In this nationwide Danish cohort study, we found a substantial increase in the incidence of ASD and ADHD over the past two decades, consistent with global trends. The rise was most pronounced for ADHD, particularly among females, who by 2024 had nearly reached the same IRs as males – narrowing a sex gap that was previously substantial. By age 18, 6.5% of males and 3.9% of females had been diagnosed with ASD, while 9.9% and 7.4% had received an ADHD diagnosis, respectively. The age at diagnosis for both ASD and ADHD changed throughout the study period towards a younger age at diagnosis in more recent birth cohorts. Using both prescription data and diagnosis codes from hospital-based care to define incident ADHD cases increased the cumulative incidence of ADHD from 6.9% to 8.7% by age 18. This suggests that relying solely on hospital diagnoses may underestimate the true incidence of individuals with ADHD, particularly among adolescents.

The main strength of our study was the use of nationwide health registers, enabling inclusion of an un-selected cohort of all children and adolescents in Denmark. Our definition of ADHD included ADHD medication use on top of hospital diagnoses, leading to improved case ascertainment and IR estimates. Our study also holds limitations. The Danish registers used in this study have high documented validity [21, 22, 26, 27], yet some degree of under-ascertainment and diagnostic misclassification is unavoidable in register-based data. In Denmark, an increasing number of ADHD and ASD assessments in children and adolescents take place outside hospital-based child and adolescent psychiatry in private psychiatric practice. Nevertheless, in 2024, 92% of registered ADHD diagnoses and 91% of registered ASD diagnoses were made in the hospital sector and thereby captured in our data [28]. As all redeemed prescriptions are recorded in the Danish National Prescription Registry, inclusion of ADHD-specific pharmacotherapy likely reduces under-ascertainment; however, individuals assessed and managed without medication may remain unidentified. Finally, while confounding is not applicable in this population-based descriptive study, future studies should aim to investigate determinants of observed trends including changes in diagnostic thresholds, referral patterns, and sociodemographic factors.

Overall, our findings align with prior studies documenting rising trends in both ASD and ADHD. The number of children diagnosed with ASD has increased steadily over recent decades in a number of countries, including Germany, the United Kingdom, and the United States [16, 29, 30]. Globally, ASD prevalence varies widely (0.1%-4.4%) across regions, likely reflecting methodological differences in measurements of ASD prevalence as well as variation in community awareness, diagnostic practices, and access to care [31]. Our estimated cumulative incidence of ASD (6.5% in males and 3.9% in females) exceeds the upper range of these global findings as well as previous Danish estimates [4]. In line with existing research, we observed a consistently higher incidence of ASD in males [16, 31, 32]. While some studies have reported a relatively steeper increase in ASD among females, leading to a decreasing male-to-female ratio [16, 32], our findings indicate parallel increases across sexes, although girls are diagnosed at a substantially older age than boys.

The incidence of clinically diagnosed ADHD has also increased in past decades, reaching global prevalence estimates of 8.0% among children and adolescents [33, 34]. Our finding that nearly 10% of boys and 7% of girls are diagnosed with ADHD by age 18, with indications of even higher prevalence in more recent birth cohorts, is notable. Several factors likely contribute to this trend, including improved access to treatment and growing awareness of sex-specific manifestations of ADHD, particularly the increased recognition of ADHD in females. Additional contributing factors include increased knowledge and awareness of ADHD among health care providers, parents, and educators [35]. The use of psychostimulant medications for ADHD has risen substantially since 2020 [9, 19, 36]. While this has partly been linked to pandemic-related stress and social restrictions [15, 37], our data show that the increase in ADHD incidence began prior to the pandemic and has continued thereafter, suggesting broader underlying drivers that warrant further investigation.

Internationally, ADHD prevalence has been reported to be approximately twice as high in boys (10%) as in girls (5%) [33], with evidence indicating that ADHD in females is often later recognized and treated [38, 39]. In our study, this sex disparity was less pronounced, with 9.8% of boys and 7.4% of girls diagnosed before age 18. Notably, from 2022 onwards, the incidence in girls nearly equaled that of boys, potentially suggesting improved recognition of ADHD among females in Denmark.

We evaluated the impact of combining both diagnostic data and redeemed prescriptions to define new cases of ADHD, which increased the estimated cumulative incidence proportion by age 18 year by 20%. This finding highlights the potential underestimation inherent in diagnosis-only definitions in register-based studies. The divergence at older ages likely reflects increasing use of private specialist assessments, where diagnoses are not recorded but prescriptions are captured. As waiting times for hospital-based child and adolescent psychiatric services increase, more families may seek evaluation and help in private settings, either through publicly subsidized providers or private insurance. This is supported by our finding of a growing discrepancy between diagnosis-based and prescription-only based ADHD definitions in recent years.

Our findings confirm that an increasing proportion of Danish children and adolescents are diagnosed with neurodevelopmental disorders, most notably young females with ADHD. Mechanisms driving this development are likely multifaceted and remain only partially understood. Possible contributors include increased public and clinical awareness, greater recognition of ADHD in girls, and changes in help-seeking and referral patterns during and after the COVID-19 period [14, 15, 40]. Greater awareness may lead to increased help-seeking and assessment of children with a broader range of symptom severity. In combination with constraints in service capacity and increased expectations for clinical support, this may in turn influence diagnostic practices [41]. With nearly 10% of children and adolescents diagnosed with ADHD and 5% with ASD in more recent birth cohorts, we need to consider whether these increases reflect improved diagnostic practices and recognition of previously undiagnosed individuals, or diagnostic inflation leading to diagnosis and medical treatment in children and adolescents with ambiguous or milder symptoms [42]. It is also possible that the Danish school system reforms in 2012 and 2013 that, respectively, introduced broader inclusion criteria for children with special needs and longer and less uniform school days, which may have worsened neurodevelopmental symptoms in some children leading to an increased demand for diagnostic clarification and potential medical treatment [43, 44]. This is particularly important as long-term safety data of the most used psychostimulant medication methylphenidate, remain limited and the association with adverse effects is uncertain [45].

In this nationwide Danish cohort, the incidence of both ASD and ADHD increased substantially over the study period, with marked temporal and sex-specific variations. While boys were diagnosed earlier and more frequently in early childhood, girls experienced a later but substantial surge in diagnoses in adolescence, effectively narrowing the historical sex gap for ADHD. This convergence suggests improved recognition of neurodevelopmental disorders, particularly in females, but may also reflect shifts in help-seeking behavior and diagnostic pathways. The inclusion of prescription data substantially increased incidence estimates, underscoring the importance of using broader case definitions in epidemiological research. The high proportion of Danish children diagnosed with ASD and ADHD highlights the importance of reflecting on current practices, including the widespread use of medication with limited knowledge of long-term side effects, and the need to consider non-pharmacological interventions.