New research looks at the reasons autistic characteristics show up at a later age in girls, and how that phenomenon affects diagnosis and treatment.

Why do girls seem to develop autism later than boys?

And does this affect the way girls are diagnosed and treated for the condition?

Those are questions being discussed intently at an annual conference in California.

A research team, led by William Mandy, PhD, senior lecturer in clinical psychology at University College London, says it has gained new insights into the different ways that autistic characteristics present themselves in girls during adolescence.

Mandy presented the findings today at the 16th Annual International Meeting for Autism Research (IMFAR) in San Francisco.

The findings are new, but they echo theories offered by Hans Asperger in 1943 that were never tested. Asperger, a medical theorist, is known for his early work into autism spectrum disorders.

Mandy’s team conducted a longitudinal study, which repeatedly gathered data for the same test subjects over a period of time.

Researchers found that while boys tend to display stable, similar autistic characteristics throughout their adolescence, girls are more likely to see these characteristics ramp up during the teen and preteen years.

The findings could help explain why boys tend to be diagnosed with autism earlier than girls, and also how guidelines for diagnosing autism in children could be biased against girls.

Read more: Biomarkers in blood may help to detect autism earlier »

Autism is not the easiest condition to diagnose.

“Unlike some physical health difficulties, we don’t have a biomarker for autism,” Mandy told Healthline. “We don’t have blood tests or brain scans. We can’t actually see autism itself, so instead we do what people do in all mental health disorders, pretty much. We diagnose it not by looking at the thing itself, but by looking at its manifestation, its size, and its symptoms.”

In short, diagnosing autism isn’t quite an exact science. The criteria for diagnosing autism includes a group of observable characteristics and behaviors that the medical community has come to a consensus on as representing autism.

Generally speaking, these characteristics come down to difficulties in the realm of social communication and flexibility when it comes to things like switching activities and focus. Other autistic characteristics include sensitivity to outside stimuli like bright lights or loud noises.

“Autism is not a black and white thing,” said Mandy. “It’s a dimensional condition. So the people that we label as having autism are really just at the extreme end of a continuum that extends all the way through the population with no clear natural cut point between those that have autism and those that don’t. And what’s become clear from the research is that having autistic traits, even if they’re not at the level where we would conventionally label someone as having a clinical diagnosis of autism, that’s still a risk factor for a range of difficulties. For example, developing social anxiety problems, conduct problems, or anorexia.”

Read more: Waiting seven years for an autism diagnosis »

Mandy’s team looked at autistic traits in the general population, rather than limiting itself just to those people who are at the extreme end of the spectrum.

Autistic traits for the same group of children and adolescents was measured at ages 7, 10, 13, and 16.

Boys who showed high levels of autistic traits at age 7 tended to remain consistent over time, demonstrating similar traits at older ages.

Girls, on the other hand, showed a marked increase in levels of autistic social difficulties between the ages of 10 and 16.

Mandy said the findings were surprising, as previous medical wisdom stated that girls and women with autistic traits tended to “camouflage” them as they got older.

“If anything, I expected to see a decline in autistic symptoms in girls over time,” he said. “What’s very interesting is that there was one person who suggested the opposite, and that was Hans Asperger himself. There’s this rather intriguing sentence from this paper he wrote in the 1940s, where he’s wondering why we never see girls with what he would call ‘autistic psychopathologies.’ And he said, ‘Well, maybe it’s because these traits don’t show onset until adolescence with females.’ And nobody tested that idea. So it’s intriguing that that would be what we appear to have found on this occasion.”

Read more: Study sheds new light on brain anatomy of girls with autism »

So, are girls getting short-changed when it comes to autism diagnoses?

“It’s possible — it’s likely, in fact — that our current diagnostic criteria are rather biased toward the male presentation, and biased against the female presentation, said Mandy. “And there’s always been a sort of circular situation, that almost all autism research is done on males, which means that your diagnostic criteria reflects males, which means that you can continue to recruit a predominance of males in your research, and so it goes on.”

Besides this apparent bias, there’s also a strong likelihood that girls with autism present autistic characteristics in ways that are different – and subtler – than what are seen in boys.

One characteristic of autism, that holds true with both sexes, is a strongly focused interest on a particular topic.

Where the sexes often differ, says Mandy, is in the nature of this interest.

“There’s emerging evidence, and this certainly fits with my clinical impression, that girls with autism, their special and focused interests, are a little bit unusual than autistic boys,” he said. “They’re less likely to focus on something technical and specific, and perhaps more likely to focus on the social realm.”

So while a boy with autism might show a preoccupation with something technical like trains or buildings, a girl with autism is more likely to focus on hierarchies or lists of family and friends.

“Often, girls are more likely to be almost stereotypically gender-specific,” said Mandy. “So you meet a lot of autistic girls who are really into animals or horses, or fashion. And those interests, of course, don’t jump out at you as much. If you get a kid who comes along and says, ‘I’m obsessed with the District Line on the London Underground,’ then that looks unusual, and you think autism might be an issue. If you have a girl who says, ‘I’m obsessed with wearing the latest styles,’ that obviously doesn’t seem as unusual, so it’s less likely to alert people to the presence of autism.”

Mandy also points out that the way girls’ autistic traits seem to accelerate between the ages of 10 and 16 mirrors a changing and complex social world.

“I think for girls, there’s a phenomenon where they can be doing fine at primary education,” he explained, “but as the social world starts to become more complex, as they transition over into secondary school and the social demands of the adolescent female social world rapidly accelerate, these girls can really struggle, and people often don’t understand.”

Read more: Playing your child’s ‘autism card’ »

While changing guidelines to reflect traits of autism in girls seems like an obvious partial solution, it’s really not that simple.

Because autism exists on a spectrum and, as Mandy tells us, it’s not a black-and-white diagnosis, changing diagnostic guidelines could shift the focus too much.

“I think the way to go is to keep the same fundamental diagnosis,” Mandy said. “Fundamentally, this is about difficulties with social communication, a tendency toward inflexibility, but I do think that people need to be more flexible in thinking in terms of how these manifest and whether the way these manifest in girls and women — especially girls and women with a normal range IQ — is a bit different.”

People with autistic traits can thrive, but it’s crucial that their needs are recognized and that they’re placed in an environment where they’re able to excel.

“I think we need better understanding of the early presentation of autism in girls, so we can identify them in a timely way, and for those that need help, we can put that support in place before things start to go wrong in adolescence,” said Mandy. “I think we need to get better clinically at thinking dimensionally, and not just thinking in these black-and-white terms. Trying to understand people in a more subtle way, and thinking about if they have traits or conditions for autism diagnosis, those are important.”

Mandy says that now that his team has gleaned some new insights into autism in girls, they’d like to go more in-depth in order to better understand the condition.

“I think what we need to do now is look at it in a bit more depth. Who are these girls who are apparently not showing autistic traits in childhood, and who are showing them in adolescence?” he said. “And asking questions like, ‘Are these social difficulties actually autistic in nature, or are they arising from something else?’ If they are autistic in nature, what were the early indicators that were being missed by this measure of autistic traits in childhood? So, it’s really about trying to get a more detailed picture so we can properly understand the meaning of this finding.”