The former is consistent with findings from the twin and molecular genetics literature, demonstrating that common externalizing disorders and behaviors (for example, alcohol dependence, other drug abuse or dependence, AAB and conduct disorder) share genetic influences,18, 44 and that this shared genetic factor is highly heritable (h2=80%).45 Supplementary analyses in our own sample were consistent with this hypothesis, and we found evidence that ABCB1 variation was associated with alcohol and cocaine dependence criterion counts. This evidence concerns the gene ABCB1 and drug dependence.