Homophily—our tendency to form ties with people like ourselves—is a near-universal feature of human social networks. Scholars often assume that ties between members of different racial, ethnic, or religious groups will not only be rarer but also weaker on average, thereby exacerbating conflict, curtailing the flow of information, and impeding the provision of public goods. Yet relationships that lack emotional depth may still be powerful in other ways. Drawing on case studies from the 1992–5 Bosnian War, the 2020 Black Lives Matter protests, and a conflict prevention experiment in 56 New Jersey middle schools, I examine the power and resilience of cross-group ties in the face of deep social cleavages. In my first chapter, I use data from a census, nationwide door-to-door survey, and 160 interviews to show that cross-group ties played an essential role in aiding and protecting tens of thousands of Bosnians faced with persecution and wartime violence. Although ties between Muslims, Serbs, and Croats were generally weaker than ties within these groups, the strength of these ties appears to have made little difference in people’s willingness to help one another.  In my second chapter, I use data from a massive online survey to show that ties between Black and non-Black Americans, both strong and weak, were instrumental in mobilizing unprecedented non-Black turnout in racial justice protests. In my third chapter, I extend Fisher randomization techniques for measuring peer effects in networks to gauge the relative effectiveness of strong, weak, and cross-group ties. I test these new causal inference methods though simulations as well as a replication analysis of an intervention aimed at reducing adolescent conflict. Specifically, I examine whether students enrolled in the anti-conflict program reshaped the attitudes of both close and non-close friends, including those of a different race or gender. Taken together, these three studies suggest that even weak cross-group ties contribute to our cross-cleavage capital, enabling us to summon assistance, mobilize allies, and transmit social norms across a conflict cleavage. As a result, policies and programs promoting intergroup contact may still be successful at reducing conflict even in the absence of deep emotional bonds.

Dissertation Committee:

David Lazer (chair), University Distinguished Professor, Network Science Institute, Northeastern University

Jennifer Larson, Associate Professor of Political Science, Vanderbilt University

Laia Balcells, Provost's Distinguished Associate Professor, Department of Government, Georgetown University

Risa Kitagawa, Assistant Professor of Political Science and International Affairs, Northeastern University

Cassie McMillan, Assistant Professor of Sociology and Criminology and Criminal Justice, Network Science Institute, Northeastern University