and peer relationships which are known to mediate genetic influence in externalizing disorders (Hicks, South, Dirago, Iacono, & McGue, 2009). Parental diagnosis of the externalizing disorder was another factor for which it was impossible to covary. Thus, the inability to control for the genetic liability due to a parent having the disorder, or a parent being alcohol dependent, or both, was a confound of this study. The ability to control for age and gender could address the confound of older, predominantly male participants in some COA studies compared to PAE participants for those with ADHD. However, since rates of ADHD are higher in males than in females, this would potentially result in an over-estimate of ADHD in COA studies compared to PAE. Thus the significantly higher rates of ADHD in PAE compared to COA studies is a conservative result. Although differences in age between PAE and COA studies for ADHD may have resulted in different ADHD criteria being met in one group compared to the other group, or in different means of assessment of the disorder (parental vs personal report), the LORs employed were for ADHD diagnosis and not based on particular criteria, sub-diagnosis, or symptom count. Maternal illicit drug
