Andrew Marr explores how far empathy, or the lack of it, can explain cruelty. Simon Baron-Cohen proposes turning the focus away from evil or specific personality disorders, and to understand human behaviour by studying the 'empathy circuit' in the brain. Gwen Adshead, a forensic psychotherapist at Broadmoor Hospital and the crime writer Val McDermid question whether this would help in their line of work, and the philosopher Julian Baggini tries to pin down what we mean when we talk about the self.

Producer: Katy Hickman.