War promotes altruism and vice-versa

I'll have to take a closer look at how they come up with "conditions likely to have been experienced by late Pleistocene and early Holocene humans" when I have more time... other than that, this seems to make intuitive sense.

The Coevolution of Parochial Altruism and War

Jung-Kyoo Choi and Samuel Bowles

Science 26 October 2007 ; 318:636-640

Abstract: Altruism—benefiting fellow group members at a cost to oneself—and parochialism—hostility toward individuals not of one's own ethnic, racial, or other group—are common human behaviors. The intersection of the two—which we term "parochial altruism"—is puzzling from an evolutionary perspective because altruistic or parochial behavior reduces one's payoffs by comparison to what one would gain by eschewing these behaviors. But parochial altruism could have evolved if parochialism promoted intergroup hostilities and the combination of altruism and parochialism contributed to success in these conflicts. Our game-theoretic analysis and agent-based simulations show that under conditions likely to have been experienced by late Pleistocene and early Holocene humans, neither parochialism nor altruism would have been viable singly, but by promoting group conflict, they could have evolved jointly.