In the past week, at least 12 people were killed in Tehran and eight killed in London by attacks claimed by the Islamic State of Iraq and the Levant (ISIL).
In an interview recorded before these attacks, we asked what drives attackers in Europe and whether ideology is a bigger factor than social or economic circumstances.
"It's not necessarily bigger, but it's very important," says Gilles Kepel, scholar and author of The Roots of Radicalism. "You cannot shun it, and you can't shun the social factor either, you know."
In this week's Headliner, Kepel speaks about what he sees as the radicalisation of some young Muslims and why he disagrees with fellow scholar Olivier Roy, who says that young idealists who in the past would have turned to Marxism or anarchism are today joining the likes of ISIL.