In-groups and out-groups
The distinction between in-groups and out-groups is a foundational concept in social psychology and sociology, describing how individuals categorize themselves and others into social groups that shape identity, perception, and behavior. An in-group is a social group with which a person psychologically identifies, while an out-group is one with which they do not identify[^c1]. Research has shown that people can form self-preferencing in-groups within minutes, even on the basis of arbitrary and meaningless characteristics[^c2], and that this categorization triggers a range of cognitive biases including in-group favoritism, out-group homogeneity perception, and out-group derogation. Historical communities organized along these lines include the German-Jewish banking dynasties of nineteenth-century New York, whose closely knit social world and relationships with both Gentile society and other Jewish communities were documented in Stephen Birmingham's 1967 book Our Crowd[^c9].
Multiple theoretical frameworks explain in-group/out-group dynamics. Social identity theory, formulated by Henri Tajfel and John Turner in the 1970s and 1980s, proposes that individuals derive part of their self-concept from perceived membership in social groups and are motivated to maintain a positive social identity through favorable intergroup comparisons[^c3]. Ethnocentrism, first systematically theorized by William Graham Sumner in 1906, describes the tendency to view one's own group as the center of everything and to regard outsiders with contempt[^c4]. However, large-scale cross-cultural evidence from 121 societies has shown that while people are slightly more prosocial toward their in-group than out-groups, those who are prosocial toward their in-group also tend to be prosocial toward out-groups, challenging the long-held premise that in-group love necessarily entails out-group hate[^c10]. The philosophical concept of othering traces how dominant groups define subordinate groups as different and inferior, from Hegel's master-slave dialectic through postcolonial theory[^c5].
In-group/out-group dynamics play a central role in contemporary politics. Populist political communication constructs an in-group of "the people" positioned against elites and out-groups[^c6], while the figure of the political outsider claims to represent ordinary people against a corrupt establishment[^c7]. The "out crowd" as a social category -- those excluded from dominant social circles -- has been documented in settings from high school hierarchies to the linguistic history of slang, where the "in crowd" was defined as "the happening people who know where it's at because they are where it's at"[^c8].
Intergroup emotions theory explains how group identification shapes emotional responses to events that affect the ingroup, driving behaviors ranging from confrontation to avoidance. Competitive victimhood -- the tendency for conflicting groups to compete over who has suffered more -- hinders reconciliation and responds dynamically to changes in intergroup threat. Collective narcissism, a defensive belief in ingroup superiority, can be triggered by categorization threat and escalates hostility toward outgroups. These contemporary extensions of the in-group/out-group framework reveal the complex ways group membership shapes cognition, emotion, and behavior across contexts.