Darwin's Rape Whistle
Have women evolved to protect themselves from sexual assault?
Women, gather round, read carefully, because this gay man—who once, long ago, feigned sexual interest in your bodies—is about to shine a spotlight on some hidden truths about your natural design. It's by no means a perfect system, but evolution has endowed you with some extraordinary, almost preternatural abilities to prevent your own sexual assault. And these abilities are especially pronounced when you're ovulating.
Although it can certainly take other forms, rape will be defined throughout this article as the use of force, or threat of force, to achieve penile-vaginal penetration of a woman without her consent. Whether or not human males evolved to rape women is, to put it mildly, a controversial topic. The flames were fanned especially with the publication, about a decade ago, of Randy Thornhill and Craig Palmer's A Natural History of Rape, which presented evidence of what appear to be biological adaptations in human males (as well as males of many other species) specialized for forcibly coercing females into copulation. They argued that rape is an adaptive behavior in certain contexts; for example, when consensual partners are unavailable. There is some evidence that convicted rapists are physically unattractive, at least as judged by women on the basis of their mug shots. And spousal rape is most likely to occur when the husband finds out (or suspects) his wife has been unfaithful, suggesting that he is attempting to supplant another man's seed. (In fact, the distinctive, mushroom-capped shape of the human penis is designed to perform the specialized function of removing competitors' sperm, which indicates an ancestral history of females having sex with multiple males within a 24-hr period.) Furthermore, UCLA psychologist Neil Malamuth and his colleagues found that one-third of men admit that they would engage in some type of sexual coercion if they could be assured they would suffer no negative consequences, and many report having related masturbatory fantasies.
Thornhill and Palmer, Malamuth, and the many other investigators studying rape through an evolutionary lens, take great pains to point out that "adaptive" does not mean "justifiable," but rather only mechanistically viable. Yet dilettante followers may still be inclined to detect a misogyny in these investigations that simply is not there. As University of Michigan psychologist William McKibbin and his colleagues write in a 2008 piece for the Review of General Psychology, "No sensible person would argue that a scientist researching the causes of cancer is thereby justifying or promoting cancer. Yet some people argue that investigating rape from an evolutionary perspective justifies or legitimizes rape."
The unfortunate demonization of this brand of inquiry is rooted in the fallacy of biological determinism (according to which men are programmed by their genes to rape and have no free will to do otherwise) and the naturalistic fallacy (that because rape is natural it must be acceptable). These are resoundingly false assumptions that reveal a profound ignorance of evolutionary biology. Yet the purpose of the remaining article is not to belabor that tired ideological dispute, but to look at things from the female genetic point of view. We've heard the argument that men may have evolved to sexually assault women. Have women evolved to protect themselves from men?
While it's debatable that a rape module lurks in the male brain, there is absolutely no question that rape is a distressingly common occurrence in our species. One study from 1992 found that about 13 percent of American women are raped; the real number is almost certainly higher since so many sexual assaults go unreported. And aside from its self-evident harms, there is no question that rape seriously impairs a woman's reproductive interests. To say that rape pregnancies are costly to a woman's genetic success would be an enormous understatement. Not only do such conceptions completely undermine the female's mate selection—and so the quality of her offsprings' genes—but rapists are unlikely to stick around and help raise children, putting such children at a significant disadvantage. In short, it's a catastrophic mess from the vantage point of the mother's genes.
Given the enormity of this adaptive problem for ancestral women, it is plausible that human females would have evolved a set of counter-adaptations to protect them from being raped, and that these anti-rape adaptations would be activated, specifically, during the woman's most fertile period, the periovulatory phase of her reproductive cycle. So with the foregoing theoretical sketch in mind, I now present to you an up-to-date list of four empirically validated "phase dependent female rape-avoidance mechanisms:"
1. When threatened by sexual assault, ovulating women display a measurable increase in physical strength. In 2002, SUNY-Albany psychologists Sandra Petralia and Gordon Gallup had 192 female undergraduate students read a story about either a female character being stalked by a suspicious male stranger in a parking lot (ending with: "As she inserts the key into her car door she feels his cold hand on her shoulder …") or a similar story in which the female character is surrounded by happy people on a warm summer's day (ending with: "She starts her car, adjusts the stereo, and as she pulls out of the parking lot those nearby can hear her music blasting"). The researchers measured the handgrip strength of each participant before and after she read the story, and compared the scores. Petralia and Gallup also knew from the results of a urine-based ovulation test kit where in their reproductive cycles each participant was, so the researchers could differentiate among women in the menstrual, follicular, ovulatory, and luteal phases. A fifth group consisted of those women who were on contraceptives at the time of the study. The results were unambiguous: Only the ovulating women who read the sexual assault scenario exhibited an increase in handgrip strength. Ovulating women who read the control passage and nonovulatory women who read the sexual assault material grasped with the same intensity as before.
