In this study, dormitory room and social group assignment data from a college are used to investigate peer effects on college students' decisions to switch majors. Results reveal strong evidence of such peer effects at both the room and the social group level. Most notably, at the room level, the dense concentration of same-major roommates deters students from switching majors; having one or two same-major roommates has no significant effect on major switching, indicating strong nonlinearity of peer effects at the room level. Such nonlinearity is not observed among social group members. Results also reveal evidence that students' choices of new majors are affected by peers' majors. Peers are more likely to choose the same destination majors than nonpeers. In choosing their new majors, students do not necessarily follow their peers indiscriminately. Their decisions seem to be influenced more by short-term academic requirements than by long-term job prospects. Finally, peer effects on major switching and major choices are stronger at the dormitory room level than at the social group level in most cases.