Rates of autism spectrum disorder have been increasing in recent decades. In 2002, 0.7% of 8-year-olds in the US had the condition, rising to 3.3% in 2022. Males are diagnosed with the condition around four times as often as females.
Various theories exist to explain the difference, including a so-called ‘female protective effect’ whereby females require a higher genetic burden than males for behaviors linked to ASD to become noticeable.
Theories also suggest that underdiagnosis or delayed diagnosis in females may be due to better social and communication skills, making certain traits linked to autism more difficult to identify.
In the current study, researchers used Swedish population data to examine changes in the male-to-female ratio in autism diagnoses over time. They analyzed data from close to 2.8 million individuals born in Sweden between 1985 and 2020. By the end of the follow-up in 2022, 78, 500 people had been diagnosed with autism.
Ultimately, the researchers found that the incidence rate for autism increased at each five-year interval throughout childhood. For males, diagnoses peaked between ages 10- 14 years at 645.5 per 100,000 person years. Meanwhile, for females, diagnoses peaked between ages 15 and 19 years old at 602.6 per 100,000 person years. By around age 20, the ratio of males to females diagnosed with autism was around 1:1.
“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,” wrote the researchers in their study.
Future research should investigate why girls and women are diagnosed with autism later than boys and men, they added.
Sources: Science Daily, BMJ