The strong evidence for the heritability of and the co-heritability between lifetime use of cannabis and cigarettes comes almost entirely from international research conducted in twin samples of European origin. In U.S. populations, this is particularly problematic given significant variations in the rates of cannabis and cigarette use across race/ethnicity (Garrett et al., 2011; Griesler and Kandel 1998; Keyes et al., 2015; Wallace, Jr. et al., 2003; Wu et al., 2014). Racial/ethnic differences are also particularly pronounced in females with AA adolescent girls and young adult women appearing to be less likely than their European American (EA) counterparts to use cigarettes and cannabis (Garrett et al., 2011; Keyes et al., 2015; SAMHSA, 2014; Wallace, Jr. et al., 2003). In addition, although cigarette use typically predates cannabis use in EAs, reverse gateways (cannabis before cigarettes/alcohol) are somewhat more common in AAs than EAs (Sartor et al., 2013; Vaughn et al., 2008). Notably, these variations in prevalence and sequence may relate to differing societal attitudes towards cannabis and cigarette use, the relative availability and exposure opportunity of the two drugs as well
