Studies of adolescent substance use initiation generate varying accounts of genetic and environmental influences, with estimates generally ranging between one-quarter to one-half of the variance for each of the factors (genetic, shared and non-shared environmental) (e.g., 7, 8, 13, 14). A study of the longitudinal implications of earlier substance use indicated that the risk of substance abuse or dependence within seven years of first drug use markedly increases as the age of first use decreases, indicating a meaningful and troubling connection between early pre-adolescent or adolescent drug experimentation and negative adult substance use outcomes (15). This relationship is likely fueled by etiologic changes in substance use during adolescence.
