Although the correlation between personal and perceived peer substance use remains among the strongest in criminology, the discriminant validity of personal and perceived peer measures remains to be formally tested via confirmatory factor analysis. Further, only limited research has attempted to discern whether substance users seek out similar others rather than being influenced by the substance use that they perceive among their peers. Finally, research has yet to isolate, via panel analysis, the reciprocal relationship between personal substance use and perceived peer attitudes. The present study addresses each of these issues using National Youth Survey data. Results reveal that personal substance-related behavior and perceived peer behavior/attitudes bear only minimal discriminant validity and that, as predicted by Gottfredson and Hirschi’s General Theory of Crime, selection provides a better explanation of their correlation than does socialization.