Another limitation is that the twin studies we analysed differed regarding the composition of the sample, the phenotypic measures, and the statistical method used. These inconsistencies between the studies are likely to be partly responsible for their inconsistent results. By combining the studies into one analysis we did not acknowledge possible differences between different samples and methods. For example, two source cohorts did not use population based samples. Firstly, in the meta-analysis for problematic use, we included a study using a treatment sample (67). Because of the small sample size of this study and the fact that the reported variance components were relatively consistent with those from the other source studies, this one study should not have strongly biased our results.
