Autism primarily affects boys, right? Officially, boys exceed girls with autism by 4 to 1 (and 10 to 1 in "high-functioning" autism).1 In fact, almost everything we know about autism comes from studying boys. But now some scientists are making discoveries that are challenging common assumptions about autism, girls and gender.

They began with questions that have dogged researchers for years. Why are so few girls diagnosed with autism? Are girls simply less susceptible to autism? Does autism look different in girls, making it harder to detect or diagnose?

A growing number of studies suggest that girls with autism, particularly those without intellectual disability, may be hiding in plain sight. They appear to have less severe symptoms than boys, and to be better able to mask their social challenges at school.2-4 Although that may sound like good news, it can have a down side. According to research by the Interactive Autism Network and others, girls with milder forms of autism are diagnosed later than boys, possibly delaying intervention.2,5,6,8 Some may not be diagnosed at all. And, particularly in the teen years, girls with autism appear to suffer anxiety and depression more often than boys with ASD or typically-developing girls.

Examining gender differences from an early age

Several researchers said they began studying girls after hearing first-hand about their struggles to get a diagnosis and find services for autism spectrum disorder (ASD).

Psychologist Rachel Hiller Ph.D., now at University of Bath in England, led two studies of gender differences in Australia.2-3 In one, her team surveyed parents or grandparents of 92 boys and 60 girls about their children's behavior. All had autism but not intellectual disability. The result? Boys and girls with ASD look different as early as the preschool years, according to their caregivers.

For one, little girls reportedly were more likely than boys to mimic others in social situations and to want to "fit-in" with other kids. "They often develop a way to camouflage their symptoms. Also they tend to obsess over friendships and can develop them, or one or two close and likeminded allies," said another team member, Robyn Young Ph.D., an associate psychology professor at Flinders University in Australia.

Boys, on the other hand, were more likely to withdraw and isolate themselves from others. In fact, the boys' parents were more worried about their sons' isolation, while the girls' parents reported more concerns about emotional outbursts such as meltdowns.2 A separate study, of children in the Simons Simplex Collection project, added irritability and lethargy to the list of problems that affect girls more than boys with ASD.7

Girls control their emotions better at school, where they act far differently than they do at home, according to several studies of children with autism and average-range IQ.2-4 Teachers are much less likely to voice concerns about girls than boys.2 That is "possibly because girls tend to withdraw and boys are more disruptive," Dr. Young said. Teachers appear to focus more of their attention on boys with autism, who reportedly struggled with hyperactivity and classroom behavior.4

Nonetheless girls with autism were just as impaired, or more impaired, in their social and communication skills, according to some studies.4,7 Boys and girls have similar problems with understanding social situations. But some girls have an edge on boys when it comes to using gestures and maintaining a conversation.3

The 4 o'clock Explosion: Home vs. School Behavior in Autism

For many kids with ASD – but especially girls – parents say that their child manages to hold it together at school, but then comes home and has to release the pressure built up during a day of pretending to be someone else.

Interestingly, teachers may miss more autistic symptoms in girls than clinicians or parents. In one of Dr. Hiller's studies, teachers reported no concerns with conversational skills in half the girls with autism; clinicians, on the other hand, had no such concerns in only 17 percent of those girls.3

It's not clear why teachers, clinicians and parents see girls with autism so differently, both during the preschool years and beyond. Are girls better able to blend in at school, while falling apart at home?

"For many kids with ASD – but especially girls – parents say that their child manages to hold it together at school, but then comes home and has to release the pressure built up during a day of pretending to be someone else. They call it the 4 o'clock explosion," said researcher William Mandy Ph.D., senior lecturer at University College London.

"So in that sense, the finding that girls are especially likely to fly under the radar at school does fit with my clinical experience, and with the reports of quite a few parents I have spoken to," Dr. Mandy said in an email interview.

And as a group, girls were more likely to control their behavior in public; they were less likely to have public meltdowns, make socially inappropriate comments, or speak too loudly.3 In other words, they may be hiding their autism.

Liane Holliday Willey described her efforts to cope with ASD in her aptly-titled autobiography, Pretending to be Normal: Living with Asperger's Syndrome. In it, she records her exhausting attempts to fit into the "neurotypical" world, with its puzzling social rules and conventions.

Pretending – or trying to blend in – at school sounds like a coping skill, but it also may hinder diagnosis. Teachers often make referrals for educational or disability testing for their students. Is this one way girls go undiagnosed and untreated, by failing to attract attention in the early school years?2 How can a family doctor decide whether to test for autism, if teachers and parents have vastly different impressions of a girl's behavior?3

Even if parents or teachers do refer a girl for testing, do her symptoms align with those commonly found in boys with autism? If they don't match, then will those diagnostic tests and descriptions of autism – developed with boys in mind – fail to detect the disorder in some girls?

