It is striking that there is no evidence of genetic association with any of the DSM clinical disorder symptom counts for externalizing disorders despite association with these outcomes in the older adult COGA participants. However, we do find evidence of association with nondiagnostic externalizing behavior as measured by the Achenbach self-reports. This is the measure for which association was previously reported with GABRA2 in an independent, community-based sample of adolescents and young adults spanning the same age range (Dick et al., 2009), but for whom clinical outcomes were not available for comparison. These findings underscore the importance of using developmentally appropriate assessments for genetic studies in individuals of different ages. If only externalizing diagnoses and not subclinical levels of externalizing behavior had been assessed, one might have erroneously concluded that there was no association with externalizing behavior in this younger COGA sample. Unlike older adult samples previously studied in COGA, there is no association at the level of diagnostic clinical symptoms in this younger sample. Instead, the association is manifest at this stage of development with subclinical levels of externalizing
