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, 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."
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
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