Despite the serious nature of their offences, psychopathic criminals get let out of prison sooner than others - in Canada, at least.
Psychopaths tend to be unusually adept at manipulating others, and even the legal system, to their advantage. "In prison, they push administrators to gain better food, resources, or to work outside on road crews," says Kent Kiehl of the University of New Mexico in Albuquerque.
To find out how effective psychopaths are at duping parole boards, Steve Porter of Dalhousie University in Halifax, Nova Scotia, studied records of 310 male inmates from a Canadian prison. He found that those termed psychopaths, based on a standard psychological test, were up to 2.5 times as likely as other prisoners to get out of jail early. They were also "much more likely to then violate their parole than non-psychopaths", he says.
Bob Hare of the University of British Columbia in Vancouver, who created the psychological tests, says parole boards may need help to tackle the problem. "Many prison officials have told me that they need better training about what a psychopath is," he says.