A prototype model of risk behavior is described and was tested in a longitudinal study of 679 college students, beginning at the start of their freshman year. Perceptions of the prototype associated with 4 health risk behaviors (smoking, drinking, reckless driving, and ineffective contraception) were assessed along with self-reports of the same behaviors. Results indicated that prototype perception was related to risk behavior in both a reactive and a prospective manner. That is, perceptions changed as a function of change in behavior, and perceptions predicted those behavior changes as well. This prospective relation was moderated by social comparison, as the link between perception and behavior change was stronger among persons who reported frequently engaging in social comparison.