Time trends in the male to female ratio for autism incidence: population based, prospectively collected, birth cohort study
Caroline Fyfe, Henric Winell, Joseph Dougherty, David H Gutmann, Alexander Kolevzon, Natasha Marrus, Kristina Tedroff, Tychele N. Turner, Lauren A. Weiss, Benjamin H K Yip, Weiyao Yin, Sven Sandin
Abstract
OBJECTIVES: To examine changes in the male to female ratio in diagnoses of autism spectrum disorder (ASD) over a 35 year period, providing temporal trends in diagnosis (incidence rate), the male to female ratio, and the age-cohort specific cumulative male to female ratio (cMFR). DESIGN: Population based, prospectively collected birth cohort study. PARTICIPANTS: 2 756 779 liveborn children recorded in the Swedish medical birth register between 1985 and 2020. SETTING: Sweden. MAIN OUTCOME MEASURE: Age-period cohort analysis investigating associations between ASD and age at diagnosis, calendar period, birth cohort, and sex, quantified by incidence rate ratios and associated two sided 95% confidence intervals. RESULTS: Among 2 756 779 individuals born in Sweden between 1985 and 2020, ASD was diagnosed in 78 522 (2.8%) by the end of follow-up (2022). The incidence rate for ASD increased with each five year age interval throughout childhood, peaking at 645.5 (per 100 000 person years) for the male cohort at age 10-14 years and 602.6 for the female cohort at age 15-19 years in 2020-2022, and then decreased. Age specific incidence of ASD increased for each calendar period and birth cohort between 1985 and 2020. The male to female ratio decreased with increasing age at diagnosis and, for those older than 10 years, by calendar period. For the final year of follow-up in 2022, the cumulative male to female ratio for incidence of ASD was 1.2 by age 20 years. Further projection of these trends suggested that the cumulative male to female ratio would reach parity at age 20 years by 2024. CONCLUSION: Findings indicate that the male to female ratio for ASD has decreased over time and with increasing age at diagnosis. This male to female ratio may therefore be substantially lower than previously thought, to the extent that, in Sweden, it may no longer be distinguishable by adulthood. This finding highlights a need to investigate why girls and women receive diagnoses of ASD later than boys and men.