Genetically informative research designs, which have been used extensively to study the onset of use for a single licit or illicit drug, offer a powerful means of identifying the source(s) of cross-substance vulnerability to initiation of substance use. Findings across twin studies (based on dichotomous indicators of lifetime use) indicate that the relative contributions of heritable and environmental factors vary somewhat by substance (Hopfer et al., 2003). Heritability estimates are generally lowest for initiation of alcohol use. Genetic factors have been found to account for between 0 and 39% of the variance in alcohol initiation in most studies (Fowler et al., 2007; Han et al., 1999; Rhee et al., 2003). One notable exception is Maes et al.’s (1999) estimate of 72% heritability when use without permission was specified. McGue et al. (2001) also reported that initiation of alcohol use was attributable to a substantial degree to genetics in males (55%) but this finding did not generalize across gender (for females, heritability was estimated at 11%). Genetic contributions to the onset of cigarette smoking appear to be somewhat higher, with most
