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Since the mid-1990s, the number of children, young people and adults diagnosed with ADHD or autism spectrum disorder (ASD) has more than quadrupled across the Western world, including Denmark.

Despite decades of intensive research, there is still no single, definitive explanation for this sharp rise. A Danish-led research team supported by the Lundbeck Foundation has now uncovered a previously unknown part of the picture: the genetic profile of people receiving these diagnoses has changed over time.

Using advanced genetic analyses of anonymised blood samples from more than 37,000 people in Denmark diagnosed with ADHD or ASD between 1994 and 2016, the researchers found that the level of genetic predisposition associated with receiving a diagnosis fell significantly during the period.

The findings were first published in the prestigious journal JAMA Psychiatry in early June.

“We found a clear decline over time for both ADHD and autism diagnoses,” says Sonja LaBianca, a postdoctoral researcher and doctor in child and adolescent psychiatry at the Institute of Biological Psychiatry, Mental Health Services in the Capital Region of Denmark.

She led the international research team, which included scientists from Denmark, Sweden, the UK, the US and Spain.

A 20 per cent decline

Genetics plays an important role in a person’s likelihood of developing ADHD or autism. DNA analyses show that people with these conditions tend to carry certain genetic risk variants more frequently than the general population.

However, neither ADHD nor autism is caused by genetics alone. Both conditions arise through a highly complex interaction between genetic predisposition and a wide range of environmental, social and individual factors.

The researchers’ findings suggest that people diagnosed more recently carry, on average, fewer of the genetic risk variants associated with ADHD or autism than people diagnosed in the past.

When the team compared the DNA profiles of 37,000 people diagnosed between 1994 and 2016, they found that the average genetic burden associated with the two conditions had fallen by approximately 20 per cent.

“We do not know exactly how many genetic risk variants are involved in ADHD and autism, but my best estimate is that the number runs into the tens of thousands,” says Andrew Schork of the Institute of Biological Psychiatry.

Schork specialises in the genetics of psychiatric disorders and is one of the lead authors of the article in JAMA Psychiatry.

The rise in diagnoses over the past 30 years means that more than 80,000 people in Denmark now have an autism diagnosis. Almost four per cent of the population – approximately 240,000 people – have been diagnosed with ADHD.

A changing definition of normal?

The growing number of diagnoses might suggest that the criteria for receiving an ADHD or autism diagnosis have simply become less stringent. According to Niels Bilenberg, however, the explanation is more complex.

Bilenberg is a professor and consultant at the Department of Child and Adolescent Psychiatry in the Region of Southern Denmark and is familiar with the study by Sonja LaBianca and her colleagues.

“In genetic terms and in terms of the number of symptoms, people may now receive a diagnosis on a less pronounced clinical basis than they did previously,” he says.

“But my clinical impression is that, even when people present with fewer symptoms, the impact on their lives can be just as profound. The conditions appear to be equally burdensome and limiting in terms of everyday functioning as they were in the past.”

Bilenberg continues: “One possible explanation for the increase in diagnoses is that many people find it more difficult to meet the demands of modern society and the education system. It may also reflect a broader cultural shift. Our definition of what is considered normal may have become narrower, leaving more people feeling that they do not fit in or are perceived as different.”

Sonja LaBianca also cautions against interpreting the results as evidence that it has simply become easier to obtain a diagnosis.

She points to several other developments that may have contributed to the increase.

“ADHD and autism are less stigmatised than they used to be, and more people are therefore willing to seek help from mental health services. Adults can also now receive treatment for ADHD, which was not an option 20 years ago.”

Sonja LaBianca also points to revisions of the official diagnostic guidelines as a possible contributing factor. Both the American DSM classification system and the World Health Organization’s ICD system are revised periodically, and these revisions can affect how conditions are defined and diagnosed.

A shifting diagnostic threshold

The study published in JAMA Psychiatry drew on 37,000 samples from the Danish iPSYCH database. These included samples from 17,000 people diagnosed with autism and 20,000 diagnosed with ADHD.

The iPSYCH database contains DNA material from 140,000 anonymised people born in Denmark between 1981 and 2008. The material was obtained from heel-prick blood samples routinely collected shortly after birth.

Of the 140,000 people included in the database, approximately 90,000 received a psychiatric diagnosis between 1994 and 2016. The remaining 50,000 form a randomly selected control group of people born during the same period who had not received a psychiatric diagnosis.

An updated study covering the period from 2016 to 2026 would be an obvious next step, but the current iPSYCH data cannot be used for this purpose.

“If such a study were possible, I suspect it would show that this trend has become even more pronounced over the past decade,” says Niels Bilenberg.

“The diagnostic threshold has continued to shift,” he says.