It has long been believed that autism affects male individuals more.
From a new study from Sweden, published by The BMJ shows that autism can actually occur at similar rates in men and women.
The results show a clear catch-up effect in women during adolescence, which the researchers say underlines the need to investigate why female individuals receive a diagnosis later than male individuals.
Autism spectrum disorder (ASD) has increased over the past thirty years, with a high diagnosis ratio between men and women of approximately 4:1.
This increase in prevalence is thought to be related to factors such as broader diagnostic criteria and societal changes (e.g. parental age), while the high male-to-female ratio has been linked to better social and communication skills in girls, making autism more difficult to recognize. However, no major study to date has examined these trends.
To address this, researchers used national registers to analyze autism diagnosis rates for 2.7 million individuals born in Sweden between 1985 and 2022, who were followed from birth to a maximum of 37 years old.
During this long follow-up period of over 35 years, 78,522 (2.8%) of the individuals were diagnosed with autism at an average age of 14.3 years.
Diagnosis rates increased every five years throughout childhood, reaching 645.5 per 100,000 person-years for male individuals aged 10–14 years and 602.6 for female individuals aged 15–19 years.
Although male individuals were more likely to be diagnosed with autism in childhood, female individuals caught up during adolescence, bringing the gender ratio to nearly 1:1 by age 20.
This is an observational study and the authors acknowledge that they did not take into account other conditions associated with autism, such as ADHD and intellectual disability. They were also unable to control for shared genetic and environmental factors such as parental mental health.
“These findings indicate that the gender ratio in autism has decreased over time and with increasing age at diagnosis. This gender ratio may therefore be significantly lower than previously thought, to the extent that it may no longer be distinguishable in adulthood in Sweden,” they wrote.
“These observations highlight the need to investigate why female individuals are diagnosed later than male individuals,” the researchers concluded.
