Bidirectional selection for high preferring vs low preferring rat and mouse lines has been conducted many times (see Table 1). Interestingly, nearly all of these studies have used almost exactly the same protocol for assigning individual phenotypic values to animals. In the rat selections, a period of a few days of forced exposure to alcohol is followed by up to 3 weeks of two-bottle preference for 10% ethanol vs water. The mouse selection studies have been conducted similarly, minus the forced exposure period. All such selections have succeeded, yielding divergent phenotypes for high and low preference drinking, and many have been replicated, following the recommendations of the authors of a selection project for activity in mice (DeFries et al. 1966). Replication has allowed us to have greater confidence that the results of these efforts are principally due to changes in the gene frequencies at trait-relevant loci rather than genetic drift due to the necessarily small population sizes. The main advantage of selective breeding is that any other traits that differ in the divergently selected lines are presumably due to pleiotropic
