succeed reproductively even when they lacked the genes or material or social resources that might have made them attractive to women. Rape exists today, on their view, because it is an evolved alternative reproductive strategy: "Over evolutionary time, some men may have succeeded in passing on their genes through rape, thus perpetuating the behavior." Rape, in short, is all about sex because sex is all about passing on genes. They then shift to policy: "We believe that only by acknowledging the evolutionary roots of rape can prevention tactics be devised that really work." For example, since rape is all about sex one way to reduce it is to discourage women from being sexy - say, by not wearing revealing clothes: "Surely the point that no woman's behavior gives a man the right to rape her can be made without encouraging women to overlook the role they themselves may be playing in compromising their safety."

This is a condensed version of the condensed version of A Natural History of Rape that appeared in The Sciences. It is also one-sided in that we focus exclusively on its problems. But these are the problems that critics of evolutionary theory seize upon and that retard the development of our only scientific theory of life. What are these problems and why are they problems?

The biggest problem with Rape is that Thornhill and Palmer ignore the conceptual advances that distinguish modern evolutionary thinking about human behaviour from the sociobiology of the '70s and '80s (so-called "vulgar" sociobiology). Early attempts to understand human behaviour in terms of the process of adaptation by natural selection focused almost exclusively on how selection worked and the traits (adaptations) that selection was expected to favour. This is biology's adaptationist perspective. Thornhill and Palmer are very much in this earlier sociobiological tradition. For them, rape is all about sex because they can imagine conditions under which rape would have been adaptive. Therefore, rape must be the product of natural selection.

This is not necessarily a bad hypothesis. The problem is that it doesn't go far enough. What's missing is any account of the psychological mechanisms that might motivate men to rape in the adaptive way they suppose. For an adaptation to exist there must be a flesh and blood mechanism for doing adaptive work. This is biology's mechanist perspective, and the reason why modern evolutionary perspectives on human behaviour (widely known as evolutionary psychology) are conceptually more sophisticated than old-fashioned sociobiology. In principle, Thornhill and Palmer's adaptationist hypothesis could be right: the capacity for rape could be part of an evolved alternative reproductive strategy. But rape doesn't just happen and not all men rape. Until they describe the psychological mechanisms that actually carry out the adaptive work of rape entailed by their hypothesis they have achieved very little. At most they have an interesting correlation between their adaptationist predictions and certain rape statistics. But correlation, as everyone knows, is not causation, and without a mechanism we don't know what causes the correlation. Nor have we reason to think that discouraging women from wearing revealing clothes is a rape prevention tactic that would "really work."

Evolutionary psychology is concerned with describing the mechanisms that cause people to behave in adaptive ways (to the extent that they do). The most important category of mechanisms for producing adaptations is development - the myriad processes that take us from the genetic information that we inherit from our parents to the flesh and blood, thinking and feeling, experience-filled creatures we are today. Evolutionary psychologists have begun to look at the development of individual differences in men's propensity to rape and a picture very different to Thornhill and Palmer's is emerging.

When we look at the mechanisms that might predispose men to rape in more or less the adaptive way that Thornhill and Palmer describe, rape no longer seems to be "in its very essence, a sexual act." Instead, men's propensity to rape seems to be in large part developmentally contingent on boys' and young men's subjective experience of risky and uncertain environments. These are environments in which the flow of material, but especially social and emotional resources is inadequate or unpredictable. There is abundant evidence that the early subjective experience of too much risk and uncertainty (ie, chronic early stress or fear) predisposes young men (and women) to impulsiveness, a taste for risky activities, or a tendency to act too quickly without fully considering the consequences. For example, although it did not look specifically at rape, a study by James Dabbs of 4,462 men in the US found that a combination of high testosterone and growing up under difficult, stressful conditions was associated with having significantly more sex partners and a lower likelihood of ever marrying - but among the men who did marry, a significantly higher incidence of extramarital sex and violence against wives. These findings are doubly intriguing because men who grew up under more difficult conditions were also more likely to have high testosterone levels.

The point is that human rape is not only about sex or passing on genes. At the very least it is also about high levels of testosterone and impulsiveness, the all-too-common consequences of growing up in risky and uncertain environments. And it is in precisely such environments that the most pressing adaptive problem would be passing on one's genes. When the flow of resources is inadequate or unpredictable, the risk of death is high. When the future is objectively risky and unsure, it can be evolutionarily rational to take even huge risks for just a small chance at passing on one's genes. Even scorpionflies do this. "Giftless males," who lack access to the resources that are attractive to female scorpionflies, take a chance at "rape," which is a chancy, second-best option, because "when females are willing, males are much more likely to achieve penetration and sperm transfer."

If rape is not all about sex (even in scorpionflies) then Thornhill and Palmer's policy of preventing rape by having women dress more sensibly is not likely to "really work." On the other hand, if rape is even partly about growing up under conditions of chronic risk and uncertainty, where access to material and social-emotional resources is inadequate and unpredictable, then maybe we should think of ways to reduce the risk and uncertainty that people encounter while they are growing up. Because inequality (including power differentials) is probably the major source of risk and uncertainty in peoples' lives then perhaps we should reduce inequality. Of course we didn't need evolutionary theory to tell us that inequality is wrong, but it's gratifying to discover that it offers reason to believe that by reducing inequality we might reduce the incidence of rape. We believe that our only scientific theory of life, far from being the racist, sexist brute of fearful imaginations, can help us think of even more ways to identify and realise our deepest social and political goals.

James S Chisholm and Victoria K Burbank are Associate Professors in (respectively) the Department of Anatomy and Human Biology and the Department of Anthropology at the University of Western Australia. Chisholm's latest book is Death, Hope, and Sex: Steps to an Evolutionary Ecology of Mind and Morality (Cambridge University Press, 1999).