Social identity theory is a theory used to explain aspects of intergroup behaviour. The term “social identity” refers to an individual’s perception of self in relation to others, given that the individual perceives that they are a member of a certain social group. Social identity theory, then, is a theory which predicts the occurrence of certain inter-group behaviours based on the perceived differences in status attributed to each individual making up the group (1). Related to social identity theory is intergroup bias, in which individuals tend to favour and are biased towards members of their own group and may display prejudices against those who are not members of that group.
(1) Tajfel, H. & Turner, J. C. (1979). “An Integrative Theory of Intergroup Conflict“. In W. G. Austin & S. Worchel (Eds.), The Social Psychology of Intergroup Relations. Monterey, Calif.: Brooks/Cole Pub. Co. pp 33-47.