Study reveals broader and more diverse diagnosis profile for ADHD and autism in Denmark over the last decade

A large Danish study indicates that children diagnosed with ADHD and autism are increasingly resembling their peers, as shifts in diagnostic criteria and awareness expand the profile of those being identified with neurodevelopmental conditions.

A large Danish study suggests that the children and teenagers being diagnosed with attention deficit/hyperactivity disorder and autism spectrum disorder have changed over time, becoming more like their peers in the general population than those diagnosed a decade earlier. Researchers from the Barcelona Institute for Global Health, known as ISGlobal, and Aarhus University said the shift may help explain why diagnosis rates have risen so sharply in recent years.

Writing in JAMA Psychiatry, the team analysed nationwide registry data from Denmark covering more than 2.1 million children and adolescents born between January 1, 1994, and December 31, 2022. They 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. The researchers found that factors previously associated with a higher likelihood of diagnosis, including premature birth, low birth weight, lower household income, lower parental education and parental psychiatric history, were still present, but the gaps narrowed steadily over the study period.

One striking example was low birth weight. At the start of the period studied, children born at a low birth weight were 54% more likely to later receive an ADHD or autism diagnosis than those born at a normal weight. By the end of the study, that difference had fallen to 17%. Similar declines were seen in the links with premature birth and several measures of socioeconomic disadvantage, suggesting that the diagnosed population has broadened over time.

Magnus Elias Tarp, the study’s first author and a doctoral researcher at Aarhus University, said the main point was not that these risk factors no longer matter, but that people receiving diagnoses in recent years look closer to the wider population than those diagnosed 10 years earlier. Oleguer Plana-Ripoll, senior author and a researcher at ISGlobal and Aarhus University, said the findings should not be taken as proof that ADHD or autism are being overdiagnosed or that the conditions have become less severe. Rather, he said, the results indicate that diagnostic trends are being shaped by changes in who is being identified, as well as any change in underlying risk.

The study points to several possible reasons for that shift. Greater awareness of ADHD and autism, wider access to assessments, improved identification in schools and healthcare settings, and changes in the threshold at which neurodevelopmental differences are judged clinically impairing could all be contributing. The authors said the data cannot show how much each factor matters, but they argued that rising diagnosis rates should not automatically be read as evidence that the underlying prevalence of these conditions has increased by the same amount. The findings may also matter for planning education, healthcare and social support, and for interpreting claims that outcomes are improving among people with ADHD or autism.