Why Do We Rape, Kill and Sleep Around?

Those studies have come under fire, however, for a long list of reasons. For instance, many child-welfare records do not indicate who the abuser was; at least some abused stepchildren are victims of their mother, not the stepfather, the National Incidence Study of Child Abuse and Neglect reported in 2005. That suggests that records inflate the number of instances of abuse by stepfathers. Also, authorities are suspicious of stepfathers; if a child living in a stepfamily dies of maltreatment, they are nine times more likely to record it as such than if the death occurs in a home with only biological parents, found a 2002 study led by Buller examining the records of every child who died in Colorado from 1990 to 1998. That suggests that child-abuse data undercount instances of abuse by biological fathers. Finally, a 2008 study in Sweden found that many men who kill stepchildren are (surprise) mentally ill. It's safe to assume that single mothers do not exactly get their pick of the field when it comes to remarrying. If the men they wed are therefore more likely to be junkies, drunks and psychotic, then any additional risk to stepchildren reflects that fact, and not a universal mental module that tells men to abuse their new mate's existing kids. Martin Daly and Margo Wilson of Canada's McMaster University, whose work led to the idea that men have a mental module for neglecting stepchildren, now disavow the claim that such abuse was ever adaptive. But, says Daly, "attempts to deny that [being a stepfather] is a risk factor for maltreatment are simply preposterous and occasionally, as in the writings of David Buller, dishonest."

If the data on child abuse by stepfathers seem inconsistent, that's exactly the point. In some circumstances, it may indeed be adaptive to get rid of the other guy's children. In other circumstances, it is more adaptive to love and support them. Again, it depends. New research in places as different as American cities and the villages of African hunter-gatherers shows that it's common for men to care and provide for their stepchildren. What seems to characterize these situations, says Hill, is marital instability: men and women pair off, have children, then break up. In such a setting, the flexible human mind finds ways "to attract or maintain mating access to the mother," Hill explains. Or, more crudely, be nice to a woman's kids and she'll sleep with you, which maximizes a man's fitness. Kill her kids and she's likely to take it badly, cutting you off and leaving your sperm unable to fulfill their Darwinian mission. And in societies that rely on relatives to help raise kids, "it doesn't make sense to destroy a 10-year-old stepkid since he could be a helper," Hill points out. "The fitness cost of raising a stepchild until he is old enough to help is much, much less than evolutionary biologists have claimed. Biology is more complicated than these simplistic scenarios saying that killing stepchildren is an adaptation that enhances a man's fitness."

Even the notion that being a brave warrior helps a man get the girls and leave many offspring has been toppled. Until missionaries moved in in 1958, the Waorani tribe of the Ecuadoran Amazon had the highest rates of homicide known to science: 39 percent of women and 54 percent of men were killed by other Waorani, often in blood feuds that lasted generations. "The conventional wisdom had been that the more raids a man participated in, the more wives he would have and the more descendants he would leave," says anthropologist Stephen Beckerman of Pennsylvania State University. But after painstakingly constructing family histories and the raiding and killing records of 95 warriors, he and his colleagues reported last month in Proceedings of the National Academy of Sciences, they turned that belief on its head. "The badass guys make terrible husband material," says Beckerman. "Women don't prefer them as husbands and they become the targets of counterraids, which tend to kill their wives and children, too." As a result, the über-warriors leave fewer descendants—the currency of evolutionary fitness—than less aggressive men. Tough-guy behavior may have conferred fitness in some environments, but not in others. It depends. "The message for the evolutionary-psychology guys," says Beckerman, "is that there was no single environment in which humans evolved" and therefore no single human nature.

I can't end the list of evo-psych claims that fall apart under scientific scrutiny without mentioning jealousy. Evo psych argues that jealousy, too, is an adaptation with a mental module all its own, designed to detect and thwart threats to reproductive success. But men's and women's jealousy modules supposedly differ. A man's is designed to detect sexual infidelity: a woman who allows another man to impregnate her takes her womb out of service for at least nine months, depriving her mate of reproductive opportunities. A woman's jealousy module is tuned to emotional infidelity, but she doesn't much care if her mate is unfaithful; a man, being a promiscuous cad, will probably stick with wife No. 1 and their kids even if he is sexually unfaithful, but may well abandon them if he actually falls in love with another woman.

Let's not speculate on the motives that (mostly male) evolutionary psychologists might have in asserting that their wives are programmed to not really care if they sleep around, and turn instead to the evidence. In questionnaires, more men than women say they'd be upset more by sexual infidelity than emotional infidelity, by a margin of more than 2-to-1, David Buss of the University of Texas found in an early study of American college students. But men are evenly split on which kind of infidelity upsets them more: half find it more upsetting to think of their mate falling in love with someone else; half find it more upsetting to think of her sleeping with someone else. Not very strong evidence for the claim that men, as a species, care more about sexual infidelity. And in some countries, notably Germany and the Netherlands, the percentage of men who say they find sexual infidelity more upsetting than the emotional kind is only 28 percent and 23 percent. Which suggests that, once again, it depends: in cultures with a relaxed view of female sexuality, men do not get all that upset if a woman has a brief, meaningless fling. It does not portend that she will leave him. It is much more likely that both men and women are wired to detect behavior that threatens their bond, but what that behavior is depends on culture. In a society where an illicit affair portends the end of a relationship, men should indeed be wired to care about that. In a society where that's no big deal, they shouldn't—and, it seems, don't. New data on what triggers jealousy in women also undercut the simplistic evo-psych story. Asked which upsets them more—imagining their partner having acrobatic sex with another woman or falling in love with her—only 13 percent of U.S. women, 12 percent of Dutch women and 8 percent of German women chose door No. 2. So much for the handy "she's wired to not really care if I sleep around" excuse.

Critics of evo psych do not doubt that men and women are wired to become jealous. A radar for infidelity would indeed be adaptive. But the evidence points toward something gender-neutral. Men and women have both evolved the ability to distinguish between behavior that portends abandonment and behavior that does not, and to get upset only at the former. Which behavior is which depends on the society.

Evolutionary psychology is not going quietly. It has had the field to itself, especially in the media, for almost two decades. In large part that was because early critics, led by the late evolutionary biologist Stephen Jay Gould, attacked it with arguments that went over the heads of everyone but about 19 experts in evolutionary theory. It isn't about to give up that hegemony. Thornhill is adamant that rape is an adaptation, despite Hill's results from his Ache study. "If a particular trait or behavior is organized to do something," as he believes rape is, "then it is an adaptation and so was selected for by evolution," he told me. And in the new book Spent, evolutionary psychologist Geoffrey Miller of the University of New Mexico reasserts the party line, arguing that "males have much more to gain from many acts of intercourse with multiple partners than do females," and there is a "universal sex difference in human mate choice criteria, with men favoring younger, fertile women, and women favoring older, higher-status, richer men."