The primary models examined in the current study treat ADHD as a single continuum of symptoms. However, previous ADHD research by Nigg (Nigg, Stavro, et al., 2005) and others (e.g., Chhabildas, Pennington, & Willcutt, 2001) led us to question whether separable components of ADHD, namely, attention problems and hyperactivity/impulsivity problems, may be differentially related to other externalizing disorders as well as to response inhibition. In post hoc analyses, we examined these two ADHD domains at ages 12 and 17 and found that conduct problems were correlated less highly with attention problems than with hyperactivity/impulsivity problems. However, the correlations with substance use and novelty seeking were nearly identical for the two ADHD components. Attention problems were also more highly correlated with response inhibition than hyperactivity/impulsivity problems, but both components showed a highly significant genetic correlation with response inhibition. Thus, our findings partially mirror previous research but suggest that conduct problems and associated reactive or motivational problems (e.g., high risk taking, low fear of punishment) are also strongly associated with deficits in response inhibition and that the relationship is largely genetic.
