We exploit the exogenous variation in peer groups generated by high school to college transitions to study the theoretical predictions of Battaglini, Benabou and Tirole's (2005) model of self-control in peer groups. We find evidence consistent with the two key predictions of this theory regarding the relationship between an agent's expected self-control problems and the size and composition of his or her social circles: (i) students embedded in social circles have more self-control than those who are alone and their self-control is increasing in the size of their social group; (ii) students' self-control is, however, a non-monotonic hump-shaped function of the average self-control of their friends.