One of the most enduring controversies in drug research and policy concerns whether early cannabis use predisposes to the use of other drugs such as cocaine and heroin (Hall & Lynskey, 2005; Kandel, 2002; Kandel & Faust, 1975; Morral et al., 2002). Several researchers have reported strong associations between early cannabis use and subsequent use of other drugs, even after control for a wide range of potentially confounding covariates (Fergusson & Horwood, 2000; Fergusson et al., 2002; Kandel et al., 1992). However, these analyses may not have adequately controlled for important family background factors, including potential genetic in-fluences, predisposing to both early cannabis use and later use of other illicit drugs. In an earlier study of this issue employing a discordant twin design we reported, contrary to expectations that, relative to their nonearly using co-twins, individuals who commenced cannabis use prior to age 17 had significantly elevated rates of illicit drug use and illicit drug dependence, suggesting that the observed associations between early onset cannabis use and later illicit drug use cannot be solely attributed to genetic and shared environmental
