onset are attributable to genetic rather than shared environmental sources. Third, due to power limitations, we were unable to test the equal environment assumption (EEA), the assumption that correlations in exposure to environmental events relevant to the trait under study are equal in MZ and DZ twins. Given the support for the validity of the EEA in studies addressing substance use and other psychiatric issues (Hettema et al., 1995; Kendler et al., 1993; Xian et al., 2000), the likelihood that there were violations and that these violations significantly impacted our results is low, but further investigation of the validity of the EEA in relation to substance use initiation appears to be warranted (Kendler and Gardner, 1998).
