We live in a world polarized by religion, nationality, political ideology, race, ethnicity, sex, social class, and many other group distinctions too numerous to mention. These social groups shape our identities and our lives. All of these social groups are characterized by membership criteria and boundaries—they include some people and exclude others. Although it is not logically necessary for these boundaries to imply any tension between groups, in practice, relations between groups are far more likely to be antagonistic than cooperative. Social identity theorists argue that one reason for intergroup antagonism is the psychological benefits derived from being a member of a social group, particularly those associated with identification with ingroups (Tajfel & Turner, 1986). These benefits include acceptance, belonging, and social support, as well as a system of roles, rules, norms, values, and beliefs to guide behavior. Social groups also provide our lives with meaning by boosting our self-esteem (Crocker & Luhtanen, 1990), increasing our sense of distinctiveness from others (Turner, Hogg, Oakes, Reicher, & Wetherell, 1987), and making us more certain of the social world and our place within it (Abrams & Hogg, 1988). Because of the needs they fill, groups are as dear to us as life itself, and we fear their destruction almost as much as we fear our own. As a result, we tend to favor our own group and exhibit hostility toward other groups, especially during dangerous or contentious times (Branscombe, Ellemers, Spears, & Doosje, 1999; Tajfel & Turner, 1986).