ADHD is a neurodevelopmental disorder characterised by persistent inattention, impulsivity, and hyperactivity that impairs daily functioning. Worldwide prevalence is roughly 5.3% in children and adolescents and 2.5% in adults. The male-to-female ratio is about 2:1 in childhood and narrows into adulthood. Around 60% of childhood cases persist in some form - though longitudinal data from the MTA study show that persistence tends to wax and wane rather than follow a clean on/off trajectory. Over 16 years, only 9% of childhood ADHD cases sustained full remission.
The downstream consequences are measurable. Adults with ADHD earn roughly 34% less than non-ADHD siblings, are 23% more likely to have vehicular crashes, and have elevated premature mortality risk from accidental causes. Up to 50% of adults with ADHD use drugs or alcohol regularly, compared to about 15% in the general population.
The Neurochemistry
ADHD is fundamentally a neurochemical condition. The prefrontal cortex - the brain region responsible for attention, planning, impulse control, and emotional regulation - operates with insufficient levels of three key neurotransmitters:
- Dopamine - regulates attention and motivation. Deficit leads to difficulty sustaining focus and pursuing delayed rewards.
- Norepinephrine - governs alertness and impulse control. Insufficient levels produce distractibility and poor inhibition.
- Serotonin - modulates mood stability. Dysregulation contributes to emotional volatility.
This is why stimulant medications work: methylphenidate (Ritalin/Concerta) blocks the dopamine transporter in the striatum and the norepinephrine transporter in the prefrontal cortex. Amphetamines (Adderall/Vyvanse) go further - they reverse dopamine transport, actively releasing dopamine into the synapse while also blocking reuptake. The response rate across stimulant classes is roughly 70%, and when both stimulant and non-stimulant medications are considered, 90–95% of patients respond to at least one class (Stanford & Sciberras, 2022).
Presentations
The DSM-5 defines two symptom dimensions - inattention (9 symptoms) and hyperactivity/impulsivity (9 symptoms) - and three presentations:
- Predominantly inattentive. Difficulty sustaining attention, poor organisation, forgetfulness, easily distracted. Historically called "ADD." More common in females, more likely to be missed in childhood.
- Predominantly hyperactive-impulsive. Fidgeting, difficulty waiting, excessive talking, interrupting. More visible, more commonly diagnosed in boys.
- Combined. Meets criteria in both domains. The most common presentation in clinical settings.
For adults aged 17 and over, the threshold drops to 5 symptoms per domain (versus 6 for children). Symptoms must be present before age 12, occur in multiple settings, and cause functional impairment. Notably, the DSM-5 now permits co-diagnosis with autism spectrum disorder, reversing the DSM-IV exclusion.
Late-Onset ADHD
Longitudinal studies have complicated the assumption that ADHD always begins in childhood. A Brazilian birth cohort found that 84.6% of young adults meeting ADHD criteria did not meet them at age 11. Similar results appeared in New Zealand and UK cohorts. Possible explanations include subthreshold childhood symptoms masked by high IQ or supportive environments, or genuinely late-onset forms. Whether late-onset ADHD shares the same pathophysiology as childhood-onset remains an open question.
Emotional Dysregulation
Russell Barkley argues that emotional impulsivity and deficient emotional self-regulation (EI-DESR) are core features of ADHD, not comorbidities. He points out that emotional symptoms were included in ADHD criteria up until the DSM-II and were only removed due to measurement difficulties, not because they stopped being relevant. Having combined-type ADHD effectively creates a borderline case of oppositional defiant disorder, because EI symptoms constitute 3–4 of the 8 ODD diagnostic criteria.
Genetics: Heritability and Polygenic Architecture
ADHD is among the most heritable psychiatric conditions. Twin studies consistently estimate heritability at 70–80% (meta-analysis: Nikolas & Burt, 2010) - comparable to autism (~80%) and schizophrenia (~80%), and substantially higher than major depression (~40%). Family studies show 4–5x increased risk in first-degree relatives.
