One approach for comparing studies of disorders having a complex mode of inheritance has been a liability, or “threshold,” model. In this model, a person’s liability to develop alcoholism is assumed to be determined by the combined effects of many separate risk factors—genetic, environmental, or both. The distribution of liability to alcoholism in the general population is assumed to be continuous and to follow a bell curve. The majority of people exhibit an intermediate risk; some, a very low risk; and some, a very high risk. The model assumes that those whose liability exceeds some critical value (i.e., threshold) will become alcoholic. Changing the definition of alcoholism merely shifts the threshold to the right (i.e., fewer but more severe cases) or to the left (i.e., more but less severe cases). (For further discussion of the liability model in twin studies, see side-bar by Prescott and Kendler, pp. 204–205).
