No offense against another human being inflicts greater costs than killing. Simply put, it's bad to be dead. Nonetheless, hundreds of thousands are murdered every year; tens of millions over the past century. From baby killing to genocide, from Susan Smith to Osama bin Laden, people in every culture experience the urge to kill. Some act on it. They do so despite legal injunctions, religious prohibitions, cultural interdictions, the risk of retaliation, and the threat of spending life in a cage. Dead bodies, a trail of grief, and a thirst for vengeance lie in their wake.
Many believe that they already know the answer to the question of cause. But existing theories woefully fail to explain why people murder. Theories that invoke violent media messages, for example, cannot explain the high rates of homicide among tribal cultures that lack media access. Theories that invoke uniquely modern causes cannot explain the paleontological record — ancient skulls and skeletons that contain arrow tips, stone projectiles, and brutally inflicted fractures. The stones and bones of the past leave no doubt that murder has been a persistent problem of social living throughout human history. We need to understand why.