An article worthy to be called science for the people

Why Do We Rape, Kill and Sleep Around?
The fault, dear Darwin, lies not in our ancestors, but in ourselves.

Sharon Begley
 From the magazine issue dated Jun 29, 2009
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.

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.

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,  

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 inProceedings 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."

On that point, the evidence instead suggests that both sexes prefer  
mates around their own age, adjusted for the fact that men mature  
later than women. If the male mind were adapted to prefer the most  
fertile women, then AARP-eligible men should marry 23-year-olds, which— 
Anna Nicole Smith and J. Howard Marshall notwithstanding—they do not,  
instead preferring women well past their peak fertility. And,  
interestingly, when Miller focuses on the science rather than tries to  
sell books, he allows that "human mate choice is much more than men  
just liking youth and beauty, and women liking status and wealth," as  
he told me by e-mail.

Yet evo psych remains hugely popular in the media and on college  
campuses, for obvious reasons. It addresses "these very sexy topics,"  
says Hill. "It's all about sex and violence," and has what he calls  
"an obsession with Pleistocene just-so stories." And few people—few  
scientists—know about the empirical data and theoretical arguments  
that undercut it. "Most scientists are too busy to read studies  
outside their own narrow field," he says.

Far from ceding anything, evolutionary psychologists have moved the  
battle from science, where they are on shaky ground, to ideology,  
where bluster and name-calling can be quite successful. UNM's Miller,  
for instance, complains that critics "have convinced a substantial  
portion of the educated public that evolutionary psychology is a  
pernicious right-wing conspiracy," and complains that believing in  
evolutionary psychology is seen "as an indicator of conservatism,  
disagreeableness and selfishness." That, sadly, is how much too much  
of the debate has gone. "Critics have been told that they're just  
Marxists motivated by a hatred of evolutionary psychology," says  
Buller. "That's one reason I'm not following the field anymore: the  
way science is being conducted is more like a political campaign."

Where, then, does the fall of evolutionary psychology leave the idea  
of human nature? Behavioral ecology replaces it with "it depends"—that  
is, the core of human nature is variability and flexibility, the  
capacity to mold behavior to the social and physical demands of the  
environment. As Buller says, human variation is not noise in the  
system; it is the system. To be sure, traits such as symbolic  
language, culture, tool use, emotions and emotional expression do  
indeed seem to be human universals. It's the behaviors that capture  
the public imagination—promiscuous men and monogamous women, stepchild- 
killing men and the like—that turn out not to be. And for a final nail  
in the coffin, geneticists have discovered that human genes evolve  
much more quickly than anyone imagined when evolutionary psychology  
was invented, when everyone assumed that "modern" humans had DNA  
almost identical to that of people 50,000 years ago. Some genes seem  
to be only 10,000 years old, and some may be even younger.

That has caught the attention of even the most ardent proponents of  
evo psych, because when the environment is changing rapidly—as when  
agriculture was invented or city-states arose—is also when natural  
selection produces the most dramatic changes in a gene pool. Yet most  
of the field's leaders, admits UNM's Miller, "have not kept up with  
the last decade's astounding progress in human evolutionary genetics."  
The discovery of genes as young as agriculture and city-states, rather  
than as old as cavemen, means "we have to rethink to foundational  
assumptions" of evo psych, says Miller, starting with the claim that  
there are human universals and that they are the result of a Stone Age  
brain. Evolution indeed sculpted the human brain. But it worked in  
malleable plastic, not stone, bequeathing us flexible minds that can  
take stock of the world and adapt to it.

With Jeneen Interlandi

© 2009