DRD4 and attention deficit-hyperactivity disorder: The two studies that assessed interactions of DAT1 with birth weight reported nominally significant effects on the occurrence of conduct problems in children with ADHD in a case-only study (Langley et al., 2008) and significant G × E effects for a genetic index including DAT1, DRD4, and DRD2 and birth weight on ADHD symptoms in a sibling sample (Jackson and Beaver, 2015).