In the twin analysis, heritabilities were estimated using standard structural equation modeling techniques that compare the tetrachoric correlations between multiple groups of twins separated by sex and zygosity (Neale & Cardon, 1992). To assess qualitative genetic sex differences a parameter was included that allowed the correlation between the genetic factors of opposite-sex DZ twins to be smaller than the same-sex DZ correlation (Neale & Cardon, 1992). If significant, this would indicate that at least partially different genetic factors culminate in AUD in males and females. Heritability in adoption studies was estimated by comparing the resemblance between biological parents and their adopted-away offspring, modeled as half the additive genetic variance. This allowed us to analyze the adoption and twin studies jointly within the SEM framework.
