This effect might also be mediated by the inclusion of only participants with ADHD combined subtype and the inclusion of oppositional defiant disorder and conduct disorder, present in three quarters of the ADHD sample, as studies have shown stronger associations with ADHD and DRD4 7R in combined ADHD (10) and ADHD with comorbid conduct problems (56). Here, DRD4 is linked to Oppositional defiant disorder.