The majority of studies investigating relative genetic and environmental effects on substance use utilize adult twin samples with broad age ranges (e.g., 4–6, 9–12). Studies of adult samples that focus on use, rather than or in addition to misuse, generate significant estimates of shared environmental influence, or those common factors that have the same effects on both twins (5), accounting for around one-quarter of the population variance. Moreover, genetic factors, which are either fully or half shared between twins, and non-shared environmental factors, those experiences which are unique to each twin (5), are generally estimated to be even more influential, each accounting for one-third to one-half of the variance in substance use (e.g., 5, 10–12). These findings are interesting in that they document both significant genetic and shared environmental components to variance in substance use, although they do not speak to the developmental trajectory which leads up to observed patterns in adult substance use.
