Of note, we did not look for epistasis within DRD1 or DRD2, nor between DRD1 and DRD2. Epitasis undoubtedly contributes to the development of a complex genetic disorder such as ADHD as well as to intermediate phenotypes such as working memory. Unfortunately, this study did not have the requisite large sample of subjects required to examine gene–gene interactions and it would have been statistically irresponsible to do so. Still, this is an important issue in psychiatric genetics to explore further in larger consortia datasets.
