Among the most chilling developments in the rise of the Islamic State is that so many citizens of Western countries have joined the group's ranks, becoming suicide bombers and beheading hostages.
Why do hundreds of Muslims, many of them educated and from middle-class backgrounds, leave comfortable Western democracies to join a brutally barbaric movement? What makes young men and women susceptible to the extremist Islamist message?
As he watched the rise of the Nazis in the 1930s, Sigmund Freud described the dangerous appeal of authoritarian leaders and the satisfying self-aggrandizement that their followers experience when they subsume their personalities in an ideology or group.