The landmark 2019 GWAS by Demontis et al. (Psychiatric Genomics Consortium + iPSYCH) analysed 20,183 ADHD cases and 35,191 controls, identifying 12 genome-wide significant loci. SNP-based heritability was 21.6%, meaning common variants explain about a fifth of the total variance. The polygenic signal was enriched for central nervous system regulatory elements.
One important finding: the widely studied candidate genes DRD4 and DAT1/SLC6A3 received little support in this GWAS. Most prior candidate gene results have failed to replicate at genome-wide significance levels.
The genetic correlation between clinical ADHD diagnosis and population ADHD trait scores is 0.94, strongly supporting a dimensional (spectrum) model rather than a categorical one. ADHD isn't a binary condition - it is the extreme end of a continuous trait distribution.
Polygenic risk scores (PRS) currently explain about 5.5% of phenotypic variance. They distinguish persistent from remitting ADHD - those with persistent symptoms carry higher polygenic burden. PRS are also associated with conduct disorder, substance use, irritability, and executive function deficits.
ADHD shares genetic liability with a broad range of conditions. Major depressive disorder has the strongest psychiatric genetic correlation (r_g = 0.44). Positive correlations exist with obesity, BMI, coronary artery disease, insomnia, and substance misuse. Negative correlations appear with educational attainment and subjective wellbeing. Mendelian randomization suggests some of these relationships are causal - ADHD genetic liability appears to increase the risk of depression, obesity, and coronary artery disease.
Rare Variants
Beyond common polygenic risk, rare genetic variants also contribute:
- Copy number variants (CNVs) - large, rare deletions and duplications - overlap with those found in autism and schizophrenia, implicating shared neurodevelopmental pathways including ion channels, glutamate receptors, and CNS development genes.
- Rare single-point mutations identified through exome sequencing overlap substantially with autism risk genes. De novo mutations tend to be more deleterious than inherited variants.
- Several developmental syndromes carry elevated ADHD risk: Fragile X, Tuberous Sclerosis, 22q11.2 deletion (velo-cardio-facial syndrome), Prader-Willi, Turner, and Klinefelter syndromes.
Epigenetics: Where Genes Meet Environment
Epigenetic mechanisms - DNA methylation, histone modification, non-coding RNA regulation - alter gene expression without changing the DNA sequence itself. They provide the biological bridge between genetic risk and environmental exposure.
Several lines of evidence connect epigenetics to ADHD:
The largest epigenome-wide association study (EWAS) to date found no genome-wide significant methylation differences for ADHD, though promising associations were found between methylation variants and both ADHD status and polygenic risk scores. The field faces significant challenges: brain tissue is inaccessible in living subjects, blood-based methylation may not reflect brain patterns, and confounders (cell composition, age, medication status, smoking) are difficult to control.
Sex Differences
Twin studies show no sex differences in genetic loading for ADHD. The molecular genetic correlation between males and females is approximately 1.0, and no PRS sex differences have been found. The clinical prevalence gap (2:1 male-to-female in childhood) is not explained by differential common variant effects. Some evidence suggests a "female protective effect" - females may require a higher genetic burden to manifest the disorder - but this remains debated.
References
- Demontis, D., et al. (2019). Discovery of the first genome-wide significant risk loci for attention deficit/hyperactivity disorder. Nature Genetics, 51, 63–75.
- Nikolas, M.A. & Burt, S.A. (2010). Genetic and environmental influences on ADHD symptom dimensions. Journal of Abnormal Psychology, 119(1), 1–17.
- Langley, K., Martin, J. & Thapar, A. (2022). Genetics of ADHD. In Stanford, S.C. & Sciberras, E. (Eds.), New Discoveries in the Behavioral Neuroscience of ADHD.
- Barkley, R.A. (2015). Attention-Deficit Hyperactivity Disorder: A Handbook for Diagnosis and Treatment (4th ed.). Guilford Press.
- Artigas, M.S., et al. (2023). GWAS analysis of cannabis use in ADHD. Molecular Psychiatry.