The liability model provides a natural framework for combining data from different studies using widely different definitions of alcoholism. It also allows us to compare the importance of genetic and/or shared environmental influences on alcoholism risk in men and women, despite the significant gender differences in the prevalence of alcoholism. In genetic studies, these liability correlations are usually expressed in terms of the causes of variation in alcoholism liability. Here, a simplified model is used that allows for the contributions of genes, family environment, and nonshared environmental experiences and ignores such complications as gene-environment correlation (i.e., the tendency of people at high genetic risk to be exposed also to high-risk environments) and, for adoption data, selective placement. Nonetheless, this simplified model provides a good starting point for comparing results from different studies.
