In July 2012, a young man with a history of erratic behavior opened fire in a movie theater in Aurora, Colorado, killing 12 people and injuring 70 more. That December, another young man — who’d struggled with psychiatric and social problems since childhood — murdered his mother and then shot and killed 20 children and six adults at Sandy Hook Elementary School in Newtown, Connecticut. As politicians, the news media and the public tried to make sense of these events, a question arose again...
