Level 2 (partially shared biological and developmental embedding) captures mechanisms through which these early adversities may become instantiated via prenatal/perinatal insults, stress–inflammation pathways or candidate dopaminergic/serotonergic systems (e.g., MAOA/COMT), which have been explored repeatedly in ADHD and, to a lesser extent, CD. The gene discussed is MAOA; the disease is attention deficit-hyperactivity disorder.