(NYTIMES) - I will never get over the fact that American society seems to produce a steady stream of young men who think it is heroic to murder innocent people. I read their histories. I look at the social science research. I've tried to understand the typical pathway they take to get to their evil behaviour. The common thing to say about mass shooters is that they have mental health issues, but that's often misleading.

This has been studied in a variety of ways. A majority of mass shooters do not have diagnosed mental illness. It's mostly the circumstances that drive them to do what they do, not an underlying disease. The more accurate place to start is with something George Bernard Shaw wrote many years ago: "The worst sin towards our fellow creatures is not to hate them, but to be indifferent to them: That's the essence of inhumanity."