In sociology and social psychology, an in-group is a social group to which a person psychologically identifies as being a member. By contrast, an out-group is a social group with which an individual does not identify. People may for example identify with their peer group, family, community, sports team, political party, gender, sexual orientation, religion, or nation. It has been found that the psychological membership of social groups and categories is associated with a wide variety of phenomena.

The terminology was made popular by Henri Tajfel and colleagues beginning in the 1970s during his work in formulating social identity theory. The significance of in-group and out-group categorization was identified using a method called the minimal group paradigm. Tajfel and colleagues found that people can form self-preferencing in-groups within a matter of minutes and that such groups can form even on the basis of completely arbitrary and invented discriminatory characteristics, such as preferences for certain paintings.

In neurology, there is an established literature about the innate propensity of the human brain to divide the world into us and them valence categories, where the exact membership of the in-group and out-group are socially contingent (hence vulnerable to the instruments of propaganda), and the intensity exists along a spectrum from mild to complete dehumanization of the "othered" group.