Diagnoses surge as Intellectual Disability becomes less common among those diagnosed, Swedish study finds
As autism diagnoses increase, intellectual disability is becoming less common among diagnosed individuals, according to a Swedish study.
The proportion of people diagnosed with autism who also have an intellectual disability has fallen dramatically over the past two decades, according to a major new Swedish study, highlighting how the understanding and diagnosis of autism has evolved over time.
Researchers analysed national health records covering more than 3 million people born in Sweden between 1988 and 2016. Among the 81,286 individuals diagnosed with autism between 2001 and 2020, the share who also had an intellectual disability dropped from 55.8% in 2001 to just 6.7% in 2020. Over the same period, autism diagnoses increased by approximately 800%, primarily in individuals without intellectual disability (approximately 1800% overall).
The findings, published in Psychiatry Research, suggest that autism today encompasses a much broader range of individuals than it did two decades ago, when intellectual disability was far more commonly associated with the condition.
“Autism diagnoses have expanded substantially while the proportion of co-occurring intellectual disability has steadily declined,” explains neuropsychologist Alen Salkić, first author and PhD student at the Gillberg Neuropsychiatry Centre, arguing that modern autism diagnoses increasingly include people with average or above-average intellectual abilities.
The study was led by researchers from the University of Gothenburg, Karolinska Institutet and several other Swedish and international institutions. Using linked national registers, the team examined trends in autism and intellectual disability diagnoses over a 20-year period. Of the more than 81,000 people diagnosed with autism, 13,403 also had an intellectual disability diagnosis, representing an overall co-occurrence rate of 16.5%. The proportion of intellectual disability in autism decreased from 55.8% in 2001 to 6.7% in 2020, with a comparable decrease in females and males.
The decline in intellectual disability among people diagnosed with autism remained evident even when researchers focused only on children diagnosed at younger ages. Among children diagnosed with autism before age 13, the proportion with intellectual disability fell from 56.2% in 2001 to 8.8% in 2020. For children diagnosed before age seven, the figure declined from 68.3% to 15.2%.
According to the researchers, the results do not mean that intellectual disability itself has become less common. Instead, they point to major changes in how autism is recognized and diagnosed. Over the past two decades, diagnostic criteria have broadened, public and professional awareness of autism has increased, and clinicians have become more likely to identify autism in people with average intelligence and less severe symptoms.
The study notes that autism was historically viewed as a relatively rare condition often accompanied by significant cognitive impairment. Earlier estimates from the 1960s and 1970s suggested that around 70% of autistic individuals also had intellectual disability. Today, autism is understood as a highly diverse spectrum encompassing people with a wide range of abilities and support needs.
The researchers caution that the findings should not be interpreted as evidence that autism is being overdiagnosed. Rather, they argue that the concept of autism has changed substantially over time.
“Observed decrease in the proportion of intellectual disability in autism over two decades should not be interpreted as autism being over-diagnosed,” explains Alen Salkić, “Rather, our study results suggest that the conception of autism has changed considerably over time.”
The findings also have important implications for healthcare, education and social services. People with both autism and intellectual disability often require extensive lifelong support and face higher rates of medical and psychiatric conditions, including epilepsy, anxiety and attention-deficit/hyperactivity disorder. Previous research has also shown that autistic individuals with intellectual disability have a greater risk of premature mortality than autistic individuals without intellectual disability.
As autism diagnoses continue to rise, the researchers say policymakers and service providers must recognise that autistic people are an increasingly diverse group with varying levels of support needs. While some individuals may require intensive assistance with daily living and communication, others may benefit more from social support, educational accommodations or mental health services.
The authors conclude that understanding these differences will be crucial as demand for autism assessments and support services continues to grow. “It is crucial to carefully identify and address the needs of different groups,” says Salkić, “to ensure that the most vulnerable individuals are adequately supported.”
Text by Anna Spyrou, Communications Officer
Link to article:
Salkić A, Tideman M, Martini MI, Larsson H, Lichtenstein P, Chang Z, D'Onofrio B, Brikell I, Kuja-Halkola R, Knez R, Gillberg C, Taylor MJ, Lundström S. The proportion of intellectual disability in autism spectrum disorder over two decades. Psychiatry Res. 2026 Sep;363:117246. doi: 10.1016/j.psychres.2026.117246. Epub 2026 May 26. PMID: 42214315 Free article.
