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 …