The model results rely on the assumptions made in twin studies including: random mating with respect to genetic liability, equal environment assumption (EEA), and additivity and independence of genetic and environmental components. Although some evidence suggests that there is non-random mating for alcoholism (e.g. Maes et al. 1998), the effect does not seem large enough to considerably bias the estimates in the current study. Previous studies of alcoholism in this sample are consistent with the validity of the EEA (Kendler & Gardner, 1998). Assumptions of additivity and independence cannot be tested without information on other relatives, but studies of alcohol consumption in extended families suggest that gene–environment correlations contribute only a small amount of variance (Eaves et al. 1989).
