Ethnic and religious conflict is an unfortunate, yet prominent, aspect of society. Research on the development of social categorization in childhood can inform our understanding of the nature of social bias in adulthood, and how it may be prevented. I plan to pursue a research program exploring the development of young children’s social reasoning based on ethno-religious identity in Northern Ireland. One series of questions in this research program will assess children’s beliefs about the stability of group membership over time. A second will assess children’s social preferences and behavioral evaluation of others based on their social category membership. The goal of this research is two-fold. First, research on the developmental origins of prejudice may be critical in both understanding and preventing segregation and violence in a society with a long history of conflict. Second, this research will offer theoretical insight into the nature of children’s social group formation and early in-group preferences more generally. I will employ quantitative methods in developmental psychology to draw empirical conclusions about children’s cognitive and social reasoning about category membership and social group division, with an eye towards how such processes may be malleable as a result of exposure to different environments.