employing complete DSM-IV CUD symptom counts. However, it should be noted that when analyses were re-run in Cohort III using the 11-item symptom count, the results were very comparable to those obtained with the restricted variable. Fourth, Cohorts II and III were assessed in different years. We regressed birth year out of all variables to control for potential cohort differences; however, it remains possible that this did not fully account for all potential secular trends in substance involvement. Fifth, we ignored the issue of censoring, i.e., the fact that particularly in Cohort II, the youngest members may not have passed through the age period of risk for initiation of substance use. However, survival analyses indicated that censoring likely did not significantly impact results. Sixth, we may have had greater statistical power to detect effects among women than men, as the present sample was 60% female. However, this “imbalance” in relative power was likely reduced by the higher prevalence of substance use disorders among men. Seventh, individuals who reported abstaining from a substance did not contribute data concerning their age of initiation for that substance. Thus, conclusions regarding the relative contributions of genetic and environmental factors to the age at first
