To test for potential zygosity differences in the association between early cannabis use and other substance use, we included in all initial analyses an interaction term between the respondent's zygosity and his early cannabis use status. As has been discussed in detail by others (Agrawal et al., 2004; Kendler et al., 1993), if this interaction term had been significant and there was an association between early cannabis use and later other substance use/abuse/dependence for dizygotic (DZ) twins only, this would have indicated that there were genetic influences that were contributing to the association between cannabis use and other drug use. This is the case because DZ twins share 50% of their segregating genes on average, thus it is possible that genetic influences not shared by the discordant twins could contribute to the association between early cannabis use and other drug use for DZ twins. In contrast, because monozygotic (MZ) twins share all of their genes, genes cannot contribute to twin pair discordance. Because all interaction terms failed to reach significance (p > .10), indicating no zygosity differences in the association between early cannabis use and other substance use, all analyses were collapsed across zygosity.
