A study found that children and adolescents diagnosed there today are more similar to the general population than those diagnosed a decade ago.

As ADHD and autism diagnoses rise in Denmark, the children and adolescents receiving them increasingly resemble the broader population in their birth histories, family backgrounds, and other measured characteristics. A new study suggests that understanding the increase requires looking at how the diagnosed population has changed, alongside possible changes in the underlying risk of developing these conditions.

Attention-deficit/hyperactivity disorder (ADHD) affects attention and impulse control, while autism spectrum disorder (ASD) affects social communication and patterns of behavior. The study found that several characteristics historically more common among young people with these diagnoses became less distinctive between 2012 and 2022.

Published in JAMA Psychiatry, the research was led by Aarhus University and the Barcelona Institute for Global Health (ISGlobal), a center supported by the “la Caixa” Foundation.

Broader recognition may explain rising diagnoses

The researchers suggest several possible explanations for the changing population. Greater awareness of ADHD and autism, better identification by healthcare and educational services, and broader access to assessments may all play a role. Another possibility is a shift in the threshold for diagnosis, meaning when traits are judged to interfere enough with everyday life to warrant a clinical diagnosis. The study could not determine how much each factor contributed.

These possibilities mean that rising diagnoses should not be interpreted solely as evidence that the conditions themselves are occurring more often.

“Our findings help us better understand why ADHD and autism diagnoses have increased so markedly in recent years,” says Oleguer Plana-Ripoll, a researcher at ISGlobal and Aarhus University and the study’s senior author. “They do not show that these conditions are being overdiagnosed or that they have become less severe. Rather,

they indicate that the population receiving these diagnoses has changed over time, and this needs to be taken into account when interpreting current trends. Further research in other countries would be needed to determine whether these findings can be replicated elsewhere,” he adds.

A weaker link to low birth weight

To examine who was receiving diagnoses over time, the team analyzed data from more than 2.1 million children and adolescents living in Denmark. Within that population, they compared more than 71,000 people diagnosed with ADHD or ASD between 2012 and 2022 with more than 713,000 peers without either diagnosis.

Earlier research had linked these diagnoses with a range of birth, family, and healthcare characteristics. These included preterm birth, low birth weight, lower parental education or household income, a parental history of psychiatric disorders, and greater use of healthcare services before diagnosis.

Young people diagnosed with ADHD or autism remained more likely than their peers to have these characteristics, but the differences progressively narrowed. At the beginning of the study period, children born at a low birth weight were 54% more likely to receive a later ADHD or ASD diagnosis than children born at a normal weight. By the end, they were 17% more likely to receive one. The researchers found similar reductions for preterm birth and several measures of socioeconomic disadvantage.

“The key message is not that these risk factors are no longer important,” says Magnus Elias Tarp, a PhD student at Aarhus University and the study’s first author. “What we found is that people diagnosed in recent years resemble the general population more closely than those who received the same diagnoses a decade ago.”

The researchers say these changes matter for planning healthcare, educational, and social services, as well as for interpreting estimates of how common ADHD and autism are.

They may also help explain why some recent studies have reported better outcomes among people with these diagnoses. Those improvements could partly reflect changes in the population being studied, rather than improvements in the expected course of the conditions themselves.