Subsequently, unsuitable studies were excluded from the analysis based on two main criteria. First, only studies specifically examining cannabis use were included, and those examining related phenotypes like general drug use were left out. Second, only studies that used twin samples and applied genetic modelling to investigate the genetics of cannabis involvement were included. This procedure identified 28 twin studies on lifetime cannabis use and 24 studies on problematic cannabis use. For the purpose of the meta-analysis only studies using independent samples could be used. Some studies measured slightly different phenotypes in a same cohort or some authors examined more than one dependent measure concerning problematic cannabis use within one study, and some authors used a (sub)sample of the same cohort. In these cases only one of the reports was included in the meta-analysis, with a preference for reports with the largest sample, separate parameter estimates for each sex, the most suitable measure of cannabis use, and estimates based on univariate models as opposed to multivariate models. Table 1 and 2 show overviews of the available studies on cannabis use initiation and problematic use, respectively.
