Why Do We Rape, Kill and Sleep Around?

That is changing. Evo psych took its first big hit in 2005, when NIU's Buller exposed flaw after fatal flaw in key studies underlying its claims, as he laid out in his book Adapting Minds. Anthropological studies such as Hill's on the Ache, shooting down the programmed-to-rape idea, have been accumulating. And brain scientists have pointed out that there is no evidence our gray matter is organized the way evo psych claims, with hundreds of specialized, preprogrammed modules. Neuroscientist Roger Bingham of the University of California, San Diego, who describes himself as a once devout "member of the Church of Evolutionary Psychology" (in 1996 he created and hosted a multimillion-dollar PBS series praising the field), has come out foursquare against it, accusing some of its adherents of an "evangelical" fervor. Says evolutionary biologist Massimo Pigliucci of Stony Brook University, "Evolutionary stories of human behavior make for a good narrative, but not good science."

Like other critics, he has no doubt that evolution shaped the human brain. How could it be otherwise, when evolution has shaped every other human organ? But evo psych's claims that human behavior is constrained by mental modules that calcified in the Stone Age make sense "only if the environmental challenges remain static enough to sculpt an instinct over evolutionary time," Pigliucci points out. If the environment, including the social environment, is instead dynamic rather than static—which all evidence suggests—then the only kind of mind that makes humans evolutionarily fit is one that is flexible and responsive, able to figure out a way to make trade-offs, survive, thrive and reproduce in whatever social and physical environment it finds itself in. In some environments it might indeed be adaptive for women to seek sugar daddies. In some, it might be adaptive for stepfathers to kill their stepchildren. In some, it might be adaptive for men to be promiscuous. But not in all. And if that's the case, then there is no universal human nature as evo psych defines it.

That is what a new wave of studies has been discovering, slaying assertions about universals right and left. One evo-psych claim that captured the public's imagination—and a 1996 cover story in NEWSWEEK—is that men have a mental module that causes them to prefer women with a waist-to-hip ratio of 0.7 (a 36-25-36 figure, for instance). Reprising the rape debate, social scientists and policymakers who worried that this would send impressionable young women scurrying for a measuring tape and a how-to book on bulimia could only sputter about how pernicious this message was, but not that it was scientifically wrong. To the contrary, proponents of this idea had gobs of data in their favor. Using their favorite guinea pigs—American college students—they found that men, shown pictures of different female body types, picked Ms. 36-25-36 as their sexual ideal. The studies, however, failed to rule out the possibility that the preference was not innate—human nature—but, rather, the product of exposure to mass culture and the messages it sends about what's beautiful. Such basic flaws, notes Bingham, "led to complaints that many of these experiments seemed a little less than rigorous to be underpinning an entire new field."

Later studies, which got almost no attention, indeed found that in isolated populations in Peru and Tanzania, men consider hourglass women sickly looking. They prefer 0.9s—heavier women. And last December, anthropologist Elizabeth Cashdan of the University of Utah reported in the journal Current Anthropology that men now prefer this non-hourglass shape in countries where women tend to be economically independent (Britain and Denmark) and in some non-Western societies where women bear the responsibility for finding food. Only in countries where women are economically dependent on men (such as Japan, Greece and Portugal) do men have a strong preference for Barbie. (The United States is in the middle.) Cashdan puts it this way: which body type men prefer "should depend on [italics added] the degree to which they want their mates to be strong, tough, economically successful and politically competitive."

Depend on? The very phrase is anathema to the dogma of a universal human nature. But it is the essence of an emerging, competing field. Called behavioral ecology, it starts from the premise that social and environmental forces select for various behaviors that optimize people's fitness in a given environment. Different environment, different behaviors—and different human "natures." That's why men prefer Ms. 36-25-36 in some cultures (where women are, to exaggerate only a bit, decorative objects) but not others (where women bring home salaries or food they've gathered in the jungle).

And it's why the evo psych tenet that men have an inherited mental module that causes them to prefer young, beautiful women while women have one that causes them to prefer older, wealthy men also falls apart. As 21st-century Western women achieve professional success and gain financial independence, their mate preference changes, scientists led by Fhionna Moore at Scotland's University of St Andrews reported in 2006 in the journal Evolutionand Human Behaviour. The more financially independent a woman is, the more likely she is to choose a partner based on looks than bank balance—kind of like (some) men. (Yes, growing sexual equality in the economic realm means that women, too, are free to choose partners based on how hot they are, as the cougar phenomenon suggests.) Although that finding undercuts evo psych, it supports the "it depends" school of behavioral ecology, which holds that natural selection chose general intelligence and flexibility, not mental modules preprogrammed with preferences and behaviors. "Evolutionary psychology ridicules the notion that the brain could have evolved to be an all-purpose fitness-maximizing mechanism," says Hill. "But that's exactly what we keep finding."

One of the uglier claims of evo psych is that men have a mental module to neglect and even kill their stepchildren. Such behavior was adaptive back when humans were evolving, goes the popular version of this argument, because men who invested in stepchildren wasted resources they could expend on their biological children. Such kindly stepfathers would, over time, leave fewer of their own descendants, causing "support your stepchildren" genes to die out. Men with genes that sculpted the "abandon stepchildren" mental module were evolutionarily fitter, so their descendants—us—also have that preprogrammed module. The key evidence for this claim comes from studies showing that stepchildren under the age of 5 are 40 times more likely to be abused than biological children.