Latest research summarized: “Flawed brain chemistry, brain damage, genetic defects, an unhealthy psychological environment— take them individually or mix them together and you may have the right ingredients for violent behavior, reports a variety of researchers.” EurekAlert! Not very profound — these are, pretty exhaustively, the domains of explanation in behavioral science. I guess what this review article reflects is that behavioral science is beginning to grapple more robustly with the problem.