Cannabis use and cannabis use disorders are frequently comorbid with other aspects of substance use and psychiatric disorders (Kandel et al., 2001). Limited genetically informative research has suggested that a substantial co-morbidity can be explained by common or shared genetic factors. For example, conduct disorder and other externalizing problems often co-aggregate with early cannabis use and cannabis use disorders (Deas & Brown, 2006; Meyers & Dick, 2010). Additionally, Shelton and colleagues (Shelton et al., 2007) found that genes influencing conduct problems during childhood were modestly but significantly correlated with subsequent liability to cannabis use and misuse. Similarly, Lynskey et al. (2004) reported a significant genetic correlation between cannabis use disorders and major depressive disorder (MDD), and suggested that previously documented associations between early onset cannabis use and MDD could be largely attributed to the influence of shared genetic risk factors.
