Is ADHD Genetic? What Twin and Genome Studies Show

Yes, ADHD is largely genetic, but no single gene causes it. The best-known answer to "is ADHD genetic" comes from twin research: the average heritability across 37 twin studies was 74%, according to a 2019 review in Molecular Psychiatry^[1].

Genes are not the whole story. The World Federation of ADHD's 2021 consensus statement concludes that for most people with ADHD, many genetic and environmental risk factors accumulate to cause the disorder^[5].

What a child inherits is a higher likelihood, not the condition itself. MedlinePlus Genetics, from the National Library of Medicine, puts the risk for first-degree relatives at about nine times that of the general public and notes that the inheritance pattern is usually unknown^[2].

CDC lists genetics first among possible risk factors and states that the causes of ADHD are still unknown^[6]. How many people have ADHD is a separate question, tracked on the ADHD statistics page and in How Many People Have ADHD?

Three kinds of study point the same way. In a family study of 894 children with ADHD and 1,135 of their siblings, the siblings had a ninefold higher risk of ADHD than siblings of children without it^[1].

Adoption studies separate genes from the home. In the most recent one reviewed by Faraone and Larsson, biological relatives of children with ADHD had higher rates of ADHD than adoptive relatives, whose risk resembled that of relatives of control children^[1].

Twin studies compare identical twins with fraternal twins. Beyond the 74% average, a study that also included full siblings and maternal and paternal half-siblings produced a similar estimate of around 80%^[1]. Heritability was similar for inattentive and hyperactive-impulsive symptoms^[1].

The largest family study used Swedish registers covering 1,656,943 people born from 1985 to 2006^[3]. Familial risk rose step by step with genetic relatedness, from a hazard ratio of 70.45 for identical twins to 1.47 for half cousins^[3].

Is adult ADHD as heritable as childhood ADHD?

Early adult twin studies that relied on self-report estimated heritability at only 30–40%^[1]. A study of clinically diagnosed ADHD in adults put it at 72%, and the review authors attribute the lower self-report figures to rater effects rather than a real drop with age^[1].

Persistent ADHD also clusters more tightly in families. Full siblings of people diagnosed at age 18 or older had a hazard ratio of 11.49, against 4.68 for siblings of people diagnosed only in childhood^[3]. Data on adults with the condition are on the adult ADHD page.

For most people with ADHD there is no single "ADHD gene." MedlinePlus Genetics notes that changes in dozens of genes have been associated with ADHD and that most of the gene variations thought to affect risk have not been identified^[2].

A genome-wide association study (GWAS) published in Nature Genetics in 2023 compared 38,691 people with ADHD and 186,843 controls^[4]. It identified 27 genome-wide significant loci pointing to 76 potential risk genes, enriched among genes expressed in early brain development^[4].

That study built on a 2019 analysis of 20,183 people with ADHD and 35,191 controls, which found 12 significant loci^[1]. Each common variant has a very small effect, which is why ADHD is described as polygenic^[5].

Common variants explain only part of the twin estimate. SNP heritability was 0.22 in the 2019 data, about one-third of twin heritability^[1], and 0.14 in the 2023 meta-analysis^[4].

What about DAT1 and DRD4?

Before GWAS, researchers tested candidate genes in the dopamine and serotonin systems that ADHD medications act on. Meta-analyses linked variants in six genes, including the dopamine transporter gene DAT1 (SLC6A3) and the dopamine D4 receptor gene DRD4, but every odds ratio was below 1.5^[1].

The 2021 consensus statement lists DAT1, DRD4 and several other genes as implicated by meta-analyses but still uncertain until validated in a genome-wide study^[5].

Rare variants and copy number changes

A smaller share of cases involves rare genetic changes. When ADHD stems from a rare gene mutation or chromosome abnormality, it is usually one feature of a broader syndrome^[2].

Large, rare copy number variants (deleted or duplicated stretches of DNA) were more common in people with ADHD than in controls in all but one study, though only three studies found a statistically significant burden^[1]. The 2023 study also found an increased load of rare protein-truncating variants in a set of ADHD risk genes, using exome data from 17,896 people^[4].

Either parent can pass on genetic risk. In the Swedish register study, the hazard ratio was 2.86 for maternal half-siblings of people with ADHD and 2.31 for paternal half-siblings^[3].

