For each zygosity group, Table 1 summarizes the prevalence, probandwise concordance rates and tetrachoric twin pair correlations for withdrawal and abuse/dependence in lifetime cannabis users. Before modeling the variance components, we tested the effects of age, sex, and zygosity on the prevalences (thresholds) of cannabis withdrawal and abuse/dependence (α=0.05). We found a significant age effect on the prevalence of withdrawal (Δχ21=11.26, P<0001), indicating that younger participants were more likely to report cannabis withdrawal than older participants. No such age effect was found for abuse/dependence (Δχ21=2.36, P=0.12).
