The combined estimate of the heritability of AUDs in the twin studies was 0.51 [95% confidence interval (CI) 0.45–0.56]. We first examined whether there was heterogeneity in the heritability estimate as a function of sex – that is testing for quantitative sex effects. As can be seen in Fig. 1 and Table 3, the genetic effects on AUD can be equated across samples both within men (χ2 = 11.63, df = 11, p = 0.39) and women (χ2 = 4.58, df = 7, p = 0.71), and across both men and women (χ2 = 0.47, df = 1, p = 0.49) without a reduction in the model fit, consistent with minimal heterogeneity. When examined separately, the heritability of AUD was estimated at 0.52 in males (95% CI 0.45–0.57) and 0.44 in females (95% CI 0.25–0.61). In addition, we found no evidence for qualitative sex differences in genetic effects for AUDs. Specifically, the ratio of the correlation between opposite-sex and same-sex twin pairs did not differ significantly from unity (rg-mf = 1, 95% CI 0.42–1.00, χ2 = 1.79, df = 4,
