But it does mean that far from being an aberrant behaviour, psychopathy may be disturbingly normal.
Further research will be needed to see how the risk and social-contract modules that govern psychopathy are actually controlled.
"There is no doubt in my mind that he passes the threshold for a diagnosis of psychopathy, " he said.
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To find out, they recruited 67 prisoners and tested them for psychopathy.
All got high scores on the Psychopathy Checklist, the "gold standard" in the field, which assesses traits like lack of remorse, callousness and grandiosity.
However, I would not accept for a minute a verdict of psychopathy based on a selective combination of characteristics of half a dozen different individuals.
While environmental factors play a part, psychopathy is rooted in biology.
On an intellectual level, it asks about the origins of psychopathy.
Claims that criminals can be identified by imaging their brains, or that there could be a gene for psychopathy are "wide of the mark", it says.
They then subdivided these children into those with psychopathic traits and those without and asked, in each case, whether an individual's twin showed bad behaviour, psychopathy, or both.
Narcissism shares aspects of psychopathy.
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