age, genetic factors accounted for one-third of the variation in drinking patterns in both sexes, and by 18 years of age genetic factors accounted for half of the variation.34 Thus, in a period of slightly more than 4 years, genetic influences changed from having virtually no detectable effect on drinking patterns to accounting for the majority of the variance. Conversely, the importance of common environmental effects decreased significantly from adolescence into adulthood, accounting for more than 70% of the variance at 14 years of age, but only approximately 15% of the variance by 18 years of age. Thus, as drinking patterns develop, differentiate, and stabilize across adolescence, genetic factors assume increasing importance to drinking patterns; however, alcohol use early in adolescence seems to be almost entirely influenced by family, school, and neighborhood.35
