Conflicts based on the often strained relationships between ethnic-cultural groups in society regularly flare up (cf. the recent Black Lives Matter protests). Similarly, the topic of intergroup relations has been on the forefront of social and political psychology research since the 1950’s. However, this scholarly focus on reducing prejudice and promoting tolerance often included either individual difference (e.g., social-ideological beliefs) or contextual factors (e.g., diversity and group norms). In this project, we bring together psychological and sociological research traditions in a unified person-within-context perspective. In Research Line 1, we propose and test that, based on pre-existing individual differences, greater diversity leads to attitude polarization between individuals (making the cleavage between the political left and right even larger), but simultaneously consolidates these attitudes within individuals (solidifying the individual’s particular intergroup opinion). In Research Line 2, we hypothesize that the social groups closest to us (e.g., family, peers) provide us norms and a sense of shared reality. Such close-group climates mobilize and ‘pull’ us towards either prejudice or tolerance, above and beyond our own social-ideological beliefs. To address these focal issues and unravel how harmonious intergroup dynamics can develop, we employ a combination of cross-sectional, multilevel, and longitudinal studies in Western Europe, South Africa, and Chile.