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Some critics see the current interest in heredity as part of an ugly political trend. "In socially conservative times," argues political scientist Diane Paul of the University of Massachusetts at Boston, "we tend to say crime and poverty are not our fault and put the blame not on society but on genes."
Even staunch believers in heredity's influence do not discount environment. In fact, the two are intimately entwined, and separating cause and effect is not easy. Biology may affect behavior, but behavior and experience also influence biology. Serotonin levels, for example, are not only controlled by genes but, according to research in monkeys, they can be lowered by regular exposure to alcohol. By the same token, says Kagan, a child with a fearless personality may turn into a criminal if reared in a chaotic home, but given a stable upbringing, "he could well become a CEO, test pilot, entrepreneur or the next Bill Clinton."
No one thinks that discovering the roots of violence will be simple. There may be as many causes as there are crimes. The issue is whether to explore all possibilities -- to search for clues in both society and biology.