Family Studies. Family studies have clearly shown that alcohol dependence is familial. The evidence for genetic factors for alcohol abuse is less well substantiated (Schuckit, 1994, 1992). About 70 percent of alcoholics have a positive family history of alcoholism in first- or second-degree relatives (M. Schuckit, personal communication, 1993). Sons and daughters of alcoholics have a threefold to fourfold increased risk for developing this disorder (Schuckit, 1986; Cotton, 1979). This suggests that either the environment of an alcoholic family or the genetic contribution from parents influences the development of alcoholism. However, family studies by themselves cannot distinguish nature versus nurture influences on etiology.
Twin Studies. The relative contribution of environment and genetics has been examined through twin studies as in the case of Alzheimer's disease and schizophrenia. If a trait has a high heritability, the concordance in monozygotic twins, who share 100 percent of their genes, will be greater than the concordance in dizygotic twins, who share only 50 percent. In general, this is what has been observed in the research (Hrubek and Omenn, 1981; Schuckit, 1981). A study with a small sample of psychiatric inpatients by Murray, Clifford, and Gurling (1983), however, showed no difference between monozygotic and dizygotic concordance using both male and female probands, so although the evidence from twin studies of the importance of genetic factors in the etiology of alcohol abuse and dependence is strong, it is not entirely conclusive.
Adoption Studies. Adoption studies are another way to untangle the environmental and genetic contributions leading to the expression of a disease state. Adopted-away sons and daughters from alcoholic families who are raised by nonalcoholic adoptive parents have a significantly increased risk for alcohol abuse and dependence. This risk may be as high as three or fourfold (Goodwin, 1985; Bohman, Sigvardsson, and Cloninger, 1981; Cadoret, 1980; Cadoret and Gath, 1978; Goodwin, Schulsinger, Hermansen, Guze, and Winokur, 1973; Schuckit, Goodwin, and Winokur, 1972).
The data from Bohman, Sigvardsson, and Cloninger's (1981) large Swedish adoption study was used to hypothesize another step in the data analyses (Cloninger, Sigvardsson, Gilligan, von Knorring, Reich, and Bohman, 1989). They have postulated two forms of alcoholism with