For most of us, violence is as inescapable as it is inexplicable. From street gangs to soccer hooligans, Charles Manson to Virginia Tech, acts of extreme violence leave all of us asking why? Traditionally we have viewed crime as a social disease and that only by improving the environment in which we raise our children will we make them less violent. But recent scientific breakthroughs reveal a more complex story, raising the possibility of treating violence like a medical condition. Do the answers to creating a more peaceful society lie in our DNA? What role does our environment play in shaping us?