In the more recent literature on terrorism (from the 1960s onwards), some of the more common explanations for the use of terror by non-state actors were psychological ones, in particular the search for the abnormal personality traits that would explain terrorism. One example of this approach is that of Ferracuti and Bruno, who conducted research on right-wing terrorist groups in Italy. Their conclusion was that those responsible were ‘frequently psychopathological’. Such approaches continue to find support today. For example, Walter Laqueur recently wrote that: ‘Madness, especially paranoia, plays a role in contemporary terrorism. Not all paranoiacs are terrorists, but all terrorists believe in conspiracies by the powerful, hostile forces and suffer from some form of delusion and persecution mania … madness plays an important role, even if many are reluctant to acknowledge it.’

The reluctance to which Laqueur refers is in fact entirely appropriate, for empirical enquiry offers very little support for such a claim. Over three decades of research by psychologists have demonstrated to all but the most determined advocates that such an approach is misplaced. There is for instance Wilfried Rasch, who examined in detail eleven German terrorists captured in the 1970s. His seminal paper, written on the basis of his expertise as a professor of psychiatry and unparalleled levels of primary investigation, found that those terrorists he examined exhibited no signs of psychological disorders. Silke reports a similar dynamic with the prosecutions of surviving Nazi leaders in Nuremberg.