Sibling rivalry 

Why the nature/nurture debate won't go away

By Steven Pinker

WHEN THE BRITISH EDUCATOR Richard Mulcaster wrote in 1582 that
''Nature makes the boy toward, nurture sees him forward,'' he gave the world
a euphonious name for an opposition that has been debated ever since. People's
beliefs about the roles of heredity and environment affect their opinions on an
astonishing range of topics. Do adolescents engage in violence and substance abuse
because of the way their parents treated them as toddlers? Are people inherently
selfish and aggressive, which would justify a market economy and a strong police, or
could they become peaceable and cooperative, allowing the state to wither and a
spontaneous socialism to blossom? Is there a universal aesthetic that allows great art
to transcend time and place, or are people's tastes determined by their era and
culture? With so much at stake, it is no surprise that debates over nature and nurture
evoke such strong feelings.
 
Much of the heat comes from framing the issues as all-or-none dichotomies, and
some of it can be transformed into light with a little nuance. Humans, of course, are
not exclusively selfish or generous (or nasty or noble); they are driven by competing
motives elicited in different circumstances. Although no aspect of the mind is
unaffected by learning, the brain has to come equipped with complex neural circuitry
to make that learning possible. And if genes affect behavior, it is not by pulling the
strings of the muscles directly, but via their intricate effects on a growing brain.

By now most thinking people have come to distrust any radical who would seem to
say that the mind is a blank slate that is filled entirely by its environment, or that
genes control our behavior like a player piano. Many scientists, particularly those who
don't study humans, have gone further and expressed the hope that the
nature-nurture debate will simply go away. Surely, they say, all behavior emerges
from an inextricable interaction between heredity and environment during
development. Trying to distinguish them can only stifle productive research and lead
to sterile polemics. 

But moderation, like all things, can be taken to extremes. The belief that it's simplistic
to distinguish nature and nurture is itself simplistic. The contributions of this
opposition to our understanding of mind and society are far from obvious, and many
supposedly reasonable compromises turn out, under closer scrutiny, to be anything
but. Let's consider some of the ''reasonable'' beliefs of the radical moderates. 

'Reasonable'' Belief No. 1: No one believes in the extreme ''nurture'' position
that the mind is a blank slate.

Certainly few people today endorse the blank slate in so many words, and I suspect
that even fewer believe it in their heart of hearts. But many people still tacitly assume
that nurture is everything when they write opinion pieces, conduct research, and
translate the research into policy. Most parenting advice, for example, is inspired by
studies that find a correlation between parents and children. Loving parents have
confident children, authoritative parents (neither too permissive nor too punitive) have
well-behaved children, parents who talk to their children have children with better
language skills, and so on. Everyone concludes that to rear the best children, parents
must be loving, authoritative, and talkative, and if children don't turn out well, it must
be the parents' fault. 

But there is a basic problem with this reasoning, and it comes from the tacit
assumption that children are blank slates. Parents, remember, provide their children
with genes, not just a home environment. The correlations between parents and
children may be telling us only that the same genes that make adults loving,
authoritative, and talkative make their children self-confident, well behaved, and
articulate. Until the studies are redone with adopted children (who get only their
environment, not their genes, from their parents), the data are compatible with the
possibility that genes make all the difference, the possibility that parenting makes all
the difference, or anything in between. Yet in almost every instance, the most
extreme position - that parents are everything - is the only one researchers entertain.

Another example: To a biologist the first question to ask in understanding conflict
between organisms of the same species is ''How are they related?'' In all social
species, relatives are more likely to help each other, and nonrelatives are more likely
to hurt each other. (That is because relatives share genes, so any gene that biases an
organism to help a close relative will also, some of the time, be helping a copy of
itself, and will thereby increase its own chances of prevailing over evolutionary time.)
But when the psychologists Martin Daly and Margo Wilson checked the literature on
child abuse to see whether stepparents were more likely to abuse their children than
biological parents, they discovered not only that no one had ever tested the possibility,
but that most statistics on child abuse did not even record the information -
stepparents and biological parents were lumped together, as if the difference couldn't
possibly matter. When Daly and Wilson did track down the relevant statistics, their
hunch was confirmed: Having a stepparent is the largest risk factor for child abuse
ever examined.

