Although books and movies might suggest otherwise, catching criminals is of course less about hunches and gut instinct, and more about gruelling analysis of hard data - including when it comes to the psychological side. David Canter is an Emeritus Professor of Psychology at the University of Liverpool, who took exactly this approach in the 1980s when he was asked to help police track down a serial killer. David had built his reputation as an environmental psychologist, studying how people behave in emergency situations and even advising the government on how to prevent disasters in football stadiums. But when he was asked to apply his unique methodology to help track down a serial killer and rapist, he realised that data-driven analysis of a suspect's behavioural patterns could be invaluable to any investigation. David tells Jim Al-Khalili about how he founded the field of investigative psychology - and went on to advise on major criminal investigations, write award-winning books and even make time to pursue a parallel life as a composer.