Structural sources of segregation: How classroom composition shapes interethnic friendships

How does the demographic composition of social contexts affect ethnic segregation in social networks? Building on formal theory in the tradition of Peter M. Blau, we provide a theoretical analysis of how ethnic diversity and consolidation—the overlap of ethnic origin with attributes like gender or social class—influence ethnic segregation. The formal model specifies the interplay of homophily preferences and opportunity structure and yields new predictions on how diversity and consolidation interact and how their influence varies across groups and consolidated attributes. We test these hypotheses using the largest data set on offline friendship networks to date, covering more than 1,300 classroom networks in Germany. We find that opportunity structure accounts for most of the variation in interethnic ties. In line with our hypotheses, the consolidation of ethnic origin and gender promotes same-ethnic ties among majority students. As predicted, this segregating effect gets amplified in more diverse classrooms. Our analysis also helps to understand why diversity and consolidation do not affect same-ethnic ties among minority students and why ethnic segregation is not driven by the consolidation of ethnic origin and socioeconomic status. We conclude by discussing the theoretical, methodological, and substantive implications for research on diversity and network segregation.