Although the parameter estimates from this meta-analysis have narrower confidence intervals than any of the source studies, their precision is still limited for several reasons. Firstly, despite the large number of twin studies into cannabis use phenotypes, they were only based on a low number of independent cohorts; our variance components estimates are based on six to nine cohorts. Also, all cohorts are from Western countries, with over half of them from the US, so the results are not necessarily generalisable to different populations. Additional and more varied cohorts would increase the generalisability as well as the precision of the estimates. On the other hand, the countries from which the different samples are drawn have different policies toward cannabis use, which might influence the reported genetic and environmental estimates. For example, in countries with a more liberal cannabis policy, like the Netherlands, cannabis is more easily available. Therefore, the relative contribution of environmental effects could be smaller and that of genetic effects larger. However, such a difference is not supported by the findings from Vink et al (44), who reported
