In sum, although many studies estimate the relative genetic and environmental effects on substance use, the majority of them collapse across broad age bands, and many also collapse across sex. This leads to a loss of information on the potential longitudinal changes and gender differences that may be important during the key developmental period of adolescence. Overall, previous studies indicate that genetic, shared environmental, and non-shared environmental factors appear to account for approximately equivalent proportions of the variance in adolescent substance use. The current study sought to expand upon previous research by investigating the possible developmental trends in relative genetic and environmental influences on the diversity of substances used during adolescence, as well as examining the role of gender. Specifically, we examined the etiology of diversity of substances used in 1386 male and female twins studied when they were 11, 14, and 17 years old. Diversity of substances used provides an important index of substance use severity (18). It also serves as a measure of substance involvement applicable across adolescence – diversity of substances used has the same meaning at
