The use of longitudinal studies of behavior across development has elucidated the dramatic changes that are evident in the importance of genetic and environmental influences throughout the lifespan, observed across multiple behavioral domains including intellectual abilities31 and depression.32 Substance use is another area in which dramatic changes in the relative importance of genetic and environmental effects across development are apparent. This finding has been documented in data from our longitudinal Finnish Twin Studies, in which the authors have found that the importance of genetic effects on drinking patterns increases dramatically from adolescence to young adulthood. At the age of 14 years, genetic influences accounted for only 18% of the variance in drinking initiation, and this was significant only in girls, with no evidence of genetic influence on drinking patterns in boys at this early age.33 However, by 16 years of 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
