A complete understanding of the psychology of social identity requires not only descriptions of how social identification processes work, but also an account of why the underlying psychological mechanisms have evolved. This chapter focuses on the evolution of coalitional (or "tribal") social identity (i.e., the type of social identity associated with nationality, ethnicity, religion, and class). Coalitional social identity appears to involve a readiness to incur costs for the collective, which

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... ay yield cooperative benefits. However, it has not been obvious why reaping the benefits of intragroup cooperation would be facilitated by social identification processes. We suggest that social identity may be related to the signaling of coalitional membership and cooperative intent. Specifically, we argue that social identity may constitute a self-represented summary of the loyalty-signaling characteristics that one has acquired. Based on this hypothesized ultimate function of social identity, we derive predictions regarding the proximate psychology of social identity. We suggest that further research may examine whether social identity involves private social identities (for balancing costs and benefits of group membership) and public social identities (for strategically influencing the behavior of others).