Krueger et al., 2002; McGue et al., 2006; Sherman et al., 1997; Slutske et al., 1997). Moreover, they share a substantial proportion of genetic variance with alcohol and cannabis-related phenotypes (Hicks et al., 2004; Miles et al., 2002; Slutske et al., 1998; True et al., 1999), so they cannot be viewed as primarily environmental. Finally, heritability estimates – and consequently, genetic correlations – may also reflect gene-by-environment interactions or gene-environment correlations (i.e., seeking out environments that promote behaviors for which one is at high genetic risk). Deviant peer affiliation, which has consistently been linked to alcohol and cannabis-related problems (Fergusson et al., 2002; Gillespie et al., 2009; Marshal et al., 2003) is one such example of gene-environment correlation: adolescents with externalizing problems, who are at high risk for substance misuse, seek out like-minded peers that encourage experimentation with substances. Exploration of gene-by-environment interplay in future studies is critical to fostering our understanding of the development of problem use of alcohol and cannabis.
