Why Do We Rape, Kill and Sleep Around?

Among scientists at the university of New Mexico that spring, rape was in the air. One of the professors, biologist Randy Thornhill, had just coauthored A Natural History of Rape: Biological Bases of Sexual Coercion, which argued that rape is (in the vernacular of evolutionary biology) an adaptation, a trait encoded by genes that confers an advantage on anyone who possesses them. Back in the late Pleistocene epoch 100,000 years ago, the 2000 book contended, men who carried rape genes had a reproductive and evolutionary edge over men who did not: they sired children not only with willing mates, but also with unwilling ones, allowing them to leave more offspring (also carrying rape genes) who were similarly more likely to survive and reproduce, unto the nth generation. That would be us. And that is why we carry rape genes today. The family trees of prehistoric men lacking rape genes petered out.

The argument was well within the bounds of evolutionary psychology. Founded in the late 1980s in the ashes of sociobiology, this field asserts that behaviors that conferred a fitness advantage during the era when modern humans were evolving are the result of hundreds of genetically based cognitive "modules" preprogrammed in the brain. Since they are genetic, these modules and the behaviors they encode are heritable—passed down to future generations—and, together, constitute a universal human nature that describes how people think, feel and act, from the nightclubs of Manhattan to the farms of the Amish, from the huts of New Guinea aborigines to the madrassas of Karachi. Evolutionary psychologists do not have a time machine, of course. So to figure out which traits were adaptive during the Stone Age, and therefore bequeathed to us like a questionable family heirloom, they make logical guesses. Men who were promiscuous back then were more evolutionarily fit, the researchers reasoned, since men who spread their seed widely left more descendants. By similar logic, evolutionary psychologists argued, women who were monogamous were fitter; by being choosy about their mates and picking only those with good genes, they could have healthier children. Men attracted to young, curvaceous babes were fitter because such women were the most fertile; mating with dumpy, barren hags is not a good way to grow a big family tree. Women attracted to high-status, wealthy males were fitter; such men could best provide for the kids, who, spared starvation, would grow up to have many children of their own. Men who neglected or even murdered their stepchildren (and killed their unfaithful wives) were fitter because they did not waste their resources on nonrelatives. And so on, to the fitness-enhancing value of rape. We in the 21st century, asserts evo psych, are operating with Stone Age minds.

Over the years these arguments have attracted legions of critics who thought the science was weak and the message (what philosopher David Buller of Northern Illinois University called "a get-out-of-jail-free card" for heinous behavior) pernicious. But the reaction to the rape book was of a whole different order. Biologist Joan Roughgarden of Stanford University called it "the latest 'evolution made me do it' excuse for criminal behavior from evolutionary psychologists." Feminists, sex-crime prosecutors and social scientists denounced it at rallies, on television and in the press.

Among those sucked into the rape debate that spring was anthropologist Kim Hill, then Thornhill's colleague at UNM and now at Arizona State University. For decades Hill has studied the Ache, hunter-gatherer tribesmen in Paraguay. "I saw Thornhill all the time," Hill told me at a barbecue at an ASU conference in April. "He kept saying that he thought rape was a special cognitive adaptation, but the arguments for that just seemed like more sloppy thinking by evolutionary psychology." But how to test the claim that rape increased a man's fitness? From its inception, evolutionary psychology had warned that behaviors that were evolutionarily advantageous 100,000 years ago (a sweet tooth, say) might be bad for survival today (causing obesity and thence infertility), so there was no point in measuring whether that trait makes people more evolutionarily fit today. Even if it doesn't, evolutionary psychologists argue, the trait might have been adaptive long ago and therefore still be our genetic legacy. An unfortunate one, perhaps, but still our legacy. Short of a time machine, the hypothesis was impossible to disprove. Game, set and match to evo psych.

Or so it seemed. But Hill had something almost as good as a time machine. He had the Ache, who live much as humans did 100,000 years ago. He and two colleagues therefore calculated how rape would affect the evolutionary prospects of a 25-year-old Ache. (They didn't observe any rapes, but did a what-if calculation based on measurements of, for instance, the odds that a woman is able to conceive on any given day.) The scientists were generous to the rape-as-adaptation claim, assuming that rapists target only women of reproductive age, for instance, even though in reality girls younger than 10 and women over 60 are often victims. Then they calculated rape's fitness costs and benefits. Rape costs a man fitness points if the victim's husband or other relatives kill him, for instance. He loses fitness points, too, if the mother refuses to raise a child of rape, and if being a known rapist (in a small hunter-gatherer tribe, rape and rapists are public knowledge) makes others less likely to help him find food. Rape increases a man's evolutionary fitness based on the chance that a rape victim is fertile (15 percent), that she will conceive (a 7 percent chance), that she will not miscarry (90 percent) and that she will not let the baby die even though it is the child of rape (90 percent). Hill then ran the numbers on the reproductive costs and benefits of rape. It wasn't even close: the cost exceeds the benefit by a factor of 10. "That makes the likelihood that rape is an evolved adaptation extremely low," says Hill. "It just wouldn't have made sense for men in the Pleistocene to use rape as a reproductive strategy, so the argument that it's preprogrammed into us doesn't hold up."

These have not been easy days for evolutionary psychology. For years the loudest critics have been social scientists, feminists and liberals offended by the argument that humans are preprogrammed to rape, to kill unfaithful girlfriends and the like. (This was a reprise of the bitter sociobiology debates of the 1970s and 1980s. When Harvard biologist Edward O. Wilson proposed that there exists a biologically based human nature, and that it included such traits as militarism and male domination of women, left-wing activists—including eminent biologists in his own department—assailed it as an attempt "to provide a genetic justification of the status quo and of existing privileges for certain groups according to class, race, or sex" analogous to the scientific justification for Nazi eugenics.) When Thornhill appeared on the Today show to talk about his rape book, for instance, he was paired with a sex-crimes prosecutor, leaving the impression that do-gooders might not like his thesis but offering no hint of how scientifically unsound it is.