All social animals fight to gain resources, using highly ritualized behaviors to assess their opponents and minimize the personal costs of injury. Changes in hormone levels and brain activity motivate and reward the winners, and minimize the costs to the losers. In a small corner of the landscape of aggressive fighting styles are an elite group of killers, animals that go beyond harming their opponents to obliterating them: ants, wolves, lions, and chimpanzees. When these species attack to kill, they typically target adult members of neighboring groups, using collaborative alliances to take out lone or otherwise vulnerable victims. The rarity and limited scope of this form of lethal aggression is indicative of monogamous thinking, and tells us something important about the economics — especially the costs and potential rewards of eliminating the enemy, as opposed to merely injuring them. Killing another adult is costly because it involves intense, prolonged combat with another individual who is fighting back. The risks of significant personal injury are therefore high, even if the potential benefit is death to an opponent. As the British anthropologist Richard Wrangham has suggested, animals can surmount these costs by attacking and killing only when there is a significant imbalance of power. This imbalance minimizes the costs to the killers and maximizes the odds of a successful kill. Still, the rarity of killing reinforces an uncontested conclusion among biologists: all animals would rather fight and injure their opponents than fight and obliterate them, assuming that obliteration is costly to the attacker. In some cases, we are just like these other animals — killophobic. Historical records, vividly summarized by Lieutenant Colonel Dave Grossman in his book On Killing, reveal that in some situations, soldiers avoid killing the enemy even though they could have. For example, despite the fact that Civil War regiments had the potential to kill 500-1000 individuals