The steep and virulent rise of terrorism ranks among the more disturbing trends in the world today. According to the 2015 Global Terrorism Index, terror-related deaths have increased nearly 10-fold since the start of the 21st century, surging from 3,329 in 2000 to 32,685 in 2014. Between 2013 and 2014 alone, they shot up 80 percent. For social psychologists, this escalation prompts a series of urgent questions, just as it does for society as a whole: How can extremist groups treat fellow human beings with such cruelty? Why do their barbaric brands of violence appeal to young people around the globe? Who are their recruits, and what are they thinking when they target innocent lives?