Chunk #25 — RESULTS — Genetically informative research approaches and their limitations — Comorbidity between abuse/ dependence on different substances
The extent to which these conclusions, which are based on adult samples, may generalize to adolescent samples or to earlier stages of substance use remains unclear. In adolescents, problem use of alcohol, tobacco and cannabis is influenced by correlated genetic, but not shared environmental, factors.36 Also, Rhee et al58 demonstrated that alcohol and drug dependence may be manifestations of a single common and heritable liability in adolescents. These authors examined multiple conceptualizations of comorbidity and determined that, in adolescents, underlying alcohol and drug dependence was a common liability and that these disorders were alternate forms of that liability. Together, these results suggest substantial overlap of both genetic and environmental factors associated with the development of abuse/ dependence across a range of licit and illicit drugs (the licit and illicit drug factors identified by Kendler et al57 were highly correlated; r = .82). In contrast, utilizing a children-of-twins design, Volk et al59 reported that after accounting for the offspring correlation between alcohol and nicotine dependence, there was evidence for the specificity of genetic transmission of vulnerability to alcohol and nicotine dependence.
