A theory of social categorization based on the concept of social identity, the part of the self-concept that derives from group membership. According to the theory, social categories, including large groups such as nations and small groups such as clubs, provide their members with a sense of who they are, and social identities not only describe but also prescribe appropriate behaviour, and membership of the social category of ‘student’, for example, determines not only how members define and evaluate themselves but also how others define and evaluate them. According to the theory, the basis of social prejudice is the enhancement of self-esteem by discrimination against out-groups. The theory was originally formulated in 1978 by the British-based Polish psychologist Henri Tajfel (1919–82). See also jigsaw classroom, minimal group situation, Robbers Cave experiment, stigmatize. SIT abbrev.