People favor members of their own group over outgroup members, even when groups are arbitrarily assigned and provide no tangible benefits, status, or resources. This ingroup bias emerges rapidly and reflects both identity-protective motivation (wanting to view one's group positively) and conformity to group norms that emphasize loyalty and ingroup favoritism.
Review minimal group paradigm experiments where trivial random assignments create favoritism; analyze how quickly ingroup identification activates these biases and whether reducing group salience decreases favoritism.
Social identity theory, your prerequisite, established that people incorporate group memberships into their self-concept and are motivated to view those groups positively. Ingroup favoritism is the behavioral expression of that motive — and the empirical evidence for it is among the most striking in social psychology precisely because of how little it takes to produce it.
The foundational demonstration comes from Henri Tajfel's minimal group paradigm. In these experiments, participants are assigned to arbitrary groups — supposedly based on aesthetic preferences or even a coin flip — with no shared history, no prior interaction, no competition, and no tangible stakes. Within minutes, participants begin allocating more resources to fellow ingroup members than to outgroup members. More striking: they sacrifice absolute gains to maximize the *difference* between what their group receives and what the outgroup receives. This is the minimal group finding: mere categorization — "you are Group A, they are Group B" — is sufficient to generate favoritism, even when the categorization is transparently meaningless.
Why is categorization alone enough? Social identity theory proposes that because group membership is part of the self-concept, the group's standing reflects on the individual. Perceiving my group as superior makes me feel better about myself. This creates a persistent, low-level motive to make favorable comparisons between ingroup and outgroup, particularly when self-esteem is threatened, when group membership is salient, or when the groups are similar enough that comparisons feel meaningful and competitive. Basking in reflected glory — feeling pride in a group's success even when one contributed nothing — is the positive side of this same dynamic.
An important distinction is that ingroup favoritism is not the same as outgroup hostility, and the two should not be conflated. The primary drive is *toward* the ingroup: to prefer, trust, and support those who share the category. Systematic derogation and hostility toward outgroups tends to require additional conditions — intergroup competition for resources, large and perceived-as-illegitimate status differences, or identity threat. Understanding this asymmetry matters for intervention: reducing ingroup bias may be possible while preserving healthy group identification, whereas reducing outgroup hostility requires addressing threat, competition, and status — a different set of levers entirely.