Autism diagnoses are more frequently recorded in boys during childhood, whereas identification among girls often occurs later, during adolescence or early adulthood, according to a recent study.
Over time, autism spectrum disorder diagnoses became increasingly common in Sweden, based on longitudinal registry data spanning more than 30 years. By 2022, cumulative diagnosis patterns showed substantially smaller differences between sexes by age 20 years, with modeling suggesting further narrowing in early adulthood beyond the observed follow-up period.
Researchers analyzed autism spectrum disorder (ASD) diagnoses using nationwide Swedish registry data from more than 2.7 million people born between 1985 and 2020. Patients were followed from birth until ASD diagnosis, death, emigration, or the end of follow-up in 2022, allowing observation up to age 37 years. Diagnoses were identified using the Swedish National Patient Register, and analyses were restricted to patients whose parents were both born in Sweden.
Age at diagnosis, calendar period, birth cohort, and sex were evaluated using age-period-cohort modeling with Poisson regression to disentangle overlapping temporal effects. ASD was diagnosed in 78,522 patients, representing 3% of the cohort. Incidence rates increased steadily across successive calendar periods for both sexes, with the highest rates observed from 2020 to 2022. During that period, incidence peaked among males aged 10 to 14 years and among females aged 15 to 19 years. The median age at diagnosis across the cohort was 14.3 years.
Sex differences were most pronounced in early childhood, with higher ASD diagnosis rates among boys before age 10 years. After that point, the male-to-female ratio declined with increasing age at diagnosis and across more recent diagnostic periods, falling below one among patients older than 15 years in 2020 to 2022. Cumulative analyses showed a rapid narrowing of the sex gap during adolescence, with ratios approaching unity by age 20 years. “Findings indicate that the male to female ratio for ASD has decreased over time and with increasing age at diagnosis,” noted lead author Caroline Fyfe, postdoctoral researcher, of the Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Solna, Sweden, and colleagues.
Several limitations warrant consideration. The study relied on register-based diagnoses, limiting detailed assessment of symptom severity, phenotypic variation, and co-occurring conditions. Changes in registry coverage over time, particularly the inclusion of outpatient diagnoses beginning in 2001, may have influenced observed age at diagnosis in earlier cohorts. Despite use of age–period–cohort models, the researchers noted limited ability to control for within-cohort confounders. Projections beyond 2022 were based on extrapolated data and carried greater uncertainty. Because analyses were restricted to patients with parents born in Sweden, findings may not be generalizable to more diverse populations.
In an accompanying editorial, the author questioned whether widely cited sex differences in autism prevalence reflect true incidence or diagnostic practice. The editorial pointed to differences in symptom expression, adaptive behaviors, and limitations of existing diagnostic instruments as contributors to later recognition in females. “This evidence seems to support the argument that systemic biases in diagnosis, rather than a true gap in incidence, underlie the commonly accepted 4:1 male to female ratio,” noted the author Anne E Cary, patient and patient advocate, of Boulder, Colorado.
Full disclosures can be found in the study. The editorial author declared no competing interests.
