Autism and ADHD: Often Together, Too Often Diagnosed Years Apart

Autism and ADHD very often come together. Depending on how and where it is measured, between a quarter and a half of autistic people also have ADHD, and around one in five children and adolescents with ADHD meets the threshold for autism. Yet until 2013 the DSM, the American Psychiatric Association’s diagnostic manual, did not allow the two to be diagnosed together — and many people still receive one diagnosis and then wait years for the other.

In ordinary life this can look like a child whose restlessness is named as ADHD early, while the social exhaustion, sensory overload and need for sameness are explained away for years. Or the reverse: an autistic teenager whose difficulty starting tasks and holding attention is put down to “just the autism”, when a second need sits beside it that deserves its own support. For many adults it means a second referral and a second waiting list, begun only after the first has ended. None of this is about collecting labels. It is about whether services see the whole person.

Between a quarter and a half of autistic people also have ADHD

The largest review of co-occurring mental health diagnoses in autistic people pooled 96 studies (1993–2019) that used confirmed DSM or ICD diagnoses. Of the eight conditions it pooled, ADHD had the highest prevalence, at 28% (95% CI 25–32%), ahead of anxiety disorders at 20%; clinical samples gave higher estimates than population and registry studies^[3]. A 2021 meta-analysis of 63 articles focused on ADHD alone put current ADHD at 38.5% and lifetime ADHD at 40.2%, with age, intellectual disability, recruitment setting and diagnostic criteria all shifting the estimate^[4].

Studies that start from whole populations find a large overlap too. In Spain, a two-phase school-population study screened children through parents and teachers, then assessed 781 in person against DSM-5 criteria: ADHD was present in 32.8% of the autistic children^[5]. In Sweden, combining the national patient register with prescription records, 48% of the 28,468 autistic people born in 1987–2006 also had ADHD^[1]. In a Stockholm student cohort, 44.4% of autistic students without intellectual disability had an ADHD diagnosis, against 4.3% of non-autistic students^[6].

The overlap does not fade in adulthood. Among more than 3.5 million US adults enrolled in Medicaid in 2008–2019, 26.7% of autistic adults without intellectual disability and 40.2% of those with intellectual disability had an ADHD diagnosis, against 2.7% of a general Medicaid sample — prevalence ratios of 5.1 and 6.8 after adjusting for age, sex, ethnicity and other factors^[7]. Medicaid enrollees qualify through low income, disability or other routes, so they are not a cross-section of all American adults.

And the other way round

Among children and adolescents with ADHD, a 2020 meta-analysis of 22 publications (combined sample 61,985) found that 21% met the threshold for autism: 19% in community samples and 24% in clinical ones, a difference that was not statistically significant^[8]. The figure was around 17% in Sweden’s registers^[1] and 9.8% in the Spanish school study^[5].

Until 2013, the DSM said you could not have both

The American Psychiatric Association’s summary of what changed in DSM-5, published in 2013, marks the turning point in one line: “a comorbid diagnosis with autism spectrum disorder is now allowed”^[9]. Under DSM-IV, a dual diagnosis was not permitted^[10], and ICD-10 carried the same exclusion^[1]. England’s independent ADHD Taskforce noted in 2025 that, with DSM-5 and ICD-11 reflected in NICE guidance, “co-diagnosis with autism is now allowed”^[11].

That history still shapes the evidence. A 2023 systematic review noted that accurate estimates had been hard to obtain because “a dual-diagnosis was not permitted until DSM-V”; across its 23 studies, reported ADHD symptoms in autistic young people without intellectual disability ranged from 2.6% to 95.5%, depending on the measure, informant, criteria and recruitment^[10]. Practice did not always follow the rulebook: in Sweden, 11,837 people had both diagnoses recorded despite the ICD-10 exclusion^[1]. And adults assessed before the change may have been given only one diagnosis under the rules then in force.

One diagnosis first, the other years later

In the US National Survey of Children’s Health for 2011–12, parents of 1,496 children aged 2 to 17 with a current autism diagnosis reported when each diagnosis was made. About 20% had been diagnosed with ADHD first, and they received their autism diagnosis around three years later (95% CI 2.3–3.5) than children whose ADHD was diagnosed at the same time or after. The delay was consistent across childhood and independent of the survey’s rating of autism severity^[2].

