The Coevolution of Parochial Altruism and War , Science
Excerpts: 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.
* 35 The Coevolution of Parochial Altruism and War, Jung-Kyoo Choi , Samuel Bowles, 07/10/26, Science : 636-640.