Research concerned with the GH, during the 35 years since it was advanced (Kandel, 1975), has commanded substantial attention due to the high practical value that has been ascribed to the observation of a sequential order in drug use initiation. GH was predated by the similar “stepping-stone” theory that first appeared in the 1930s and assumed that consumption of a “soft” drug such as marijuana inexorably sets an individual on a trajectory to addiction to hard drugs. GH relaxes the inevitability assumption, but still posits that substance use starting with a licit substance and progressing to marijuana leads to use of other, “harder” drugs. Marijuana is thus designated the “gateway” drug in this progression, although originally, and sometimes still, this role has been assigned to alcohol and tobacco (Grunberg and Faraday, 2002; Kandel, 1975). Because it is frequently observed that “[v]ery few individuals who have tried cocaine and heroin have not already used marijuana; the majority have previously used alcohol or tobacco” (Kandel, 2003, p. 482), the conclusion is drawn (Kandel, 2002a) that “there is a progressive and hierarchical sequence
