Are Your Genes To Blame?

Readers of the science pages could be forgiven for thinking that the conversation in the cartoon on the opposite page really took place. Study after study has shown that genes can affect behavior and mental life. Identical twins separated at birth (who share their genes but not their environment) are similar in their intellectual talents, their personality traits (such as introversion, conscientiousness and antagonism), their average level of lifelong happiness and such personal quirks as giggling incessantly or flushing the toilet both before and after using it. Identical twins (who share all their genes) are more similar than fraternal twins (who share half their genes). And biological siblings (who also share half their genes) are more similar, of course, than adopted siblings (who share none of their genes). Not only are personality and intelligence partly heritable, but so is susceptibility to psychological maladies such as schizophrenia, obsessive-compulsive disorder and major depression.

The discovery that genes have something to do with behavior came as a shock in the second half of the 20th century, when most people still thought that the mind of a newborn was a blank slate and that anyone could do anything if only he or she strove hard enough. And the link continues to set off alarm bells about what it will lead to. Many people are worried about a Brave New World in which parents or governments will try to re-engineer human nature. Others see genes as a threat to free will and personal responsibility, citing headlines like MAN'S GENES MADE HIM KILL, HIS LAWYERS CLAIM. Behavioral geneticists are sometimes picketed, censored or compared with Nazis.

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As we increase our knowledge of how the genome works, many beliefs about ourselves will indeed have to be rethought. But the worst fears of the genophobes are misplaced. It is easy to exaggerate the significance of behavioral genetics for our lives. For one thing, genes cannot pull the strings of behavior directly. Behavior is caused by the activity of the brain, and the most genes can do is affect its wiring, size, shape and sensitivity to hormones and other molecules. Among the brain circuits laid down by genes are the ones that reflect on memories, current circumstances and the anticipated consequences of various courses of action and that select behavior accordingly in an intricate and not entirely predictable way. These circuits are what we call "free will," and providing them with information about the likely consequences of behavioral options is what we call "holding people responsible." All normal people have this circuitry, and that is why the existence of genes with effects on behavior should not be allowed to erode responsibility in the legal system or in everyday life.

Also, don't count on the I'll-let-you-get-off-the-phone-now gene or any other single gene with a large behavioral effect being identified anytime soon. Behavioral genetics has uncovered a paradox. Studies that measure similarities among twins and adoptees reliably show strong effects of sharing a large group of genes. The outcome is so reliable that behavioral geneticists now speak of the First Law of their field: that all behavioral traits are partly heritable. But studies that try to isolate a single gene for a behavioral trait have been fickle; many of putative genes-for-X have not held up in replications. Genes must exert their effects by acting together in complex combinations. A rough analogy: a computer program can have a trait, such as being easy to use, without necessarily having a single magical programming instruction that makes any program easy to use when added and any program hard to use when omitted.

So psychological engineering is more remote than the futurologists would have you believe. Though musical talent may be partly heritable, there is probably no single gene for musical talent that ambitious parents can have implanted into their unborn children. It might take hundreds or thousands of the right genes, with a different combination needed for each child.

Finally, the fact that genes matter doesn't mean that other things don't matter. Some of the causes of the differences in personality, intellect and pathology are obvious. There are no genes for speaking English or for being a Presbyterian (though there may be sets of genes for verbal skill and religiosity). One's choice of language or religion depends almost entirely on one's culture. Less obvious are some of the other possible causes. These include germs, accidents, chance encounters in life and random events in the development of the brain in utero.

And still other environmental factors may not be acting as we think they do. It is easy to assume that a variation in behavior not caused by genes must be caused by parents. But it's been surprisingly hard to demonstrate any long-term effects of growing up in any particular family within a culture. Identical twins reared together are similar, but they are not literally identical: one may be more anxious than the other; one may be gay and the other heterosexual. This shows that genes are not everything but because these twins grow up in the same family, it also shows that what isn't explained by genes isn't explained by family influences either. Similarly, children need to hear English to acquire it. But if their parents are immigrants, they end up with the accent of their peers, not their parents.