Who theorized the social self?
Social identity theory
Henri Tajfel and John Turner

Social identity is the portion of an individual's self-concept derived from perceived membership in a relevant social group.[1] As originally formulated by social psychologists Henri Tajfel and John Turner in the 1970s and the 1980s,[2] social identity theory introduced the concept of a social identity as a way in which to explain intergroup behaviour.[3][4][5]