"I had to do it. What other choice did you give me?"

These words, spoken by Cho Seung-Hui on a video in between the two sets of killings at Virginia Tech last week, raise more questions than answers. What made him believe that such a tragic act was necessary? Was he a psychopath, a man who killed in cold blood or in anger set off by the slightest provocation? Did he embody what most religions would simply classify as "evil"?

Psychiatrists and neuroscientists are making extraordinary advances in understanding the psychopathic or sociopathic mind, a mind that lacks empathy, compassion, fear, or remorse. In some of the most exciting research, advanced brain-imaging techniques are revealing that certain sections of psychopaths' brains seem to be misfiring.

Using functional magnetic resonance imaging (fMRI), researchers in the United States, Germany, and elsewhere have started taking scans of the brains of psychopaths while the patients view horrific images, such as photographs of bloody stabbings, shootings, or evisceration. When normal people view these images, fMRI scans light up to indicate heavy brain activity in sections of the emotion-generating limbic system, primarily the amygdala, which is believed to generate feelings of empathy. But in psychopathic patients, these sections of the amygdala remain dark, showing greatly reduced activity or none at all. This phenomenon, known as limbic underactivation, may indicate that some of these people lack the ability to generate the basic emotions that keep primitive killer instincts in check.

Other researchers see similar deficits from fMRI scans of the frontal cortex, part of the reasoning center of the brain, which helps regulate impulsive and irrational actions. These researchers say that frontal-deficit syndrome creates a psychopathic inability to rein in overly emotional, impulsive, and violent reactions to the slightest provocation.

James Blair, head of the National Institute of Mental Health's Unit on Affective Cognitive Neuroscience, believes that a dysfunctional amygdala affects the frontal cortex. In just-completed studies of psychopathic brains, to be published late this year or early next, Blair's fMRI scans show that a lack of normal activity in the amygdala is mirrored in the frontal cortex. He believes that the amygdala forwards the wrong signals to the frontal cortex.

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."

Copyright Technology Review 2007.