Several researchers have reported that early substance use is a risk factor for substance abuse/dependence (Agrawal et al., 2006; Anthony & Petronis, 1995; Grant & Dawson, 1998; Wagner, Velasco-Mondragon, Herrera-Vasquez, Borges, & Lazcano-Ponce, 2005) even when using a cotwin-control design (Agrawal et al., 2004; Grant et al., 2006; Lessem et al., 2006; Lynskey et al., 2003, 2006) and, in two reports, even when early-users are compared to later users (Agrawal et al., 2004; Lynskey et al., 2003). Agrawal et al. (2004) reported smaller but still significant odds ratios when cannabis abstainers were removed from their analyses, but Lynskey et al.'s (2003) analyses suggested that the effect was comparable when early-users were compared to late/never-users and when they were compared to later-users. In our study the association with early-use was dramatically reduced when comparing early-users to later-users (versus later/never-users). Differences between our results and Lynskey et al.'s (2003) could be related to sampling (e.g., country of residence, age, birth cohort, military service history, gender) and/or methodology (e.g., DSM-III-R vs. DSM-IV). Although Lynskey et al. (2003) found no evidence of gender differences
