The widely documented “outgroup homogeneity effect” in psychology refers to people’s tendency to view members of groups to which they do not belong (outgroups) as more similar to one another than members of their own groups (ingroups). Here, I present evidence for a novel but related phenomenon: people tend to view members across different minority groups as collectively more similar to one another than members of the majority group are to one another. Across 13 studies, I demonstrate a robust “minority-groups homogeneity effect” among participants from both majority and minority groups (albeit less consistently among the latter) and along different dimensions of identity. In Chapter 1, I provide an overview of the theoretical underpinnings of people’s perceptions of majority and minority groups, especially in the context of shifting demographics in the United States. In Chapter 2, I describe studies that examined this effect among members of different majority groups in terms of race and ethnicity, sexual orientation and gender identity, and minimal group assignment (i.e., fictional groups), finding evidence of a minority-groups homogeneity effect in all contexts. In Chapter 3, I present results from studies that examined this effect among minority group members, finding a minority-groups homogeneity effect among minority participants based on sexual orientation and nationality but not based on race/ethnicity. In Chapter 4, I describe studies designed to investigate the mechanisms underlying the minority-groups homogeneity effect and some of its implications. Finally, in Chapter 5, I discuss pathways for future research aimed at furthering our understanding of why this effect emerges and how perceptions of the similarity of minority groups might influence intergroup attitudes and behaviors.