Evolutionarily stable in-group favoritism and out-group spite in intergroup conflict.

We study conflict between two groups of individuals. Using Schaffer's (1988) concept of evolutionary stability we provide an evolutionary underpinning for in-group altruism combined with spiteful behavior towards members of the rival out-group. We characterize the set of evolutionarily stable combinations of in-group favoritism and out-group spite and find that an increase in in-group altruism can be balanced by a decrease in spiteful behavior towards the out-group.