substances ranges from 40% to 60%.27 Heritabilities in the range of 30% to 60% are also observed for illicit drug dependences.28,29 Variability in the exact estimates is likely a function of the age of the participants (genetic influences often increase across development30), cohort differences, and the exact measure of the phenotype being studied. Current research on the heritability of youth drug and alcohol problems suggests that these disordered behaviors are manifestations 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.
