Alcohol consumption and related problems are classic examples of complex behavioral outcomes that likely involve many genes of small effect [1]. Twin studies, which infer genetic influences by comparing the phenotypic similarity between monozygotic (MZ) twins (who share all of their genetic variation) and dizygotic (DZ) twins (who share half of their genetic variation, on average), have been crucial for demonstrating that latent genetic influences account for a considerable amount of the variation in measures of alcohol consumption and problems, with heritability estimates in the range of 50%–60% [2,3,4,5]. Twin studies have also been critical for demonstrating that environmental factors moderate the importance of genetic influences. In adolescents, for example, genetic influences on alcohol use and other closely related externalizing problems (e.g., conduct problems) increase under conditions of low parental knowledge (i.e., the degree to which parents know about one’s daily activities and associates) or high peer deviance (i.e., the degree to which one’s peer group engages in substance use and antisocial behavior) [6,7,8,9]. Thus, genetic influences appear to become more important under environmental conditions characterized by more social opportunity and less social control [10].