The Netherlands shows the same pattern. On the Netherlands Autism Register, children and adolescents first diagnosed with ADHD received their autism diagnosis an average of 1.8 years later than those with autism alone — 1.5 years later for boys and 2.6 years for girls. No such delay was found among adults, though the researchers urged caution about that result^[12]. The longer wait for girls fits the wider pattern described in our article on autistic women and girls.

For many children the second diagnosis had not come at all. In the Spanish school study, only 15.8% of children found to have both conditions had previously been diagnosed with both, even though a parent or teacher had reported signs of both in 95% of them^[5].

Autism and ADHD run together in families, with partly shared genetic roots

A 2010 review of family, twin and gene studies concluded that family and twin studies “do provide support for the hypothesis that ADHD and ASD originate from partly similar familial/genetic factors”, while noting that very few gene-level studies had examined the overlap^[13]. The Swedish register study then tested the family link directly. Autistic people had about 22 times the odds of having ADHD recorded (a diagnosis or prescribed ADHD medication) compared with non-autistic people, and the association was stronger for autistic people without intellectual disability. Their relatives were also more likely to have ADHD, most of all where relatives share the most genes: the identical twin of an autistic person had about 18 times the odds of ADHD compared with twins of non-autistic people, against four to five times for non-identical twins and full siblings. The authors concluded that this pattern “supports the existence of genetic overlap”^[1].

In 6,866 Swedish twins aged 20 to 28, ADHD and autistic traits were correlated, with genetic correlations between trait dimensions of 0.33 to 0.56 — but genetic and non-shared environmental influences accounted for similar shares of the overlap^[14]. Genes are part of the story, not all of it. None of this means one condition causes the other, or predicts any one person’s future. It does mean that where one condition is present, in a person or their family, services should be alert to the other.

Having both is linked with harder school years and higher health risks

The clearest education evidence comes from Stockholm, where researchers followed all 364,957 young people old enough to finish compulsory school in 2001–2011, including 6,138 autistic students without intellectual disability, 42% of whom also had ADHD. By age 16, 86.3% of non-autistic students qualified for upper secondary education, compared with 61.0% of autistic students without ADHD and 52.4% of autistic students with ADHD. After adjusting for family income, parental education and other factors, having ADHD as well was associated with an additional 8 in every 100 autistic students not qualifying at 16, and nearly 6 in 100 at 20. Autistic students were more likely to finish compulsory school when given extra time, and the authors called for “supportive interventions” during compulsory school^[15]. A follow-up found the same gradient in completing upper secondary school: compared with students with neither condition, autistic students with ADHD had 6.03 times the odds of not completing (adjusted), against 4.99 for autism alone and 3.95 for ADHD alone. The authors framed this as a question about how well inclusive school policies serve autistic students’ needs^[6] — a question about schools, not about the students.

Mental health follows a similar pattern. In 556 young adult twins from the Twins Early Development Study (born in England and Wales in 1994–96; the sample was enriched for autism and ADHD), high autistic traits and high ADHD traits were each independently linked with more depression and anxiety, and having high levels of both “exhibited the greatest risk for mental health challenges”^[16]. That study measured traits, not diagnoses. Our article on autism and anxiety covers anxiety in depth.

The same gradient appears in the most serious outcomes. Swedish register data on 54,168 autistic people recorded in 1987–2013, compared with 270,840 matched controls, found the highest odds of suicidal behaviour among autistic people without intellectual disability who also had ADHD (43.7% of autistic people without intellectual disability in that cohort had ADHD). Compared with controls, this group had around seven times the odds of a recorded suicide attempt (OR 7.25). Taking other psychiatric diagnoses into account reduced these estimates, and the authors’ message was for services: be vigilant and “consider treatment of psychiatric comorbidity”^[17]. These are population figures, not predictions about any individual, and they point to needs services can meet. If this subject touches your life, support is available now. Our article on suicide and autism research sets out the wider evidence on prevention.

Wider health outcomes point the same way. In the US Medicaid study, 13.2% of autistic adults without intellectual disability who also had ADHD had a recorded substance use disorder, against 5.7% of those without ADHD. After adjustment, autistic adults with ADHD were 2.4 times as likely as autistic adults without ADHD to have a recorded substance use disorder, 1.4 times as likely to have a recorded injury and 1.3 times as likely to have a cardiovascular condition^[7]. Evidence on employment for adults with both is much thinner.

