Contributors to the co-occurring use of cannabis and cigarettes include risk and protective influences that shape a general liability to experimentation with multiple substances (Hawkins et al., 1992) as well as influences specific to cigarette and cannabis co-use (e.g., shared route of administration; (Agrawal et al., 2012)). Both genetic and environmental influences play a role in the shared vulnerability to cannabis and cigarette use (Agrawal et al., 2010; Han et al., 1999; Young et al., 2006). One study suggested a genetic correlation as high as 0.75 (Agrawal et al., 2010) between cannabis and cigarette use while another suggested a more modest overlap of r = 0.31 (Young et al., 2006). Environmental contributions on these early stages of substance use can be further parsed into those that make members of twin and sibling pairs similar to each other (i.e., shared environment) and those that are individual-specific, with more robust evidence for the shared influences being correlated than the non-shared (Young et al., 2006). However, a study using a subset of the data from this study showed that in African American (AA)
