Reduced P3 amplitude has been consistently linked to a spectrum of externalizing disorders. Utilizing data from a large sample of adolescent male twins (N=1196), we used biometric modeling to assess the genetic and environmental contributions to the association between reduced P3 amplitude and a general vulnerability to externalizing disorders. Externalizing vulnerability was indexed by a composite of symptoms of conduct disorder, adult antisocial behavior, and alcohol, nicotine, and drug dependence. The sample included two independent age cohorts, providing an internal replication of the findings. For the best-fitting model, genetic influences alone accounted for the association between P3 amplitude and externalizing disorders, with an estimated genetic correlation of rg=−.22. Results replicated across the two age cohorts and demonstrate that reduced P3 amplitude is a marker of the biological vulnerability to externalizing disorders.