DRD4 and attention deficit-hyperactivity disorder: There is ample evidence that the DRD4 7+ variant is associated with increased difficulties in self-control, executive functioning, or signs of ADHD in children (Schmidt et al., 2001; DiLalla et al., 2009; Pappa et al., 2015), but also for DRD4 × environment interaction effects (Martel et al., 2011).