Figure 1a and 1b display the results of the meta-analysis for genetic contributions to cannabis use initiation for males and females, respectively. The horizontal lines represent the 95% confidence intervals around the heritability estimates (diamonds) from the different cohorts. When confidence intervals were not reported by the source studies, they were estimated (dotted lines), based on a logarithmic curve regression of the confidence intervals and sample size from the other studies. The bottom line shows the results of our meta-analysis, displaying the narrower confidence interval of the estimates as compared to the intervals from the source studies. As can be seen in the Figure, the point estimates from the meta-analyses fall within all confidence intervals from the source studies, suggesting the source studies are homogenous.
