Human assemblages to which a person could "belong".
(Eric Hobsbawm. (1994). Age of Extremes. London: Michael Joseph ltd. p. 428.)
Social identity is commonly defined as a person's sense of self derived from perceived membership in social groups. When we belong to a group, we are likely to derive our sense of identity, at least in part, from that group. While standard economic analysis focuses on individual-level incentives in decision making, group identity has been shown to be a central concept in under- standing phenomena in social psychology, sociology, anthropology, and political science. It is used to explain such phenomena as ethnic and racial conflicts, discrimination, political campaigns (Rose McDermott, forthcoming), and the formation of human capital (James Coleman 1961 ). Social identity theory was developed by Henri Tajfel and John Turner (1979) to understand the psychological basis for intergroup discrimination. According to this theory, social identity has three major components: categorization, identification, and comparison. The first component, categorization, is the process of putting people, including ourselves, into categories. Labelling someone as a Muslim, a female, or a soldier is a way of defining these people. Similarly, our self-image is associated with what categories we belong to. Social psychology experiments show that people quickly and easily put themselves and others into basic categories. The second component, identification, is the process by which we associate ourselves with certain groups. Ingroups are groups we identify with, and outgroups are ones we do not identify with. The third component, comparison, is the process by which we compare our groups with other groups, creating a favorable bias toward the group to which we belong.
Chen,Yan and Sherry Xin Li. (2009) “Groups Identity and Social Preferences”. American Economic Review, 99:1, 431-457.