Scientists who scanned the brains of men convicted of murder, rape and violent assaults have found the strongest evidence yet that psychopaths have structural abnormalities in their brains.
The researchers, based at King's College London's Institute of Psychiatry, said the differences in psychopaths' brains mark them out even from other violent criminals with anti-social personality disorders (ASPD), and from healthy non-offenders.
Nigel Blackwood, who led the study, said the ability to use brain scans to identify and diagnose this sub-group of violent criminals has important implications for treatment.
The study showed that psychopaths, who are characterized by a lack of empathy, had less grey matter in the areas of the brain important for understanding other peoples' emotions.
While cognitive and behavioral treatments may benefit people with anti-social personality disorders, the same approach may not work for psychopaths with brain damage, Blackwood said.
"To get a clear idea of which treatments are working, you've got to clearly define what people are like going into the treatment programs," he said in a telephone interview.
Essi Viding a professor in the psychology and language sciences department of University College London, who was not involved in Blackwood's study, said it provided "weighty new evidence" about the importance of distinguishing psychopathic from non-psychopathic people rather than grouping them together.
The findings also have implications for the justice system, because linking psychopathy to brain function raises the prospect of arguing a defense of insanity.
Interest in what goes on inside the heads of violent criminals has been sharpened by the trial of Anders Behring Breivik, the Norwegian who massacred 77 people last July.
Two court-appointed psychiatric teams who examined Breivik came to opposite conclusions about his mental health. The killer himself has railed being called insane.
Blackwood's team used magnetic resonance imaging (MRI) to scan the brains of 44 violent adult male offenders in Britain who had already been diagnosed with anti-social personality disorders.
The crimes they had committed included murder, rape, attempted murder and grievous bodily harm.
Of the 44 men scanned, 17 met the diagnosis for ASPD plus psychopathy and 27 did not. The researchers also scanned the brains of 22 healthy non-offenders.
The results showed that the psychopaths' brains had significantly less grey matter in the anterior rostral prefrontal cortex and temporal poles than the brains of the non-psychopathic offenders and non-offenders.
These areas of the brain are important for understanding other people's emotions and intentions, and are activated when people think about moral behavior, the researchers said.
Damage to these areas is linked with a lack of empathy, a poor response to fear and distress and a lack of self-conscious emotions such as guilt or embarrassment.
Lindsay Thomson, a professor of forensic psychiatry at the University of Edinburgh who was not involved in this study, said Blackwood's findings add to evidence that psychopathy is a distinct neurodevelopmental brain disorder.
Research shows that most violent crimes are committed by a small group of persistent male offenders with ASPD.
In England and Wales, for example, around half of male prisoners meet diagnostic criteria for ASPD. A major review of studies covering 23,000 prisoners from 62 countries conducted in 2002 found that 47 percent had anti-social personality disorder.
Such people typically react in an aggressive way to frustration or perceived threats, but most are not psychopaths, the researchers wrote in a summary of their study, which was published in the Archives of General Psychiatry journal.
There are clear behavior differences among people with ASPD depending on whether they also have psychopathy. Their patterns of offending are different, suggesting the need for a separate approach to treatment.
"We describe those without psychopathy as 'hot-headed' and those with psychopathy as 'cold-hearted,' " Blackwood explained.
"The 'cold-hearted' psychopathic group begin offending earlier, engage in a broader range and greater density of offending behaviors, and respond less well to treatment programs in adulthood compared to the 'hot-headed' group."