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For decades researchers, academics and psychologists have wanted to know what kind of person becomes a terrorist. If there are pre-existing traits which make someone more likely to kill for their beliefs \u2013 well, that would be worth knowing. In this edition of The Inquiry \u2013 part of the BBC World Service Identity Season \u2013 we tell the story of that search for a \u2018terrorist type\u2019. It\u2019s a story which begins decades ago. But, with the threat from killers acting for so-called Islamic State, finding an answer has never felt more pressing.

(Photo: Somali soldiers stand at the scene of car bomb at a restaurant in Mogadishu, 2016. Militant Islamist group al-Shabab claimed responsibility for the attack. Credit: Getty Images)