Who theorized the social self?
Social identity theory
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]
Kurt Lewin
Lewin is often recognized as the "founder of social psychology" and was one of the first to study group dynamics and organizational development. A "Review of General Psychology" survey, published in 2002, ranked Lewin as the 18th-most cited psychologist of the 20th century.