Causes of ADHD: Genetics, Brain Biology, and Environmental Factors

ADHD (Attention-Deficit/Hyperactivity Disorder) is a neurodevelopmental disorder with no single cause. Research supports a multifactorial model involving genetic, neurological, and environmental influences.

1. Genetic Factors (Heritability ~74–88%)

• Family/twin/adoption studies confirm strong genetic component.
• Polygenic: Many common variants (SNPs) each with small effect — dopamine (DRD4, DRD5, DAT1), norepinephrine (NET), serotonin, synaptic development genes.
• Rare copy number variants (CNVs) also contribute.
• Genes ≠ destiny: They confer susceptibility; expression depends on environment.

2. Neurobiological Differences

• Structural: Reduced total brain volume (~3–5%), especially prefrontal cortex, basal ganglia, cerebellum, corpus callosum.
• Functional: Hypoactivation in fronto-striatal circuits (executive function), default mode network dysregulation (attention lapses).
• Neurotransmitter: Dopamine and norepinephrine signaling deficits in reward/motivation and attention networks.
• Developmental delay: Cortical maturation delayed ~2–3 years (peak thickness later).

3. Environmental Risk Factors (Non-Genetic)

• Prenatal: Maternal smoking, alcohol, stress, infection; preterm birth (<37 wks), low birth weight (<2500g).
• Perinatal: Hypoxia, neonatal complications.
• Early childhood: Lead exposure, severe psychosocial deprivation, traumatic brain injury.
• Diet/nutrition: Artificial food colors (small effect in sensitive subset), omega-3 deficiency (weak evidence).
• Not caused by: Parenting style, sugar, screen time (though excess screens may worsen symptoms), vaccines.

Gene-Environment Interaction

Genetic susceptibility + environmental insults → altered neurodevelopment → ADHD phenotype. Epigenetic mechanisms (DNA methylation, histone modification) may mediate this interplay.

Clinical Implication

ADHD is a real, biologically based disorder — not a choice or character flaw. Comprehensive evaluation (clinical interview, rating scales, rule-outs) guides individualized treatment (behavioral therapy, medication, educational supports, lifestyle).