Autism has long been viewed as a condition that predominantly affects male individuals, but a study from Sweden published by The BMJ shows that autism may actually occur at comparable rates among male and female individuals.
The results show a clear female catch-up effect during adolescence, which the researchers say highlights the need to investigate why female individuals receive diagnoses later than male individuals.
The prevalence of autism spectrum disorder (ASD) has increased over the past three decades, with a high male-to-female diagnosis ratio of around 4:1.
The increase in prevalence is thought to be linked to factors including wider diagnostic criteria and societal changes (eg, parental age), whilst the high male to female ratio has been attributed to better social and communication skills among girls, making autism more difficult to spot. However so far no large study has examined these trends over the life course.
To address this, researchers used national registers to analyse diagnosis rates of autism for 2.7 million individuals born in Sweden between 1985 and 2022 who were tracked from birth to a maximum of 37 years of age.
During this follow-up period of more than 35 years, autism was diagnosed in 78,522 (2.8%) of individuals at an average age of 14.3 years.
Diagnosis rates increased with each five year age interval throughout childhood, peaking at 645.5 per 100,000 person years for male individuals at age 10-14 years and 602.6 for female individuals at age 15-19 years.
However, while male individuals were more likely to have a diagnosis of autism in childhood, female individuals caught up during adolescence, giving a male to female ratio approaching 1:1 by age 20 years.
This is an observational study and the authors acknowledge that they did not consider other conditions associated with autism, such as ADHD and intellectual disability. Nor were they able to control for shared genetic and environmental conditions like parental mental health.
However, they say the study size and duration enabled them to link data for a whole population and disentangle the effects of three different time scales: age, calendar period and birth cohort.
As such, they write: "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."
"These observations highlight the need to investigate why female individuals receive diagnoses later than male individuals," they conclude.
These findings align with recent research and seem to support the argument that current practices may be failing to recognise autism in many women until later in life, if at all, says Anne Cary, patient and patient advocate, in a linked editorial.
She notes that studies like this are essential to changing the assumption that autism is more prevalent in male individuals than in female individuals, but points out that as autistic female individuals await proper 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."
