"Our key findings were that 1) there are no unique factors distinguishing the gateway sequence and the reverse sequence that is, the sequence is opportunistic; 2) the gateway sequence and the reverse sequence have the same prognostic accuracy; and 3) a sizable proportion of substance users begin regular consumption with an illicit drug. These results, considered in the aggregate, indicate that the gateway sequence is not an invariant pathway and, when manifest, is not related to specific risk factors and does not have prognostic utility. The results of this study as well as other studies demonstrate that abusable drugs occupy neither a specific place in a hierarchy nor a discrete position in a temporal sequence. These latter presumptions of the gateway hypothesis constitute what Whitehead referred to as the 'fallacy of misplaced connectedness,' namely, asserting 'assumptions about categories that do not correspond with the empirical world.'"
Ralph E. Tarter, Ph.D., Michael Vanyukov, Ph.D., Levent Kirisci, Ph.D., Maureen Reynolds, Ph.D., and Duncan B. Clark, M.D., Ph.D. Predictors of Marijuana Use in Adolescents Before and After Licit Drug Use: Examination of the Gateway Hypothesis. American Journal of Psychiatry 2006 163:12, 2134-2140.