Warfare was sufficiently common and lethal among our ancestors to favor the evolution of what Sam Bowles, SFI Professor, calls parochial altruism, a predisposition to be cooperative towards group members and hostile towards outsiders. Biologists and economists have doubted that a genetic predisposition to behave altruistically (help others at a cost to oneself) could evolve (excepting the help extended to close genetic relatives). Skepticism among biologists arose primarily because most think that groups are not sufficiently different genetically to favor group selection (the most obvious evolutionary mechanism promoting altruism beyond the family). But both observation in natural settings and experiments (some of them by Bowles and his co authors) show that altruism is quite common among humans (much more so than in most other animals). In a series of recent papers Bowles shows that altruism could have evolved among humans as a result of the advantages that altruistic groups have in military and other competition with other groups.