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 increase in non-shared environmental effects observed in Figure 3 could contain etiology analogous to a gateway process, in that there are environments experienced by an individual that contribute to dependence to multiple drugs in that person, but not that person’s co-twin. While the range of possible environments is vast and the effect is small, it could very well include drug use (e.g., impaired cognition caused by neurotoxicity) and/or other risk factors for non-specific drug use such as occupational, social, and legal problems.
