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.
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.