Young Girls Are Still Less Likely To Be Diagnosed Autistic Than Boys, And We Still Don't Fully Know Why

Autism is a form of life-long neurodivergence that influences how people perceive the world around them and how they interact with others. This can manifest in differences in social communication, focused interests, and repetitive behaviors. 

The prevalence of autism has been increasing over the last 30 years. For instance, in 2002, around 0.7 percent of 8-year-olds in the US were diagnosed with the condition, while 3.3 percent were diagnosed in 2022. However, this rise has not been equal across the sexes, with the male-to-female diagnosis ratio being around 4:1.

Overall, the reasons for this increase in prevalence have been put down to multiple factors. Rather than there being a sudden autism “epidemic” as some may have us believe, the study authors suggest the increase is largely due to a mix of societal and diagnostic changes (though some environmental factors likely play a role too). 

Firstly, there is now greater awareness of the condition and far less social stigma surrounding it. More people have an understanding of it, allowing parents and teachers to recognize traits in their children (or even themselves) so they can seek a diagnosis.

At the same time, there are now broader diagnostic criteria available which take into account subtler signs that have previously been overlooked.

But why is there such a large disparity between the sexes, especially when there is clear evidence that females undergo a “catch-up effect” during adolescence? Traditional explanations have included the idea that girls are better at communicating and have better social skills when they’re young. This, the argument goes, makes makes their symptoms more difficult to spot.

However, there have not been any large studies that have examined these trends over a life duration. That’s where this new study from Sweden comes in. The researchers used national registers to analyze diagnosis rates of autism for 2.7 million individuals born in Sweden between 1985 and 2022. These individuals had data that tracked them from birth to a maximum age of 37 years.

The team found that over this long period, autism was diagnosed in 78,522 individuals (2.8 percent of the study population) at the average age of 14.3 years. They also found that diagnosis rates increased with each five-year age interval throughout childhood. This peaked at 645.5 per 100,000 person years for males at age 10-14 years, and 602.6 for females at 15-19 years of age.

It was clear from the data that males were more likely to have a diagnosis during childhood, whereas females caught up during adolescence, resulting in a male to female ratio around 1:1 by age 20.

The authors acknowledge that this observational study did not account for other conditions closely associated with autism, such as ADHD or intellectual disability. They also did not control for shared genetic and environmental conditions, such as parental mental health. However, their study size and the duration they analyzed does allow them to link data across a large population. It also lets them disentangle the effects of timescales, such as age, calendar period, and birth cohort.

“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,” the team explain in a statement.

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

The results further demonstrate that current practices are failing to recognize autism in many young girls and women until they are much older than their male counterparts.

Clearly, autism is just as prevalent in females as it is in males, and this study goes a long way to help challenge existing assumptions and biases. 

In particular, Anne Cary, a patient and patient advocate, argues in an associated editorial that, as many autistic females 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.”

The study has received largely positive responses from other experts. Although a few have commented on the language the study used to discuss autism, as well as the limitations of the data the authors obtained, most praised it for its timeliness. 

“This paper is timely and provides important support for what autism researchers have known for a long-time: that autism is markedly undiagnosed in people assigned female at birth,” Dr Rachel Moseley, Principal Academic in Psychology at Bournemouth University, who was not involved in the study, said in a comment to the Science Media Centre.

“The study’s longitudinal follow-up of young people is essential, because the signs of autism are often only visible at a slightly later age in girls; this means that studies which only look at toddlers may well miss autistic features which are already present in males but not yet visible in females. Moreover, since the study uses national data, this means that it’s unaffected by biases in who chooses to opt in or opt out to the data.”

"Autism is not a male condition, yet girls struggle to get a diagnosis in a timely or robust manner,” Dr Monique Botha, Associate Professor in Social and Developmental Psychology at Durham University, who was also not involved in the study, added.

“Importantly, these diagnostic disparities are not unique to Sweden. We see parallel patterns in the UK and given the UK’s more fragmented diagnostic pathways and constrained infrastructure, there is good reason to think these inequities may be even more pronounced here.”

The study is published in The BMJ.