The combined estimate of the unique environment proportion of variance is 0.39 (95% CI 0.38–0.42). There was strong heterogeneity in the estimates of the non-shared environment in males (χ2 = 54.70, df = 11, p < 0.001). Given that the Kaij sample had the smallest non-shared parameter estimate, the heterogeneity analysis was re-run with a fixed effect for the Kaij study parameter. While the heterogeneity in the unique environmental parameter was considerably reduced, it was still significant (χ2 = 31.30, df = 10, p < 0.001) and therefore not a function of a single study. In contrast, in females there was no evidence of heterogeneity (χ2 = 3.11, df = 7, p = 0.87). While there was heterogeneity within the male unique environmental estimate, when the estimates of the unique environmental parameter across sexes was constrained to equality, there were no significant differences between the sexes (χ2 = 0.04, df = 1, p = 0.83). Thus, there are no significant differences in estimates of the unique environmental variance for males and females.
