that ADHD in the absence of other externalizing disorders may reflect a different underlying genetic mechanism than comorbid ADHD because individuals with “pure” ADHD did not show the characteristic P3AR. Thus, ES twins have helped us to further refine this endophenotype. Moreover, in Gilmore et al. (in press), we showed that the EROs comprising the P300 response can be used to capture risk for externalizing disorders beyond that which is possible using P300 alone.
