In contrast to the variability in estimates of E on individual differences in and between cannabis and cigarette use, the role of genetic influences was comparable across substances and racial/ethnic groups. Heritability estimates (43–66%; Table 3) were similar across racial/ethnic groups and approximated reports from other studies of European twin cohorts (Madden et al., 2004; Maes et al., 2006; Pergadia et al., 2006; Verweij et al., 2010; Vink et al., 2005). Results from the bivariate model suggested that these genetic influences were highly and possibly, perfectly correlated across the substances (Table 4). Such shared genetic factors could include predisposition to a third, heritable trait, such as a general liability to disinhibited behaviors (Hicks et al., 2011) or a shared vulnerability to the use of drugs that utilize combustion/inhalation as the main route of administration (Agrawal and Lynskey 2009; van Leeuwen et al., 2011). Alternatively, genes related to cigarette use may also be linked to onset of cannabis use, such as brain-derived neurotrophic factor (BDNF). The BDNF variant rs6265 has been linked to cigarette use at p = 1.8 × 10−8
