Criminals such as Paul Bernardo, Ted Bundy and Clifford Olson, who scored high on psychopathy checklists, were known to be callous and unemotional. Psychopaths derive pleasure from being manipulative and use premeditated aggression to get what they want with no regard for those who are hurt.
The search for what makes them tick has shown some physical differences in their brains such as reductions in grey matter.
Now researchers in London, Montreal and Bethseda, Md., have used functional MRI imaging to assess how the brains of 12 violent criminals with psychopathy, 20 violent criminals with antisocial personality disorder but not psychopathy (such as those with a history of impulsivity and risk-taking), and 18 healthy people who were not criminals responded differently to rewards and punishment.
"In the room with them, there's the sense that the weight of what they've done and the deleterious effect this is having on their lives doesn't really hold for them," said Dr. Nigel Blackwood of King's College London, a senior author of the paper in Wednesday's issue of Lancet Psychiatry.
It's only at the moment in the scanner when the sanction of lost points cues them to change their behaviour that the differences between violent psychopaths and those with antisocial personality disorder appear.
"They're not simply insensitive to punishment," Blackwood said. "There's a very different organization of their reinforcement learning system that shapes their behaviour."
The findings could have implications both for treating incarcerated psychopaths and to prevent children showing callous tendencies from progressing to psychopathy.
In Canada, psychopathy occurs in about one per cent of the population. In federal prisons, it's about 25 per cent, said Michael Woodworth, a psychologist at the University of British Columbia Okanagan, who has worked on research projects with Correctional Service Canada.
Standard cognitive behavioural treatments aren't considered effective in criminals with psychopathy.
"Perhaps at the youth level, if we can start to learn more about some of these underlying motivations and processing of information that we could develop some way to recognize and prevent them from behaving in that manner," Woodworth said.
The goal is to find ways to help those with psychopathic tendencies to fulfil their needs in less criminal ways to prevent harm to others.
Rather than being "doomed from the womb," Woodworth said the brain is particularly plastic early in life. The biological fact offers an opportunity to intervene in children with conduct disorders before the path to psychopathy is set.
For instance, perhaps children with these tendencies would benefit if their parents stressed rewards for good behaviour instead of just sanctions like timeouts, the researchers said.