Our final analyses tested for heterogeneity across twin and adoption studies. Specifically, two models were estimated that simultaneously evaluated the twin and adoption data. In the first model, two heritability coefficients were estimated: one each for twin and adoption studies. As indicated above, these parameters can be equated across sexes in both types of study. In the second model, we constrained these parameters to equality. When the two models are compared using a using a likelihood ratio test, the heterogeneity in the heritability estimates of the twin and adoption studies was not statistical significant (χ2 = 3.51, df = 1, p = 0.06). The overall best estimate of the heritability of AUDs, derived from both twin and adoption studies, was 0.49 with relatively tight CIs (0.43–0.53).
