COMT and conduction system disorder: In part one of this paper, we therefore try to create a comprehensive overview and integration of the findings so far, by reviewing 53 published cG × E studies including interactions between the six most studied candidate genes (i.e., the monoamine oxidase A (MAOA), the dopamine receptors D4 (DRD4) and D2 (DRD2), the dopamine transporter 1 (DAT1), the 5′ serotonin transporter linked polymorphic region (5-HTTLPR), and the catechol-O-methyltransferase (COMT)) and postnatal family adversity in externalizing behaviors, such as aggression, and conduct disorder.