Two conditions, too often two separate queues

In England, the independent ADHD Taskforce reported in 2025 that “ADHD, autism and other neurodevelopmental services in England are often separated and siloed by diagnosis type and by age, so are not person-centric”^[11]. Its final report added that even though co-occurrence is very common, “diagnoses, services and budgets are siloed”, which “is neither evidence-based nor person-centred and can lead to clinical risk, inefficiencies and duplication of work”^[18]. For a family, that can mean one referral, wait and assessment for autism, then another for ADHD. The scale of England’s autism queue alone is set out in our analysis of NHS waiting data.

The guidelines already point the other way. In 2017 NICE added ADHD to the factors associated with a higher prevalence of autism in its under-19s guideline, noting that this “allows for joint assessment for both conditions in children and young people with signs or symptoms that could be caused by either condition”; the guideline also asks clinicians to consider ADHD as a possible coexisting condition as part of an autism assessment^[19]. NICE’s ADHD guideline lists autistic people among groups who “may have increased prevalence of ADHD”^[20], and its adult autism guideline lists a history of ADHD among the factors that, alongside persistent social and communication difficulties, should prompt consideration of an autism assessment, and asks for coexisting conditions to be assessed^[21].

Scotland’s 2021 National Neurodevelopmental Specification for children and young people starts from needs rather than single diagnoses: it recognises that co-existing needs are common, that assessment “may result in overlapping diagnoses, including: Autism, ADHD”, and that understanding of support needs “should not wait for diagnosis”^[22].

What the evidence asks of us

• Assess for both, every time. When someone is referred for autism or ADHD, the other should be actively considered, as NICE already recommends^[19][20][21]. The US and Dutch data show the cost of not doing so: years of delay, longest for girls in the Dutch data^[2][12].
• One front door, not two queues. England’s Taskforce called for “a single front door for holistic support and, where needed, for clinical assessment”^[11]. Commissioners should fund neurodevelopmental pathways that can assess and support both conditions together.
• Support that does not wait for the second diagnosis. Scotland’s principle^[22] matters most for people who have one diagnosis and are waiting for the other.
• Schools planned for students with both. In Stockholm, autistic students with ADHD were the least likely to qualify for or complete upper secondary school^[15][6].
• Mental health care that looks for both. The highest risks in the Swedish and British data sit with people who have both conditions^[16][17]; as the largest review put it, careful assessment of mental health “is an essential component of care for all people on the autism spectrum”^[3].
• Better data. Waiting-list and outcome statistics should make people with, or waiting for, both diagnoses visible. Research funders should prioritise adults, women, employment, autism among adults with ADHD, and studies outside Europe and North America.

More evidence is collected in our Autism & Health research library.

Language: this piece uses identity-first language (“autistic people”), the preference of most autistic-led organisations, and uses clinical terms such as “autism spectrum disorder” only when naming a study or diagnostic category. Co-occurring conditions are described as health needs that deserve care.

Why we're publishing this

Health Insurance UK is a commercial health-insurance resource, not a charity and not a campaign. We compiled this evidence because autistic people, families and autistic-led organisations pushing for joined-up assessment deserve the research in one clean, sourced place — ready for a commissioner, a school leader, an MP or a journalist. The case is theirs. Our part is to stand alongside it and say plainly what the figures show: autism and ADHD often come together, the systems built to recognise them too often work apart, and the people caught between two queues deserve better.

How to read this data

The figures come from different kinds of study and are not directly comparable. Clinical samples tend to give higher estimates than population or register studies^[3]. Register studies count recorded diagnoses, so they reflect who reached services and how clinicians diagnosed; the Swedish ADHD figures also drew on prescription records^[1]. The US childhood timing data are parent-reported^[2], and some studies measured traits or symptoms rather than diagnoses^[10][16]. Odds ratios are described as odds, and prevalence ratios as “times as likely”.

Separating the two conditions is itself contested: some clinicians question whether attention differences in autistic people are always ADHD, and call the reality of the overlap “a subject of debate”^[23]. Most evidence comes from Sweden, the US and a few other European countries; we found little from low- and middle-income countries, few large studies of ADHD diagnoses in autistic adults beyond US Medicaid, and no verified pooled estimate of autism among adults with ADHD. This is analysis of published research, not new research.

Use this data

Free to cite with attribution to Health Insurance UK. Charities, campaigners and journalists are welcome to reuse these figures and charts. For the underlying figures as a spreadsheet, get in touch.

Important: This article summarises published research for general information. It is not medical advice and cannot be used to diagnose, treat or predict anything about an individual. If you have concerns about your own or someone else's health, please speak to a qualified health professional.

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