The study, led by the Barcelona Institute for Global Health (ISGlobal), a center supported by the La Caixa Foundation, and Aarhus University, found that young people diagnosed with ADHD or ASD in Denmark now more closely resemble the broader population than those who received the same diagnoses 10 years ago.

Published in JAMA Psychiatry, the study suggests that the rise in ADHD and ASD diagnoses may partly reflect changes in who is identified and diagnosed, as well as possible changes in the likelihood of developing related symptoms.

How ADHD and autism diagnoses have changed

Previous research has identified several characteristics that occur more frequently among people diagnosed with ADHD or ASD than in the general population. These include being born prematurely or with low birth weight, having lower parental education or household income, having a parent with a history of mental illness, and using health services more often before diagnosis.

The researchers wanted to determine whether these patterns changed as ADHD and ASD diagnoses became more common.

For the study, the team analyzed information from more than 2.1 million children and adolescents in Denmark. Researchers compared more than 71,000 young people diagnosed with ADHD or ASD between 2012 and 2022 with more than 713,000 young people who had neither diagnosis.

The gap between diagnosed youth and the general population narrowed

Young people diagnosed with ADHD or ASD were still more likely than their peers to have many of the characteristics previously associated with these conditions. However, the size of those differences steadily decreased during the study period.

Low birth weight provides one example. At the beginning of the study period, children born with low birth weight were 54% more likely to later receive an ADHD or ASD diagnosis than children born at a normal weight. By the end of the study, that difference had fallen to 17%.

The researchers found similar reductions in the association between preterm birth, socioeconomic disadvantage, and later ADHD or ASD diagnosis.

“The important message is not that these risk factors are no longer important,” says Magnus Elias Tharp, a doctoral student at Aarhus University and lead author of the study. “What we found is that people diagnosed in recent years are more similar to the general population than people who had the same diagnosis 10 years ago.”

Why ADHD and autism diagnoses may be increasing

Much of the research on ADHD and ASD focuses on why diagnosis rates are rising or which factors may increase the likelihood of developing these conditions. This study takes a different approach by examining whether the population receiving a diagnosis has changed over time.

Researchers say several developments could contribute to this shift. Increased awareness of ADHD and ASD may have helped health care and education systems identify cases more effectively. Expanded access to diagnostic evaluations and changes in the point at which neurodevelopmental characteristics are considered clinically impairing may also play a role.

The study cannot determine how much each factor contributed. However, the findings indicate that increases in ADHD and ASD diagnosis rates should not automatically be interpreted as evidence that the incidence of subclinical ADHD or ASD is increasing by the same amount.

“Our findings help us better understand why diagnoses of ADHD and autism have increased so markedly in recent years,” says Oregel Plana Ripoll, a researcher at ISGlobal and Aarhus University and lead author of the study. “These findings do not indicate that these conditions are being overdiagnosed or are becoming less severe. Rather, they indicate that the number of people receiving these diagnoses is changing over time, and this should be taken into account when interpreting current trends.”

Further research will be needed in other countries to determine whether the findings can be replicated elsewhere.

What the findings mean for ADHD and autism research

The results may affect how scientists interpret changes in the prevalence of ADHD and ASD. They may also help inform the planning of medical, educational, and social services for people with these diagnoses.

The findings could also influence how researchers interpret reports of improved outcomes among people with ADHD or ASD. Some apparent improvement may partly reflect changes in the characteristics of the population currently being diagnosed, rather than improvements in the prognosis of the symptoms themselves.