The maternal figure was significantly higher. Because maternal and paternal half-siblings are equally related genetically, the gap fits the study's conclusion that familial clustering reflects genes plus a small amount of shared environment^[3].

The sex of the person with ADHD did not appear to change their relatives' risk^[3]. How ADHD presents differently in girls and women is covered in Female ADHD Symptoms.

Does ADHD skip a generation?

Not in the way a single-gene trait can. People inherit an increased risk of ADHD rather than the condition itself, and the inheritance pattern is usually unknown^[2].

Twin research treats diagnosed ADHD as the far end of a continuous trait; a study of 16,366 Swedish twins found a strong genetic link between clinical-level and sub-threshold symptoms^[1]. Polygenic risk for ADHD also predicts lower levels of ADHD symptoms in the general population^[5].

A parent or grandparent can therefore carry risk variants without meeting diagnostic criteria, or without ever having been assessed. That can look like a skipped generation in a family tree.

Both, and the two are hard to separate. Faraone and Larsson note that the high heritability estimate itself encompasses gene-by-environment interaction^[1].

The consensus statement says environmental risks act very early, during the fetal or early postnatal period^[5]. For most of them, it adds, the association may still reflect correlated genetic and environmental effects rather than a direct cause^[5].

Smoking in pregnancy shows why this matters. Three meta-analyses covering more than three million people linked it to a more than 50% higher incidence of ADHD, but the association disappears after adjusting for family history of ADHD^[5].

Some exposures hold up better. Very or extremely preterm birth or very low birth weight was associated with a threefold higher rate of ADHD across twelve studies of more than 6,000 participants^[5]. A Swedish register study of 1.2 million children found the stepwise link with prematurity was not explained by having a relative with ADHD^[5].

Higher blood lead levels were associated with quadrupled odds of ADHD in a meta-analysis of 14 studies with more than 17,000 children^[5]. A meta-analysis of seven studies with more than 25,000 participants found no association between sugar consumption and ADHD in young people^[5].

CDC's list of possible risk factors also includes alcohol and tobacco use during pregnancy, head injuries in childhood, and parental mental health and family environment^[6]. NIMH-supported research continues to examine genes, hormones, brain structure and activity, and prenatal and early-life exposures^[7].

Can someone have ADHD with no family history?

Yes. In rare cases, ADHD-like symptoms follow extreme deprivation early in life, a single genetic abnormality or a traumatic brain injury early in life^[5].

New mutations play a part in some cases. An exome-sequencing study of ADHD patients without a family history reported six de novo missense variants in brain-expressed genes^[1].

No genetic test can diagnose ADHD. CDC states there is no single test for ADHD; diagnosis is a multi-step clinical process^[6].

Polygenic risk scores add up the small effects of many variants. In the 2019 genome-wide data they predicted ADHD in a dose-dependent way but explained only 5.5% of the variance^[1]. The consensus statement adds that findings on genetic and environmental causes help explain ADHD but are not useful for diagnosing it^[5].

Genes can matter for treatment. Variants in the CYP2D6 gene make some people slow metabolizers of the non-stimulant atomoxetine, whose half-life ranges from 5.2 hours in rapid metabolizers to 21.6 hours in slow metabolizers^[1].

How clinicians actually assess ADHD is covered in ADHD Test: How Screening and Diagnosis Work, and the diagnostic criteria are listed in ADHD Symptoms in Adults and Kids. Medication options are compared in ADHD Meds.

Much of the genetic risk for ADHD is shared with other conditions. The 2023 GWAS estimated that 84–98% of ADHD-influencing variants are shared with other psychiatric disorders^[4].

The overlap with autism spectrum disorder is strong. Twin studies have found genetic correlations between ADHD and autism above 0.50^[1], and in the 2023 analysis 84% of ADHD risk variants also appeared to influence autism^[4].

Over 90% of ADHD risk variants also appeared to influence schizophrenia and major depression^[4]. The same paper notes that 12–16% of people with ADHD are also diagnosed with autism and around 40% with depression^[4].

Shared genetic or familial causes extend to some physical conditions too, including migraine, obesity and autoimmune diseases^[5].