We develop a Roy model of social interactions in which individuals sort into peer groups based on comparative advantage. Two key results emerge: First, when comparative advantage is the guiding principle of peer group organization, the effect of moving a student into an environment with...

This paper develops a model of social interactions and endogenous poverty traps. The key idea is captured in a framework in which the likelihood of future social interactions with members of one's group is partly determined by group-specific investments made by individuals. I prove three main...