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The Evolution of Parochial Altruism in Scarcity Systems
Parochial Altruism is the interplay between altruism towards ones own tribe, and hostility towards outside groups [1]. Individually, the act of either altruism or parochialism and the benefits thereof are weighed against the cost of such an act. In other words, the decision to engage in either one depends on whether it is more beneficial to not do it. From an evolutionary perspective, this is a weak mechanism for suitability in the gene pool. However both acts engaged in together would make for a much stronger viability in the long term, and help ensure the persistence and evolution of both.
[1]Choi, Jung-Kyoo, & Bowles, Samuel (2007). The Coevolution of Parochial Altruism and War. Science 26 October 2007: Vol. 318 no. 5850 pp. 636-640 DOI: 10.1126/science.1144237