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 substance use may not generalize to the phenotype of any lifetime use. Finally, participants reported on their ages of first substance use in adulthood, which may decrease reliability. For instance, individuals have been shown to report progressively later ages of initiation over time (Parra, O’Neill, & Sher, 2003). Despite this, correlations between respondents’ ages at interview and their reported ages of initiation were minimal (rs = −.01 to .08), suggesting limited age-related bias. Nevertheless, accuracy in reporting may be increased via prospective assessment.
