Time trends in the male to female ratio for autism incidence: population based, prospectively collected, birth cohort study.

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.