The out-group homogeneity effect is the perception of out-group members as more similar to one another than are in-group members, e.g. "they are alike; we are diverse". Perceivers tend to have impressions about the diversity or variability of group members around those central tendencies or typical attributes of those group members. Thus, outgroup stereotypicality judgments are overestimated, supporting the view that out-group stereotypes are overgeneralizations. The term "outgroup homogeneity effect", "outgroup homogeneity bias" or "relative outgroup homogeneity" have been explicitly contrasted with "outgroup homogeneity" in general, the latter referring to perceived outgroup variability unrelated to perceptions of the ingroup.

The outgroup homogeneity effect is sometimes referred to as "outgroup homogeneity bias". Such nomenclature hints at a broader meta-theoretical debate that is present in the field of social psychology. This debate centres on the validity of heightened perceptions of ingroup and outgroup homogeneity, where some researchers view the homogeneity effect as an example of cognitive bias and error, while other researchers view the effect as an example of normal and often adaptive social perception. The out-group homogeneity effect has been found using a wide variety of different social groups, from political and racial groups to age and gender groups.

The out-group homogeneity effect is part of a broader field of research that examines perceived group variability. This area includes in-group homogeneity effects as well as out-group homogeneity effects, and it also deals with perceived group variability effects that are not linked to in-group/out-group membership, such as effects that are related to the power, status, and size of groups. The out-group homogeneity effect has been found using a wide variety of different social groups, from political and racial groups to age and gender groups. The implications of this effect on stereotyping have been noted.