"The gateway effect, if it exists, has at least two potential and quite different sources (MacCoun, 1998). One interpretation is that it is an effect of the drug use itself (e.g., trying marijuana increases the taste for other drugs or leads users to believe that other substances are more pleasurable or less risky than previously supposed). A second interpretation stresses peer groups and social interactions. Acquiring and using marijuana regularly may lead to differentially associating with peers who have attitudes and behaviors that are prodrug generally, not only with respect to marijuana. One version of this is the possibility that those peers will include people who sell other drugs, reducing the difficulty of locating potential supplies. If the latter is the explanation, then legalization might reduce the likelihood of moving on to harder drugs compared to the current situation."
Kilmer, Beau; Caulkins, Jonathan P.; Pacula, Rosalie Liccardo; MacCoun, Robert J.; Reuter, Peter H., "Altered State? Assessing How Marijuana Legalization in California Could Influence Marijuana Consumption and Public Budgets" Drug Policy Research Center (Santa Monica, CA: RAND Corporation, 2010), p. 42.