Our initial work examining older-cohort twins with a lifetime diagnosis of ADHD indicated significant reduction in P300 amplitude (Iacono et al., 2002b). However, when “pure” ADHD cases (those with no comorbid externalizing diagnosis) were examined, P300 amplitude was not significantly reduced, suggesting that the P3 effect observed in ADHD may be attributable to its overlap with other externalizing disorders. Further, because this study was carried out with 17-year-old twins, many of whom had already initiated substance use, it was not possible to rule out substance exposure as a factor in the P3AR of the comorbid group. We therefore extended this work using 11-year-old ES and MTFS twins for whom substance use is uncommon. In this younger-cohort sample, Yoon et al. (2008) found that ADHD cases showed reduced P300 only in the presence of comorbid CD and/or ODD. These results suggest 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
