The finding that 23% of this sample used cannabis before alcohol contrasts with the widely held belief that the sequence of drug initiation invariantly proceeds from legal to illegal drugs. Although this is the cornerstone assumption in the gateway hypothesis (Kandel & Yamaguchi, 1999), it is important to note that several studies have revealed that many youths begin using illegal drugs before legal drugs (Young et al., 1995; Golub & Johnson, 1994; Tarter et al., 2003). Furthermore, although order of initiation of different substances has garnered enormous interest, it is noteworthy that this variable does not improve prediction of SUD beyond transmissible liability (Tarter, Kirisci, Mezzich, et al., 2012).
