Everyone thought autism mostly affected boys. This study says otherwise

The researchers observed a clear pattern in which females begin to close the gap during adolescence. They say this trend points to an urgent need to better understand why females are often diagnosed later than males.

Rising Autism Diagnoses Over Time

The prevalence of autism spectrum disorder (ASD) has risen steadily over the past 30 years. Throughout this period, diagnosis rates have shown a pronounced imbalance, with males diagnosed about four times as often as females.

Experts believe the overall rise in autism diagnoses is linked to broader diagnostic definitions and social factors (eg, parental age). The large difference between male and female diagnoses has often been explained by the fact that girls tend to have stronger social and communication skills, which can make autism harder to identify. Until now, however, no large study had followed these patterns across different stages of life.

Following Millions Across the Lifespan

To fill this gap, researchers analyzed national health records covering 2.7 million individuals born in Sweden between 1985 and 2022. Participants were followed from birth for as long as 37 years.

Over more than 35 years of observation, autism was diagnosed in 78,522 individuals, representing 2.8% of the population studied. The average age at diagnosis was 14.3 years.

How Autism Diagnosis Rates Change With Age

Autism diagnosis rates rose with each five year age group throughout childhood. Among males, the highest rate occurred between ages 10-14 years, reaching 645.5 per 100,000 person years. For females, the peak came later, between ages 15-19 years, at 602.6 per 100,000 person years.

While males were more likely to be diagnosed during childhood, females showed a strong increase in diagnoses during adolescence. By about age 20 years, the ratio of males to females diagnosed with autism approached 1:1.

Study Limitations and Strengths

The authors noted that this research was observational. They did not account for other conditions often linked to autism, such as ADHD and intellectual disability. The study also could not fully adjust for shared genetic or environmental influences, including parental mental health.

At the same time, the researchers emphasized that the scale and duration of the study made it possible to analyze data from an entire population. This allowed them to separate the influence of age, calendar period, and birth cohort.

Autism Rates May Equalize by Adulthood

Based on their analysis, 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 Consequences

The findings are consistent with recent research suggesting that autism in women is frequently missed or identified much later in life. In a linked editorial, patient and patient advocate Anne Cary said the results support concerns about gaps in current diagnostic practices.

She emphasized that studies like this help challenge the long-standing belief that autism is more common in males than in females. However, she also warned that while autistic female individuals wait for 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."