Millions Studied and One Surprising Autism Pattern Emerged

A sweeping study from Sweden is challenging the long-held belief that autism mostly affects boys.

Autism has traditionally been seen as a condition that mainly affects males. However, a large study from Sweden published by The BMJ suggests that autism may be nearly as common in females as it is in males.

Researchers found a clear pattern in which females begin to catch up during adolescence. According to the study team, this trend raises important questions about why females are often diagnosed later than males.

Rising Autism Rates and a Persistent Gender Gap

Over the past 30 years, diagnoses of autism spectrum disorder (ASD) have increased substantially. During that time, the reported ratio of diagnoses between males and females has remained high, at roughly 4:1.

This overall rise in autism diagnoses is believed to be driven by factors such as broader diagnostic criteria and social changes (eg, parental age). The large difference between male and female diagnosis rates has often been explained by stronger social and communication skills among girls, which can make autism harder to recognize. Until now, no large-scale study had followed these patterns across the entire lifespan.

Tracking Autism Across the Life Course

To better understand these trends, researchers analyzed national health records from Sweden. The study followed 2.7 million individuals born between 1985 and 2022, tracking them from birth through as many as 37 years of life.

Over more than 35 years of follow-up, 78,522 people were diagnosed with autism. This represented 2.8% of the study population, with the average age at diagnosis being 14.3 years.

Diagnosis Patterns Shift With Age

Autism diagnosis rates increased steadily as children grew older. The highest rates were seen among males ages 10-14 years, at 645.5 per 100,000 person years. For females, the peak occurred later, at ages 15-19 years, reaching 602.6 per 100,000 person years.

Although males were more likely to be diagnosed during childhood, females began to close the gap during adolescence. By around age 20 years, the ratio of diagnosed males to females approached 1:1.

Study Limits and Strengths

The researchers noted that this was an observational study. They did not account for other conditions linked to autism, such as ADHD and intellectual disability, and they were unable to adjust for shared genetic or environmental factors like parental mental health.

At the same time, the authors emphasized that the study’s large size and long duration made it possible to examine an entire population. This allowed them to separate the effects of age, calendar period, and birth cohort.

A Changing Picture of Autism by Adulthood

Based on these findings, the authors wrote: “These findings indicate that the male to female ratio for autism 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.”

They added that “These observations highlight the need to investigate why female individuals receive diagnoses later than male individuals.”

Missed Diagnoses and Real-World Impact

The results are consistent with other recent studies and support concerns that many women with autism are not identified until later in life, or may not be diagnosed at all. Anne Cary, a patient and patient advocate, discussed these issues in a linked editorial.

She wrote that research like this is critical for challenging the belief that autism is more common in males than in females. She also warned that while autistic female individuals wait for an accurate diagnosis, “they are likely to be (mis)diagnosed with psychiatric conditions, especially mood and personality disorders, and they are forced to self-advocate to be seen and treated appropriately: as autistic patients, just as autistic as their male counterparts.”

Reference: “Time trends in the male to female ratio for autism incidence: population based, prospectively collected, birth cohort study” by 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 and Sven Sandin, 4 February 2026, BMJ.
DOI: 10.1136/bmj-2025-084164

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