Consistent with previous studies (e.g. 1–3), the diversity of substances used increased over the period of adolescence and males used a greater diversity of substances than females. However, the sex difference was not significant at age 14, potentially due to the later onset of puberty in males than in females (22), such that females begin to “catch-up” with their adult substance use levels earlier than males. Similar to a wide range of other phenotypes (16, 17), both males and females demonstrated a pattern of increasing genetic influence over time on diversity of substances used. The genetic effects at one age were highly correlated with genetic effects at other ages for both males and females, although their relative influence increased. Additionally, genetic factors impacted the diversity of substances used more in males than in females at age 14 and 17 and shared environmental factors exerted greater influence over females than males, again at ages 14 and 17. It is also interesting to note that the confidence intervals on the biometric estimates shown in Figure 2 suggest that 14-year-old females are the
