Far more people are being diagnosed with ADHD and autism today than in the past, and a new study from the University of Copenhagen shows that the genetic contributions have decreased over the past 20 years.
The number of people diagnosed with ADHD and autism in Denmark has increased substantially in recent decades. In 2000, around 0.1% of the population had an ADHD diagnosis; by 2022, this figure had risen to over 3%. A new study from the University of Copenhagen suggests that this trend is largely driven by changes in how diagnoses are made today.
The researchers analysed genetic data from more than 37,000 individuals diagnosed with ADHD or autism in Denmark between 1994 and 2016. The results show that people diagnosed in more recent years, on average, have a lower genetic predisposition for these conditions than those diagnosed earlier.
This suggests that today's diagnoses encompass a broader group of individuals and milder symptom profiles than before.
"Genetics is our most objective measure, because the frequency of genes causing ASD or ADHD has not changed over the last 100 years - evolution just doesn't work that fast," says Andrew J. Schork, Associate Professor at the Department of Neuroscience at the University of Copenhagen and senior author of the study.
"The criteria and culture for how diagnoses in psychiatry are made - that can change rapidly, over even just a few years. Our study shows that individuals diagnosed in 2016, where rates are higher, carry less genetic risk than individuals diagnosed in 1996, where rates are lower."
Genes have not changed - diagnoses have
The researchers used so-called polygenic risk scores, which combine the effects of thousands of small genetic variations associated with ADHD and autism. Both conditions are highly heritable - around 80% - meaning genetics plays a major role, although individuals can have high genetic risk without developing the disorder.
Because the genetic makeup of a population does not change substantially over a few decades, this approach provides a unique window into how diagnostic practices have evolved.
The study shows that more recently diagnosed patients have around a 10-15% lower genetic risk for ADHD and up to 25% lower for ASD than those diagnosed in 1994. Importantly, the kind of genetic risk - the pattern of risk for ADHD, ASD and other disorders like schizophrenia or bipolar - was very similar across the decades.
The researchers therefore conclude that the increase in diagnoses is best explained by a lower diagnostic threshold - meaning that more individuals with milder, but similar, symptoms are identified and diagnosed today.
Not evidence of overdiagnosis
- More than 37,000 individuals with ADHD or autism were included in the analysis
- The researchers used data from blood samples from infants born between 1981 and 2008 as part of the iPsych research project. This data was combined with diagnoses from 1994 to 2016 from Danish health registries
- For each person, the researchers calculated a polygenic risk score-an overall measure of genetic predisposition based on thousands of genetic variations.
- The study is published in the renowned psychiatric journal JAMA Psychiatry
Andrew J. Schork emphasizes that the findings should not be interpreted as evidence that people are being over- or misdiagnosed.
Rather, it shows that individuals diagnosed with ADHD or autism still carry a higher genetic risk than the general population and a pattern of risk that was relatively stable.
"Our results do not invalidate people's diagnosis of ADHD or autism. Rather, they show that clinical practice, diagnostic manuals and society's understanding of these diagnoses have changed to be broader over time. ," says Sonja LaBianca, postdoc at the Institute of Biological Psychiatry at Copenhagen University Hospital and the study's lead author.
However, the study ends at 2016, and the diagnostic rates have increased even further over the past 10 years.
"Unfortunately, we cannot speak to changes in the polygenic profile beyond 2016," says Sonja LaBianca.
Could reshape the public debate
The rise in ADHD and autism diagnoses has been at the centre of intense public debate in recent years, touching on issues such as pressure on psychiatric resources, the role of schools, social media and screen use, and concerns about overdiagnosis.
The researchers believe their findings add an important layer of nuance.
"A challenge is a lack of objective data - there is no confirmatory blood test for a psychiatric disorder. As such, we believe genetics has an important role to play in these debates, and our data point to a process by which diagnoses have gradually become broader and more inclusive, while maintaining a consistent core genetic profile" says Andrew J. Schork.
