Several addictive disorders tend to co-occur in the same individual.23–25 Studies on genetically informative samples (eg, adoptive or twin studies) can measure the relative contribution of genes and environment to this comorbidity by evaluating the frequency of cross-transmission.26 Twin studies reveal an overlap between genetic influences for alcoholism and illicit drug use disorders 27,28 and between alcoholism and smoking.29 Kendler and colleagues30 explored the effect of shared genetic influences on addiction to alcohol, caffeine, nicotine, cannabis, and cocaine in a portion of the Virginia twin sample including 5000 participants. In this study genetic risk could not be explained by one factor acting across all substances. Rather, two shared factors were found: an illicit agent factor mainly explaining vulnerability to cannabis and cocaine dependence and a licit agent factor mainly explaining vulnerability to alcohol, caffeine, and nicotine.
