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 studies producing heritability estimates around 40% (Fowler et al., 2007; Koopmans et al., 1997; McGue et al., 2000; Rhee et al., 2003), although both considerably lower (11%, Han et al., 1999) and higher (84%, Maes et al., 1999) estimates have been reported. Approximately 30% of the variance in the onset of cannabis use is attributable to genetic factors, but as in the case of both alcohol use and cigarette smoking, heritability estimates cover a wide range (0–72%) (Fowler et al., 2007; Han et al., 1999; Kendler and Prescott, 1998; Maes et al., 1999; McGue et al., 2000; Rhee et al., 2003; Tsuang et al., 1999).
