Numerous studies consistently report that the diversity of substances used increases throughout high school and is generally greater for males than for females (e.g., 1–3). Breaking down the relative impact of genetic and environmental factors by age and sex may allow us to better understand the potentially complex etiology of substance use during this developmental period. In particular, a twin study design provides a means to estimate the extent to which environmental and genetic predispositions influence substance use behavior. Furthermore, a longitudinal study design provides the unique ability to separate developmental effects from secular trends in substance use. Although most previous studies have investigated genetic and environmental effects on substance use collapsing across age and/or sex (e.g., 4–8), it is likely that both play a role in determining the specific etiologic vulnerabilities to substance use across adolescent development, as this is a period of substantial change in substance related behavior.
