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In the wake of tragedy, come the inevitable questions: What makes a killer? Is there a switch that turns on a rampage? And why, why, did they do it?
"You can just say the person's evil," said Adrian Raine, criminologist and the author of a new book "The Anatomy of Violence." "I think that's 13th century thinking. I think we've moved beyond that."
Raine has spent more than three decades studying cold-blooded killers. He says there are biological explanations for violence. Raine is convinced that brain dysfunction may – in part – explain the terror unleashed in Boston.
"Were they completely normal people ... who just decided one day, 'You know what, we want to create mayhem?'" said Raine. "I don't think so. I think it's more complicated than that."
Raine says he first saw echoes of his work with violent criminals when the FBI released images and video of Dzhokhar Tsarnaev in the aftermath of the attack.
"What struck me quite forcibly was the notion of one of these bombers setting the bomb ... and while others were running away, he was just walking away as cool as a cucumber. I mean, that really struck me, because I've seen this before in psychopaths and murderers in prison," said Raine.
And then there were the boxing photos of Tamerlan Tsarnaev.
"We've found a neurological abnormality in the brain that predisposes to violence and psychopathy that's also been found in boxers," said Raine.
When areas of the brain responsible for emotions don’t grow to normal size, emotional response to what should be fearful situations changes.
"That gives rise to a lack of fear, and a psychopathic like personality who could go and kill a number of people and maybe not a have a sense of shame or remorse or guilt about doing that," said Raine.
Another seat of fear in the brain is an almond-shaped structure called the amygdala. According to Raine's studies, certain areas of the brain have shrunk in psychopaths, making the amygdala dramatically smaller.
Raine pointed to a part of the brain that he said is involved with making decisions as to whether an act is right, or wrong.
"But if that emotional engine that holds many of us back from something terrible ... if that's broken, then perhaps an individual is more likely to perpetrate a horrific act like the Boston bombings," said Raine.
Raine is quick to point out that his musings are just that. No one knows what is going on in the brains of the alleged Boston bombers. And he says mass killers, terrorists, are a rare breed, not as well studied as people who commit single, impulsive acts of violence.
Still, this early science may, in the near future, provide answers to the questions that nag us when violence strikes.