2. Ovulating women overestimate strange males' probability of being rapists. Add this one to a growing list of adaptive cognitive biases—evolved psychological distortions that orient people toward strategic decision-making. These findings come from a 2007 report by Christine Garver-Apgar and her colleagues. "When the costs of being sexually victimized are highest," reason these investigators, "women should shift their perceptions to decrease false negative errors at the expense of making more false positive errors. Thus, we predicted that women perceive men as more sexually coercive at fertile points of their cycle than at non-fertile points." The researchers showed 169 normally ovulating women videotaped interviews with various men and asked them to rate the men on several dimensions, including their tendencies toward sexual aggression, kindness, or faithfulness. The more fertile the woman was at the time of her judging, the more likely she was to describe the men as "sexually coercive." Ovulating women didn't see these men as being less kind, faithful, or likely to commit—only more inclined to rape them.
3. Ovulating women play it safe by avoiding situations that place them at increased risk of being raped. Fending off would-be rapists and pigeonholing strange men as potential sex fiends sounds exhausting—wouldn't it make more sense to avoid dangerous places and unknown males altogether? That is exactly what ovulating women tend to do. At least two studies have demonstrated that women at the peak of their fertility are less likely than their peers to have engaged in high-risk activities such as walking alone in a park or forest, letting a stranger into the house, or stopping their cars in a remote place over the preceding 24 hours. Importantly, as German investigators Arndt Bröder and Natalia Hohmann established, ovulating women are not less active in general—they're still busy shopping, going to church, visiting friends, and so on—but they avoid doing those things that make them sexually vulnerable.
4. Women become more racist when they're ovulating. At least white American ovulating women do when it comes to thinking about black American men. Those are the jaw-dropping, politically incorrect findings of Michigan State University's Carlos Navarrete and colleagues. White, undergraduate females were evaluated for race bias using several variants of an implicit association test, which asks participants to perform a word-matching task that indicates the relative accessibility of certain stereotypes. The women who happened to be ovulating scored especially high when it came to fear of black (as opposed to white) men, a fact that the authors interpret as reflecting an evolved disposition to avoid so-called "out-group males," who "may not have been subject to the same social controls as in-group members and would have constituted a threat in antagonistic situations." In this case, skin color serves as a convenient marker of group identity. (The authors concede that people of different skin colors came into contact with one another only in recent times, evolutionarily speaking, but propose that any physical trait that serves to demarcate an out-group member would be processed by ovulating females as a sort of "hazard heuristic.") Stereotypes about the particular out-group being prone to violence may also play a role, so, at least in American society, cultural transmission works alongside evolutionary biology in promoting racism. It remains unclear if the same race bias occurs in ovulating women from other races: Do black women show heightened fear of white men?
I don't know about you, but I'm riveted, and convinced, by much of the logic in this anti-rape area. And researchers are just getting started. Above is a set of astonishing truths that, had an evolutionary approach to studying complex social behavior not been adopted so rigorously over the past quarter-century and applied to human sexuality, would have gone entirely unnoticed—not least of which by a Kinsey-6 gay man who wouldn't know what to do with an ovulating woman if she came with instructions.
Jesse Bering is the author of The Belief Instinct and Why Is the Penis Shaped Like That? (July 2012). He is a frequent contributor to Slate and writes the "Bering in Mind" column for scientificamerican.com. His next book will be on the curiously scandalous science of human sexuality. Follow him at www.jessebering.com, on Twitter @JesseBering, or try adding him on Facebook.
The Rape of the Sabine Women painting by Pietro da Cortona is in the public domain.