When do individuals radicalize?

For more than a century now, observers have been trying to understand why and how individuals become terrorists. Every time the international community was confronted with a significant wave of terrorist attacks, a sense of urgency boosted this quest for answers. Each time, fierce academic and public debates on the causes of terrorism ensued and answers varied widely, ranging from mental health problems to social injustice. What do we now know – and what don’t we known (yet)?