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 only group to display significant shared environmental effects. These differing proportions of variance attributable to genetic versus environmental factors may be explained by varying motivations for use. In females, substance use may be linked to a “self-medicating” orientation in reaction to greater rates of depressive symptoms, while in males substance use may be more part of a broad continuum of externalizing behaviors (23).
