A forest plot of the shared environmental variance components is presented in the middle panels of Fig. 1 and in Table 3. The combined estimate of the shared environmental variance proportion for AUDs across all studies was modest 0.10 (95% CI 0.03–0.16) but statistically significant (p < 0.01). Heterogeneity in the common environmental variance component was also tested as a function of both sex and assessment technique. Across all studies, the estimate of shared environmental effects on AUDs in males was modest (0.083, 95% CI 0.01–0.15) and statistically heterogeneous (χ2 = 22.44, df = 11, p = 0.02). As is evident in the figure, this heterogeneity derived largely from a single study: Kaij (1960). When this study was removed, the heterogeneity decreased substantially and became non-significant (χ2 = 9.97, df = 10, p = 0.44). However, when this study was excluded from our analysis, the shared environment effect in males was no longer statistically significant (0.05, 95% CI 0.00–0.12, χ2 = 1.92, df = 1, p = 0.17). In females, the estimate of shared environmental effects was relatively large and