The finding was by no means banal: Many parenting experts insist that the hostile
stepparent is a myth originating in Cinderella stories, and that parenting is a ''role''
that anyone can take on. For agencies that monitor and seek to prevent child abuse
the finding of a greater risk with stepparents could be critical information. But
because of the refusal to entertain the idea that human emotions are products of
evolution, no one had ever thought to check.

''Reasonable'' Belief No. 2: For every question about nature and nurture, the
correct answer is ''Some of each.'' 

Not so. Take the question, ''Why do people in England speak English, and people in
Japan Japanese?'' The ''reasonable compromise'' would be that the Japanese have
genes that make it easier for them to learn Japanese (and vice versa for the English),
but both groups must be exposed to the language to acquire it fully. This compromise,
of course, is not reasonable at all; it's false. Immigrant children acquire the language
of their adopted home perfectly, showing that people are not predisposed to learn the
language of their ancestors (though they may be predisposed to learn language in
general). The explanation for why people in different countries speak different
languages is 100 percent environmental.

And sometimes the answer goes the other way. Autism, for example, used to be
blamed on ''refrigerator mothers'' who did not emotionally engage with their children.
Schizophrenia was thought to be caused by mothers who put their children in ''double
binds'' (such as the Jewish mother who gave her son two shirts for his birthday, and
when he turned up wearing one of them, said, ''The other one you didn't like?'').
Today we know that autism and schizophrenia are highly heritable, and though they
are not completely determined by genes, the other likely contributors (toxins,
pathogens, chance events in brain development) have nothing to do with parenting.
Mothers don't deserve ''some'' of the blame if their children have these disorders, as
a nature-nurture compromise would imply; they deserve none of it.

''Reasonable'' Belief No. 3: Disentangling nature and nurture is a hopeless
task, so we shouldn't even try.

On the contrary, perhaps the most unexpected and provocative discovery in
20th-century psychology came from an effort to distinguish nature and nurture in
human development. For a long time, psychologists have studied individual
differences in intellect and personality. They have assessed cognitive abilities using
IQ tests, statistics on performance in school and on the job, and measurements of
brain activity. They have assessed people's personalities using questionnaires, ratings
by other people who know them well, and tallies of actual behavior such as divorces
and brushes with the law. The measures suggest that our personalities differ in five
major ways. We are to varying degrees introverted or extroverted, neurotic or stable,
incurious or open to experience, agreeable or antagonistic, and conscientious or
undirected. 

Where do these differences come from? Recall those flawed studies that test for the
effects of parenting but forget to control for genetic relatedness. Behavioral
geneticists have done studies that remedy those flaws and have discovered that
intelligence, personality, overall happiness, and many other traits are partly (though
never completely) heritable. That is, some of the variation in the traits among people
in a given culture can be attributed to differences in their genes. The conclusion
comes from three different kinds of research, each teasing apart genes and
environment in a different way. First, identical twins reared apart (who share their
genes but not their family environment) are far more similar to each other than
randomly selected pairs of people. Second, identical twins reared together (who
share their environment and all their genes) are more similar than fraternal twins
reared together (who share their environment but only half their genes). Third,
biological siblings reared together (who share their environment and half their genes)
are more similar than adoptive siblings (who share their environment but none of their
genes).

In each comparison, the more genes a pair of people share (holding environment
more or less constant), the more similar they are. These studies have been replicated
in large samples from many countries, and have ruled out the alternative explanations
that have been proposed. Of course, concrete traits that patently depend on content
provided by the home or culture are not heritable at all, such as the language you
speak, the religion you worship in, and the political party you belong to. But the
underlying talents and temperaments are heritable: how proficient with language you
are, how receptive to religion, how hidebound or open to change. 

So genes play a role in making us different from our neighbors, and our environments
play an equally important role. At this point most people leap to the following
conclusion: We are shaped both by our genes and by our family upbringing: how our
parents treated us and what kind of home we grew up in.

Not so fast. ''The environment'' and ''our parents and home'' are not the same thing.
Behavioral genetics allows us to distinguish two very different ways in which our
environments might affect us. The shared environment is what impinges on us and
our siblings alike: our parents, our home life, and our neighborhood (as compared with
other parents and neighborhoods). The unique environment is everything else:
anything that happens to us over the course of our lives that does not necessarily
happen to our siblings. 

