Well, why has violence declined? No one really knows, but I have read four explanations, all of which, I think, have some grain of plausibility. The first is, maybe Thomas Hobbes got it right. He was the one who said that life in a state of nature was "" solitary, poor, nasty, brutish and short. "" Not because, he argued, humans have some primordial thirst for blood or aggressive instinct or territorial imperative, but because of the logic of anarchy. In a state of anarchy, there's a constant temptation to invade your neighbors preemptively, before they invade you. More recently, Thomas Schelling gives the analogy of a homeowner who hears a rustling