The observation that people from similar environmental backgrounds often differ considerably in ethanol consumption and the well-documented familial incidence of alcoholism indicate that heredity contributes to alcohol use disorders (AUDs) (Cloninger, 1987; Cotton, 1979; Schuckit, 1986). Similarly, heterogeneous stock rats display a wide-range of ethanol-consumption levels (Richter and Campbell, 1940). In the late 1940’s, Williams and associates (Williams et al., 1949) as well as Mardones and Segovia-Riquelme (1983) proposed a genetic influence on ethanol intake in rodents. From their early work and that of three other sites, bidirectional selective breeding has resulted in at least five high alcohol-consuming vs. their respective low alcohol-consuming rat lines. Bidirectional selection, from a heterogeneous foundation stock, is accomplished through systematic mating of animals from the same extreme of the normal distribution (alcohol-preferring on the one hand vs. alcohol-avoiding on the other) over successive generations. This results in divergent lines that exhibit these extreme phenotypes in ethanol preference and alcohol consumption levels that exceed the range displayed by the foundation population. A major advantage is that this ethanol-drinking phenotype is observed without environmental manipulations. Moreover,
