Finally, in a study with a focus rather different than the ones just considered, Seeger et al.47 evaluated whether the season of the year in which a child was born interacted with the dopamine DRD4 polymorphism in predicting the hyperkinetic conduct disorder (ADHD). Employing a cross-sectional design involving 64 children with the disorder and 163 healthy controls (mean age 11–12±3 years), they found that it did—and in ways consistent with what is known about photoperiod exposure during pregnancy. When comparing patients with controls, children with one copy of the DRD4 7-repeat allele born in autumn and winter (that is, long photoperiod during pregnancy) had a 5.4-fold decreased relative risk for hyperkinetic conduct disorder, whereas children with the same genotype born in spring and summer (that is, short photoperiod) had a 2.8-fold increased relative risk for hyperkinetic conduct disorder. Neither season of birth nor the presence of DRD4 7-repeat allele represented a risk factor for hyperkinetic conduct disorder per se.
