Genetic factors are important for the development of AD, as established by the usual relevant methods of genetic epidemiology: twin, family, and adoption studies. Twin studies have generally yielded heritability estimates in the range of 50–60% (e.g., Kendler et al. 1992, 1997; Prescott and Kendler 1999). We describe two illustrative studies that included large samples. Heath et al. (1997) studied AD risk in an Australian twin registry sample that included nearly 6,000 subjects. This study considered both male and female twins (like-sex and unlike-sex twin pairs), and found a consistent heritability of 64%, using standard alcohol dependence diagnostic criteria. These investigators were also able to evaluate heritability by birth cohort, and found consistent values over a >20-year range of cohorts. Another notable study (Kendler et al. 1997) considered the intersection between the Swedish Twin Registry, which logged almost all twins born in that country from 1902 to 1949 (nearly 9,000 male pairs), and Swedish temperance board registrations from 1929 to 1974 (about 2,500 twins). Subjects came to the attention of temperance boards mostly because of alcoholism or crimes related to
