Extensive research suggests that marijuana use tends to precede the use of other illicit substances among adolescents. At the same time, there remain two viable interpretations of such research. First, marijuana use may cause an increase in one's probability of using other drugs. Second, the correlation between marijuana use and other drug use may be spurious, reflecting the influence of one or more “third variables” that simultaneously cause both behaviors. The present paper provides an empirical assessment of each view using panel data from three waves of the National Youth Survey. Even after adjusting for the influence of variables derived from strain theory, social bonding theory, and differential association theory, a series of longitudinal logistic regression analyses fail to disconfirm the hypothesis that marijuana use exerts a causal influence on one's probability of using other illicit substances. A three-wave panel model adjusting for the influence of unmeasured variables yields similar results.