Remarkably, study after study has failed to turn up appreciable effects of the shared
environment - often to the shock and dismay of the researchers themselves, who
started out convinced that the nongenetic variation in personality had to come from
the family. First, they've found, adult siblings are equally similar whether they grew
up together or apart. Second, adoptive siblings are no more similar than two people
plucked off the street at random. And third, identical twins who grew up in the same
home are no more similar than one would expect from the effects of their shared
genes. Whatever experiences siblings share by growing up in the same home in a
given culture makes little or no difference in the kind of people they turn out to be.

The implications, drawn out most clearly by Judith Rich Harris in her 1998 book ''The
Nurture Assumption,'' are mind-boggling. According to a popular saying, ''as the twig
is bent, so grows the branch.'' Patients in traditional forms of psychotherapy while
away their 50 minutes reliving childhood conflicts and learning to blame their
unhappiness on how their parents treated them. Many biographies scavenge through
the subject's childhood for the roots of the grown-up's tragedies and triumphs.
''Parenting experts'' make women feel like ogres if they slip out of the house to work
or skip a reading of ''Goodnight Moon. '' All these deeply held beliefs will have to be
rethought. It's not that parents don't matter at all. Extreme cases of abuse and
neglect can leave permanent scars. Skills like reading and playing a musical
instrument can be imparted by parents. And parents affect their children's happiness
in the home, their memories of how they were treated, and the quality of the lifelong
relationship between parent and child. But parents don't seem to mold their children's
intellects, personalities, or overall happiness for the rest of their lives.

The implications for science are profound as well. Here is a puzzle: Identical twins
growing up together have the same genes, family environments, and peer groups, but
the correlations in their traits are only around 50 percent. Ergo, neither genes nor
families nor peer groups, nor the interactions among these factors, can explain what
makes them different. Researchers have hunted for other possible causes, such as
sibling rivalry or differential treatment by parents, but none has panned out. As with
Bob Dylan's Mister Jones, something is happening here but we don't know what it is.

My own hunch is that the differences come largely from chance events in
development. One twin lies one way in the womb and stakes out her share of the
placenta, the other has to squeeze around her. A cosmic ray mutates a stretch of
DNA, a neurotransmitter zigs instead of zags, the growth cone of an axon goes left
instead of right, and one person's brain might gel into a slightly different configuration
from another's, regardless of their genes. 

If chance in development is to explain the less-than-perfect similarity of identical
twins, it says something interesting about development in general. One can imagine a
developmental process in which millions of small chance events cancel one another
out, leaving no difference in the end product. One can imagine a different process in
which a chance event could derail development entirely, or send it on a chaotic path
resulting in a freak or a monster. Neither of these results occurs with a pair of
identical twins. They are distinct enough that our instruments can pick up the
differences, yet both are healthy instances of that staggeringly improbable, exquisitely
engineered system we call a human being. The development of organisms must use
complex feedback loops rather than prespecified blueprints. Random events can
divert the trajectory of growth, but the trajectories are confined within an envelope of
functioning designs for the species. 

These profound questions are not about nature vs. nurture. They are about nurture
vs. nurture: about what, exactly, are the nongenetic causes of personality and
intelligence. But the questions would never have come to light if researchers had not
first taken measures to factor out the influence of nature, by showing that
correlations between parents and children cannot glibly be attributed to parenting but
might be attributable to shared genes. That was the first step that led them to
measure the possible effects of parenting empirically, rather than simply assuming
that those effects had to be all-powerful.

The human brain has been called the most complex object in the known universe. No
doubt many hypotheses that pit nature against nurture as a dichotomy, or that fail to
distinguish the ways in which they might interact, will turn out to be simplistic or
wrong. But that complexity does not mean we should fuzz up the issues by saying
that it's all just too complicated to think about, or that some hypotheses should be
treated a priori as necessarily true, necessarily false, or too dangerous to mention. As
with other complex phenomena like inflation, cancer, and global warming, when it
comes to the development of a human being we have no choice but to try to
disentangle the causes. 

Steven Pinker is Peter de Florez Professor of Psychology at MIT and author of
''The Language Instinct,'' and ''How the Mind Works." This essay is adapted in
part from his latest book, ''The Blank Slate.''