The study, led by the Barcelona Institute for Global Health (ISGlobal), a centre 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 earlier.

The findings suggest that the increase in ADHD and ASD diagnoses may partly reflect changes in who is being identified and diagnosed, alongside any possible changes in the underlying likelihood of developing these conditions. The research was published in JAMA Psychiatry.

Tracking changes in ADHD and autism diagnoses

Previous studies have identified several characteristics that occur more often among people diagnosed with ADHD or ASD than among the general population. These include being born prematurely or at a low birth weight, having parents with lower levels of education or lower household income, having a parent with a history of psychiatric disorders, and making greater use of healthcare services before receiving a diagnosis.

Researchers wanted to know whether those patterns had changed as ADHD and ASD diagnoses became more common.

To investigate, the team analyzed information from more than 2.1 million children and adolescents in Denmark. The researchers compared more than 71,000 young people diagnosed with ADHD or ASD between 2012 and 2022 with more than 713,000 individuals who had not received either diagnosis.

Differences narrowed over 10 years

Young people diagnosed with ADHD or ASD were still more likely than their peers to have many of the characteristics previously linked with these conditions. However, the size of those differences steadily declined 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, the difference had dropped to 17%.

Researchers found comparable declines in the associations involving premature birth and several measures of socioeconomic disadvantage.

"The key message is not that these risk factors are no longer important," says Magnus Elias Tarp, PhD student at Aarhus University and first author of the study. "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."

A different way to understand rising diagnoses

Much of the research surrounding ADHD and ASD has examined why diagnosis rates are increasing or which factors are associated with a greater likelihood of developing these conditions. The new study approaches the trend from another direction by asking whether the population receiving diagnoses has itself changed.

The researchers say several developments could potentially contribute to the shift. Awareness of ADHD and ASD has increased, healthcare and education systems may be identifying cases more effectively, access to diagnostic evaluations may have expanded, and the point at which neurodevelopmental characteristics are considered clinically impairing may have changed.

The study cannot determine how much each of these factors contributed. However, the findings indicate that rising diagnosis rates should not automatically be interpreted as evidence that the underlying occurrence of ADHD or ASD has increased by the same amount.

"Our findings help us better understand why ADHD and autism diagnoses have increased so markedly in recent years," says Oleguer Plana-Ripoll, researcher at ISGlobal and Aarhus University and senior author of the study. "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.

What the findings could mean

Researchers say the results could affect how scientists interpret changes in the prevalence of ADHD and ASD. They may also be important for planning healthcare, education, and social services for people with these diagnoses.

The findings could also influence how researchers interpret reports of improving outcomes among people with ADHD or ASD. Some apparent improvements may result partly from changes in the characteristics of the population now receiving diagnoses, rather than from an improvement in the prognosis of the conditions themselves.