The Anatomy of Violence

The "violence gene" theory soon found itself on shaky ground, however. In 2002, scientists who had followed 442 New Zealand men since their birth found that the MAOA link was not nearly as straightforward as the Dutch study suggested. Yes, men with the low-activity form of the MAOA gene were more likely to engage in persistent fighting, bullying, cruelty and violent crime than were men with the high-activity version. But that was so only if they had been neglected or abused as children. If they had not been mistreated, men with the low-activity MAOA gene were not much likelier to be violent. The gene alone was not sufficient. It was not strictly deterministic in the sense of always causing someone to become violent, but merely "permissive": if two boys are severely abused, the one with the low-activity gene is more likely to grow up to commit violent crimes, and even then only if society provides fertile ground for this weed to grow.

The road from genes to behavior travels through the brain. In his research on killers, Adrian Raine of the University of Southern California classifies them as either reactive, those who murder in response to an insult or slight (real or imagined), or proactive, who kill to achieve a thought-out goal such as robbery. Proactive killers show brain-activity patterns no different from that of normal, nonviolent volunteers, Raine reported in 1998. But the brains of reactive killers have clearly reduced activity in the prefrontal cortex, the site of such "executive" functions as judgment, planning, abstract reasoning, inhibiting inappropriate or impulsive behavior and self-monitoring. "This is the part of the brain that says, 'Let's stop and think about this again'," says Raine. "It has a calming effect on the emotional regions of the brain that give rise to pent-up anger and rage." Low prefrontal activity "also means that empathy will be off," says neuropsychiatrist Daniel Amen, who heads a chain of four psychiatric clinics and who found this pattern in the brain of Kip Kinkel, who killed his parents and then shot two dozen fellow students in Springfield, Ore., in 1998 when he was 15. "How do you kill 32 people and have any kind of empathy?" he asks. "That's highly associated with decreased activity in the prefrontal cortex."

In the brains of reactive killers the eerie quiet in the prefrontal regions is paired with increased activity in the limbic regions, site of emotions. "That gives rise to aggression and less prefrontal control over that aggression," Raine says. "It's a sort of double hit that may make them more likely to act out aggressively." Also overactive is a region involved in shifting attention, called the cingulate gyrus. "You become obsessive," Amen says. "Someone with violent thoughts can't let them go. Stalking is one sign of that."

Since the early days of research on the brains of violent felons, however, neuroscience has undergone a paradigm shift. Researchers now know that life experiences and even introspection can alter patterns of brain activity. When people suffering from obsessive-compulsive disorder learn to think about their thoughts differently, for instance, they can quiet activity in the cingulate. That raises the possibility that killers' aberrant brain activity is itself the result of experiences they had or thoughts they thought, rather than something that was wired in at birth. Similarly, it used to be thought that testosterone drove aggression, and more testosterone drove more aggression. In fact, however, individual differences in testosterone levels (as long as they are within 20 percent to 200 percent of normal) do not cause differences in levels of aggression, nor do changes in a man's testosterone levels over time predict changes in aggression, Stanford University neuroscientist Robert Sapolsky noted in his book "The Trouble With Testosterone." Only levels at least four times the norm (as can occur with "roid rage") spell trouble. And just as experiences can alter brain circuitry, so behavior can alter biology: aggression can raise testosterone levels even more strongly than testosterone raises aggression.

"It would be so nice if there were a single gene or hormone or neurotransmitter or part of the brain that was it, the cause, the explanation" of violence, Sapolsky wrote. But "our behavioral biology is usually meaningless outside the context of the social factors and environment in which it occurs." Which means that the search for the root causes of violence must move up one level, to psychology—the interaction of biology and the life one leads.

Forensic psychologists have tried to create a profile of a "typical" mass murderer, with some success. More than 90 percent of killers, and even more mass murderers, are male. (Though that may change, just as there are now female suicide bombers in the Mideast.) Mass killers are usually 25 to 35, though school shooters are younger. Few have a serious criminal record. Some kill for revenge, others for fame. Some give off obvious warning signs like Cho's violence-filled writings, others strike unexpectedly. Some kill people they know, others target anyone handy. Rather than being smooth, manipulative psychopaths, says Louis Schlesinger, professor of forensic psychology at the John Jay College of Criminal Justice in New York, mass killers tend to be aggrieved, hurt, clinically depressed, socially isolated and, above all, paranoid.