HEN 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.''
For comments and suggestions, email ideas@globe.com
This story ran on page D1 of the Boston Globe on
10/13/2002.
© Copyright
2002 Globe Newspaper Company.
