While Grossman presents strong evidence, there are equally strong arguments against the idea that our brutality is a recent phenomenon. In Jared Diamond's book The Third Chimpanzee, he quotes studies showing that in pre-civilized societies, murder is the single greatest cause of death in males. The fact that the first example drawn on to show our modern barbarity is nearly two thousand years old shows that it is hardly a recent change. Humans have reveled in violence against others from time immemorial. Ancient sacrifice rituals, gladiatorial matches, autos-de-fe, contact sports; there is really no end to the number of examples. I think that the only conclusion is that humans are innately violent. Whether it evolved as a self-defense mechanism, a way to ensure the elimination of rivals, or even an accident of neural wiring, we are now by our nature potential killers, with or without cultural encouragement.