The widely held assumption that autism spectrum disorder (ASD) predominantly affects males may reflect diagnostic patterns rather than true differences in prevalence.

A large population-based study showed females are more frequently diagnosed later — rather than in early childhood like boys — narrowing the gender gap in ASD prevalence over time and resulting in nearly equal lifetime rates between the sexes by adulthood.

While boys were initially about four times more likely to be diagnosed with ASD than girls, that gap began to close at around age 10 and continued to shrink as diagnosis among girls increased during adolescence. By age 20, the ASD prevalence was approximately 1:1 between males and females.

“The male-to-female ratio for autism spectrum disorder has decreased over time and with age at diagnosis, demonstrating a pattern of female catch-up. These observations highlight the need to investigate why female individuals receive diagnoses later than male individuals,” Caroline Fyfe, PhD, Karolinska Institutet in Solana, Sweden, and her colleagues wrote.

The study was published online on February 4 in The BMJ.

Longitudinal Analysis

Comparison of ASD rates between males and females has typically been limited to childhood incidence in most studies and surveillance programs. In the US, for example, the surveillance system used by the CDC only tracks diagnoses through age 8 years.

To better understand the longitudinal trends in ASD diagnoses across a broader age range, the authors conducted a birth cohort study using the Swedish medical birth register, including 2,756,779 children born in Sweden from 1985 to 2020 who had complete records and whose parents were also born in Sweden.

The prospectively collected data included all ASD diagnoses based on International Classification of Diseases ( ICD)-9 (for 1987-1997) and ICD-10 (after 1997) for the children from age 2 years to a maximum age of 37 years (median age, 20.1 years) at the end of follow-up in December 2022.

Overall, 2.8% of the study population were diagnosed with ASD at a median age of 14.3 years. Incidence increased for each 5-year age interval throughout childhood, reaching the highest rates between 2020 and 2022 at ages 10-14 years for males and 15-19 years for females.

Incidence per 100,000 person years at these ages was 645.5 for males and 602.6 for females, declining in both sexes as age increased. Incidence during the study period also increased with each birth cohort and calendar period for both sexes.

The male-female ratio remained around 3:1 until children were aged 10 years, after which the ratio decreased with increasing age and calendar period. By age 20, the ratio had dropped to nearly 1:1.

Earlier research linked changes in ASD incidence over time to such effects as broadened diagnostic criteria. But controlling for that factor did not fully explain the sex disparity in prevalence, the authors noted.

The different finding here may be due to the inclusion of diagnoses in childhood, adolescence, and young adulthood. The longer follow-up period and increased awareness of ASD leading to more diagnoses in adolescence and young adults could also be factors, they suggested.

Delay in Diagnosis Worrisome

That females were diagnosed much later than males is a key finding in the study, researchers reported.

The diagnostic delay could be due to criteria favoring detection in boys or because females are usually better at masking their ASD symptoms, they wrote. Diagnostic overshadowing may also play a role, whereby females’ ASD diagnoses are overlooked or attributed to other conditions, such as anxiety, depression, or attention-deficit/hyperactivity disorder.

“The harms of underdiagnosis and misdiagnosis of autism in women — harms that are infrequently reported in medical research but are often discussed in the autistic community — extend beyond barriers to appropriate interventions, supports, and accommodations afforded to correctly diagnosed autism in women,” Anne E. Cary, an autistic patient and advocate, wrote in an accompanying editorial.

The study’s 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,” Cary added.

However, the ratio skew in childhood “may or may not be misleading,” she acknowledged. “It could be that the onset of autistic traits is delayed in female individuals,” she wrote.

“If that is the case, it may be unreasonable to assume that autism is being missed in young girls. It might, however, suggest that the assessment tools contain sex biases and need reworking,” Cary added.

Mixed Reaction to Findings

Autism experts who were not part of the study offered mixed reactions to the results.

Commenting for Medscape Medical News, James C. McPartland, PhD, Harris Professor of Child Psychiatry and Psychology at the Yale Child Study Center, Yale University in New Haven, Connecticut, said he was struck by the results “because the sex difference in diagnostic rate is something that’s been pretty consistent across countries and cultures and even across different diagnostic criteria as they’ve changed over time.”

Because ASD diagnostic criteria is similar in both Sweden and the US, the results likely aren’t due to differences in Sweden’s diagnostic criteria, said McPartland, who also is director of the Yale Developmental Disabilities Clinic and the Yale Center for Brain and Mind Health.

“It would be interesting to see if other datasets also show this decreasing [of a male-female ratio] over time,” McPartland said, adding that there are several potential explanations for the results.

“As awareness of autism increases or stigma associated with the diagnosis decreases, it might be these people are coming forward to get diagnoses more so later in life. Another possibility would be that, if bias and ascertainment is a factor, diagnosticians are getting better at picking up the signs of autism in girls and women,” he noted.

Given the overall increasing diagnostic rate, McPartland added that one clinical implication of the findings is that “supports and services for autistic people really are valuable and essential for a large and increasing segment of the population.”

Offering a different view, Diana Schendel, PhD, professor at the A.J. Drexel Autism Institute, Drexel University in Philadelphia, told Medscape Medical News that she was not surprised by the results because other studies have shown indications of these patterns.

While those studies showed individual components of trends seen here, this one is “trying to pull it together into a single story,” said Schendel, who also leads the Research Program in Modifiable Factors in Autism.

She found the methods appropriately rigorous and, like McPartland, believes they are likely generalizable to the US, if national surveillance by the CDC continued into adolescence instead of stopping at age 8 as is currently the case.

Schendel agreed with the potential reasons for delayed diagnoses in females that the study authors proposed, including diagnostic overshadowing that can happen with female patients — one of the key clinical implications of the findings, Schendel said.

When talking with patients about their health, clinicians “should be sensitive to [whether] they’re seeing the potential for autistic behaviors in that patient that may not have been detected at a younger age,” Schendel said. “Maybe those problems are helping to explain the health conditions that they’re coming to them for.”

Tara Haelle is a science and medical journalist based in Dallas.