What makes us good or evil? It’s a simple but deeply unsettling question. One that scientists are now starting to answer.

Horizon meets the researchers who have studied some of the most terrifying people behind bars – psychopathic killers.

But there was a shock in store for one of these scientists, Professor Jim Fallon, when he discovered that he had the profile of a psychopath. We meet the scientist who believes he has found the moral molecule and the man who is using this new understanding to rewrite our ideas of crime and punishment.

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