Changes in Characteristics Associated With ADHD and ASD Diagnoses Over Time

Importance Diagnoses of attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) have increased substantially in recent decades, but it is unclear whether associations of prediagnostic characteristics with these diagnoses have changed over time. Objective To examine temporal trends in associations between well-defined risk factors and subsequent diagnoses of ADHD and ASD. Design, Setting, and Participants This population-based matched case-control study was conducted using nationwide Danish registry data for individuals born between January 1, 1994, and December 31, 2022, followed up to age 18 years. Individuals diagnosed with ADHD or ASD before age 18 years between 2012 and 2022 were included as cases. Each case was matched to 10 controls without these diagnoses by sex, birth year, municipality of residence, and country of birth. Data were analyzed from April 2025 through June 2026. Exposure A list of 19 prediagnostic characteristics, including parental and family factors, birth and perinatal factors, and health care use. Main Outcomes and Measures Associations between prediagnostic characteristics and ADHD or ASD diagnoses by year of diagnosis, estimated as odds ratios (ORs) with 95% CIs using conditional logistic regression. Results Of 2 194 951 children and adolescents in the registry, 100 323 individuals (4.6%) were diagnosed with ADHD or ASD before age 18 years, among whom 71 317 children and adolescents were diagnosed between 2012 and 2022 and included as cases (26 452 female [37.1%]; median [IQR] age at diagnosis, 11.4 [8.1 to 14.9] years), matched with 713 170 controls. Individuals diagnosed with ADHD or ASD differed from controls across all characteristics, but differences diminished over time, with ORs attenuating toward the null, particularly for socioeconomic and perinatal factors. For example, the OR for low birth weight decreased from 1.54 (95% CI, 1.41 to 1.68) in 2012 to 2013 to 1.17 (95% CI, 1.10 to 1.24) in 2020 to 2022. Attenuation was greater for ADHD than ASD (eg, household income: mean yearly change in OR, 6.5% [95% CI, 5.7% to 7.3%] for ADHD; mean yearly change in OR, 0.7% [95% C, −0.1% to 1.5%] for ASD) and more pronounced among individuals diagnosed at ages 10 to 17 years than at younger ages. Patterns were similar across sexes and in analyses restricted to cases with higher diagnostic certainty. Conclusions and relevance This study found that from 2012 to 2022, children and adolescents diagnosed with ADHD or ASD became increasingly similar to peers without these diagnoses. These findings suggest that increasing diagnostic rates may reflect changes in diagnostic practices or health care capacity rather than changes in underlying risk alone and should not be interpreted as evidence that ADHD or ASD have become less impairing at the individual level.