DRD4 and attention deficit-hyperactivity disorder: Results of two studies (Ding et al., 2002; Wang et al., 2004) support this theory as they found that the increased 7R frequency in DRD4 gene related to ADHD may be the result of positive selection since it is associated with both ADHD and the personality trait of novelty seeking (Benjamin et al., 1996; Gizer et al., 2009).