How evolutionary psychology gets evolution wrong.
This spring, New York Times columnist John Tierney asserted that men must be innately more competitive than women since they monopolize the trophies in—hold onto your vowels—world Scrabble competitions. To bolster his case, Tierney turned to evolutionary psychology. In the distant past, he argued, a no-holds-barred desire to win would have been an adaptive advantage for many men, allowing them to get more girls, have more kids, and pass on their competitive genes to today's word-memorizing, vowel-hoarding Scrabble champs.
Tierney's peculiar, pseudo-scientific claim—not the first from him—reflects the extent to which evolutionary psychology has metastasized throughout public discourse. EP-ers' basic claim is that human behavior stems from psychological mechanisms that are the products of natural selection during the Stone Age. Researchers often focus on how evolution produced mental differences between men and women. One of EP's academic stars, David Buss, argues in his salacious new book The Murderer Next Door that men are wired to kill unfaithful wives because this response would have benefited their distant forefathers. Larry Summers took some cover from EP this winter after his remarks about women's lesser capacity to become top scientists. And adaptive explanations of old sexist hobbyhorses—men like young women with perky breasts and can't stop themselves from philandering because these urges aided ancestral reproduction—are commonly marshaled in defense of ever-more-ridiculous playboys.
Evolutionary psychologists have long taken heat from critics for overplaying innate characteristics—nature at the expense of nurture—and for reinforcing gender stereotypes. But they've dismissed many detractors, fairly or no, as softheaded feminists and sociologists who refuse to acknowledge the true power of natural selection. Increasingly, however, attacks on EP come from academics well-versed in the hard-nosed details of evolutionary biology. A case in point is the new book Adapting Minds by philosopher David Buller, which was supported by a research grant from the National Science Foundation and published by MIT Press and has been getting glowing reviews like this one (paid link) from biologists. Buller persuasively argues that while evolutionary forces likely did play a role in shaping our minds, the assumptions and methods that have dominated EP are weak. Much of the work of pioneers like Buss, Steven Pinker, John Tooby, Leda Cosmides, Martin Daly, and Margo Wilson turns out to be vulnerable on evolutionary grounds.
EP claims that our minds contain hundreds or thousands of "mental organs" or "modules," which come with innate information on how to solve particular problems—how to interpret nuanced facial expressions, how to tell when someone's lying or cheating. These problem-solving modules evolved between 1.8 million and 10,000 years ago, during the Pleistocene epoch. And there the selection story ends. There has not been enough time in the intervening millenia, EP-ers say, for natural selection to have further resculpted our psyches. "Our modern skulls house a Stone Age mind," as Cosmides' and Tooby's primer on evolutionary psychology puts it. The way forward for research is to generate hypotheses about the urges that would have been helpful to Stone Age baby-making and then try to test whether these tendencies are widespread today.
What's wrong with this approach? To begin with, we know very little about the specific adaptive problems faced by our distant forebears. As Buller points out, "We don't even know the number of species in the genus Homo"—our direct ancestors—"let alone details about the lifestyles led by those species." This makes it hard to generate good hypotheses. Some EP-ers have suggested looking to modern-day hunter-gatherers as proxies, studying them for clues about our ancestors. But this doesn't get them far. For instance, in some contemporary African groups, men gather the bulk of the food; in other groups, women do. Which groups are representative of our ancestors? Surely there's a whole lot of guesswork involved when evolutionary psychologists hypothesize about the human brain's supposedly formative years.
In addition, we are probably not psychological fossils. New research suggests that evolutionary change can occur much faster than was previously believed. Natural selection is thought to effect rapid change especially when a species' environment is in flux—precisely the situation in the last 10,000 years as humans learned to farm, domesticate animals, and live in larger communal groups. Crucially, Buller notes, in order for significant change to have occurred in the human mind in the last 10 millennia, evolution need not have built complex brain structures from scratch but simply modified existing ones.
Finally, the central, underlying assumption of EP—that humans have hundreds or thousands of mental problem-solving organs produced by natural selection—is questionable. Many cognitive scientists believe that such modules exist for processing sensory information and for acquiring language. It does not follow, however, that there are a plethora of other ones specifically designed for tasks like detecting cheaters. In fact, considering how much dramatic change our forebears faced, it makes more sense that their problem-solving faculties would have evolved to be flexible in response to their immediate surroundings. (A well-argued book from philosopher Kim Sterelny fleshes out this claim.) Indeed, our mental flexibility, or cortical plasticity, may be evolution's greatest gift.
So, if evolutionary psychology has so many cracks in its foundations, why is it so stubbornly influential? It helps that EP-ers like Buss and Pinker are lively, media-friendly writers who present topics like sex, love, and fear in simple terms. More to the point for scientists, EP's conclusions can be quite difficult to falsify. Even if its methods of generating hypotheses are suspect, there is always the possibility that on any given topic, an EP-er will turn out to be partly right. That forces critics to delve into the details of particular empirical claims. Buller does this in the latter part of his book and successfully dismantles several major EP findings.
For instance, EP-ers have asserted that stepparents are more likely to abuse their stepchildren than their own sons and daughters because in the Stone Age, the parents who selectively devoted love and resources to their own progeny would have had a leg up in passing on their own genes. The proof is data that purport to show a higher rate of modern-day abuse by stepparents than by parents. When Buller dissects the data, however, this conclusion begins to fall apart. To begin with, most of the relevant studies on abuse do not say whether the abuser was a parent or stepparent. The EP assumption that the abuser is always the stepparent creates an artificial and entirely absurd confirmation of the field's hypothesis. In addition, research has shown that when a stepfather is present, a child's bruises are more likely attributed to abuse rather than to accidents, whereas when a biological father is present, the opposite tendency exists. Buller has to wade in deep to unravel this, but the effort pays off.
Ultimately, the biggest problem with EP may be that it underestimates the power of evolutionary forces—both to tinker continually with the human brain, and to have created ingenious and flexible problem-solving structures in the first place. There's a nice irony here, since for years EP-ers have ridiculed opponents for not appreciating evolutionary theory's core tenets. Buller goes so far as to note an eerie resemblance between EP and intelligent design, which also treats human nature as fixed and complete. The more persuasive claim is that there is no single human nature, and that we're works in progress.
Amanda Schaffer is a science and medical columnist for Slate.
Illustration by Robert Neubecker.