Still, some scientists say that this focus on the amygdala is too simplistic. "I'm not sure if the amygdala is the core of the problem," says Joshua Greene, assistant professor of psychology at Harvard University. Greene says that while the amygdala may be "one of the areas compromised," the affected part of the brain might be different in different patients. Greene has not studied psychopathic patients, but he has used fMRI to look at the brains of people as they make moral decisions. He has found that either an emotional center or a reasoning center may play the dominant role, depending on the kind of moral decision being pondered.

Of course, not everyone demonstrating these brain abnormalities ends up a killer. Some individuals with limbic underactivation end up in heroic professions, becoming firefighters, police officers, or fighter pilots, possibly because of a reduced fear response and a need for strong emotional stimuli. One theory is that other triggers, such as severe childhood abuse or neglect, are needed to turn people with already suppressed emotions into cold-blooded killers.

And of course not all killers are psychopaths. Thomas Lewis, a psychiatrist who has extensively studied the research on psychopathy and who specializes in the neurochemistry of depression at the University of California, San Francisco, describes an extraordinarily rare condition in which a nonpsychopathic person can become a "rampage killer." This individual starts out severely depressed, traumatized, and suicidal, a condition that could be caused by anything from genetics to a brain tumor. Then some perceived crisis causes him or her to snap and go on a killing spree before taking his or her own life. "It's kind of like throwing a temper tantrum--only with automatic weapons," says Lewis.

Using neuroscience to understand seemingly evil acts of violence is still in its very early days. Indeed, diagnosis and prediction of killing behavior are far off into the future, if at all possible. But many brain researchers see enormous potential in the new imaging work. "We've always regarded psychopathy as completely untreatable," says Blair. "This could absolutely change that."