drug dependence also showed some disorder-specific genetic influences. However, 69% of the total genetic variance on alcohol dependence, and 64% of the genetic variance on illicit drug dependence resulted from the genetic factor shared across externalizing psychopathology. This finding indicates that much of the genetic predisposition to alcohol and other drug dependence is not specific to that disorder. Using only male twins from the same population, Kendler and colleagues40 found that a common genetic factor loaded strongly onto abuse/dependence of all classes of illicit drugs. There has now been much evidence to suggest that youth drug and alcohol problems are a manifestation of risk to a spectrum of externalizing disorders, and that to consider each of these disorders separately may lead us to miss important etiologic clues.
