Adolescent girls and boys are diagnosed with autism at nearly equal rates even though, earlier in childhood, boys are about three times as likely to receive this diagnosis, according to a new study.
The results add to mounting evidence that autistic girls tend to be diagnosed later in life than autistic boys. This disparity could reflect variability in the way autism traits manifest in boys versus girls, diagnostic issues or other factors, says study investigator Caroline Fyfe, a postdoctoral research associate at the University of Edinburgh, who conducted the work while at the Karolinska Institutet.
The new analysis, published in The BMJ last week, involved records from almost 3 million Swedish children over nearly 40 years.
The work represents “one of the biggest studies to date” of autism’s boy-to-girl ratio, says Casey Burrows, associate professor of pediatrics at the University of Minnesota, who was not involved in the research.
The researchers used Sweden’s medical birth register to track the rate of new diagnoses of autism among nearly everyone born in the country to Swedish parents between 1985 and 2020. “In a country where you can enumerate everyone, you avoid selection biases to a large extent,” says study investigator Sven Sandin, associate professor of epidemiology at the Icahn School of Medicine at Mount Sinai and a senior researcher at the Karolinska Institutet.
More than 78,500 participants, or 2.8 percent of the cohort, had an autism diagnosis by the end of 2022, the study shows.
Among children diagnosed before age 10, there are three boys for every one girl with autism. But among those diagnosed in adolescence, “the girls essentially caught up,” with the ratio shrinking to 1.2-to-1 by age 20, says Fyfe.
Changes in the autism diagnostic criteria over the years do not appear to fully explain the ratio’s shift, Sandin says.
The study didn’t explore why so many girls and women are diagnosed later in life. Autism traits may appear differently because of genetic and environmental factors, Sandin says, such that women and girls are less likely than men and boys to present themselves in ways that align with the diagnostic criteria.
For example, autism in men is commonly associated with language difficulty, but “females have slightly more advanced language development,” Burrows says. And women with autism struggle less with maintaining eye contact than do men with the condition, her work has found.
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Lauren Kenworthy, division chief of neuropsychology at Children’s National Hospital praises the “really rich data” the study provides but notes that the picture is far from complete. For example, she says, “the people included in the study do not represent the racial and ethnic diversity of the U.S.”
Sandin says their work excluded anyone whose parents were born outside of Sweden “to improve the statistical power and reduce the variation in the data” but that he and his colleagues are keen to explore how autism prevalence differs among women and men with varied backgrounds and experiences.
A postdoctoral researcher is currently exploring the prevalence of autism among second-generation immigrants in Sweden, he says. The team would also like to look into trends in autism diagnosis alongside co-occurring conditions such as attention-deficit/hyperactivity disorder and preterm birth.
“The sex ratio is much lower in children with intellectual disability as a comorbidity with autism,” Fyfe says.
Given the value of early diagnosis in terms of targeted intervention and treatment, the sex differences in children warrant further study. By studying more communities, Burrows says, “hopefully these different kinds of research can inform a more personalized identification” of autism traits.
