We meta-analysed the standardised variance components for the two phenotypes by calculating the weighted average genetic (A), shared environmental, (C) and unshared environmental (E) estimates. An explanation of this method can be found in Li et al. (45) and Sutton et al. (46). Briefly, to estimate the weighted mean, the male/female parameter estimates for each cohort were weighted by the number of males/females in the sample. In some cases the reports we used did not report separate parameter estimates for each sex (because they did not differ significantly). In these cases we used the equated estimates for both sexes. Calculations were done in Microsoft Office Excel 2007. Estimates were made separately for each sex and phenotype (cannabis use initiation and problematic cannabis use). We also calculated the 95% confidence interval (CI) around each estimate, calculated from the variance in the sample of source studies.
