While consistent with the theory that disinhibition accounts for comorbidity among substance use disorders, the results are not entirely inconsistent with the gateway hypothesis (34). The gateway hypothesis holds that using one drug leads to using other drugs, perhaps due to the experienced high and an increased desire to obtain bigger and better highs. If correct, the theory would predict the opposite pattern of comorbidity we observed; that is, correlations should be low at younger ages and increase over time. At younger ages, most people would not yet have used their first or second gateway drug, and would not have had time to explore other drugs. Over time, the correlations among drugs should increase as the initial drug use would cause them to use other drugs. In fact, we found that as drug dependence symptoms increased (Figure 1), the magnitude of the associations among different drugs declined (Figure 3); opposite the pattern predicted by the gateway theory. This conclusion is consistent with a growing body of literature inconsistent with parts of the gateway theory (1, 9, 35–37). That said, the