"Everyone said, 'She's fine.'"

Some parents of girls say their daughters' symptoms differed from typical (meaning male) autism, making it harder to convince people of a problem.

Leslie knew something was different about her two-year-old daughter, Ann. The girl wouldn't respond to her name or make eye contact, but her outward behavior did not seem very unusual to others. "Everyone said, 'She's fine. There's nothing wrong with her.' But I was very persistent."

She enrolled Ann in a preschool that her brother had attended, and asked the teacher to keep an eye out for developmental problems. By January of that school year, the teacher told her, "I absolutely agree with you," Leslie recalled. Her daughter didn't line up toys, like many boys with autism do. Instead, "she played with them in a markedly different way than other children. She also did not interact with other children in an age-appropriate way," said Leslie, who participated in the Simons Simplex Collection autism research program. (She did not want their real names used to protect her daughter's privacy).

With the teacher's observations in hand, Leslie took her daughter for testing, which led to an ASD diagnosis. "I knew how a typically developing child should behave since I had an older son; my daughter's behaviors were different. Because I was very insistent on pursuing answers, I was able to get her diagnosed relatively early," she said.

Are girls less "obsessed" than boys with autism?

Research backs up the observations of parents like Leslie. Some girls with autism really do have less obvious autistic symptoms than boys with the same diagnosis.

The list of common autistic mannerisms – which experts call "repetitive and restricted behaviors" – comes primarily from studying boys with autism. These behaviors include lining up toys, a fascination with spinning wheels or parts of objects, or obsessions with trains, motors, video games, or mechanical objects. But now researchers say girls with autism have either milder repetitive behaviors,4,7 or simply different ones.2-3

For one, girls with ASD are less likely than boys to line up or sort things.2-4 One study of children in the Simons Simplex Collection, which includes Ann, found lower levels of obsessive interests in girls than in boys with ASD.7

A study by the Interactive Autism Network also found that girls in its large, U.S-based registry have milder autistic mannerisms than the boys.8 "This and other studies are suggesting that girls with ASD … differ from males in key symptoms and behaviors, particularly around social interactions," said IAN Director Paul H. Lipkin M.D. "We must consider whether the girls are not only being recognized later but also may be under-identified due to less pronounced symptoms."

Most studies only include girls who have been diagnosed with autism. One large study in the United Kingdom looked at girls with autistic symptoms but no ASD diagnosis. It found that girls with high levels of autistic traits – but without behavior or learning problems – may be missed by the tools used to diagnose ASD. That may be due to a "gender bias" in diagnosis or because girls are able to adapt better to their condition than the boys, those scientists concluded.9

Can fashion or shell collections be a sign of (female) autism?

Some researchers wonder if girls' obsessions and rituals may not show up on autism screenings or be flagged by parents because they seem less odd or disruptive.

Dr. Hiller said that girls are more likely to have "seemingly random obsessional interests (e.g., collecting shells, pencils) from the pre-school years right through to adolescence. Perhaps these interests are harder for clinicians and family doctors to identify when the girl is young as they are more in-line with the interests of typically-developing girls. However, as the girl grows older it may become more apparent that the interest is very different from her same-aged peers. It is also possible that, in the pre-school years, girls' interests are less intense or less disruptive on family life, and thus less likely to be identified as problematic."

Dr. Mandy said girls with autism may share the same interests as other girls – but with a twist. "For example, I have met quite a few females with ASD with very, very strong interest in fashion, hair and appearance in general. Nothing unusual about the topic of interest, but in these instances it was pursued with an autistic intensity."

American psychiatrists now require someone to have some repetitive behaviors and obsessions in order to receive an autism spectrum disorder. Researchers have wondered whether girls and women will find it harder to get an ASD diagnosis under that new criteria.7

If diagnostic tests – and doctors – sharpened their focus on "female autism," would the gender ratio be different? The verdict is out.

Dr. Mandy said, "I do believe that males are more susceptible to ASD than females, but I think we exaggerate the extent of this gender difference, especially amongst people with normal-range IQ and fluent speech. If you asked me to take a bet on the true gender ratio in ASD, I would say it is two males to every female. And I think that one reason why [autism] clinics tend to find a higher male-to-female ratio is that some females never come to the attention of ASD services, because they mask their autistic difficulties more effectively that males."

If girls can learn to mask their symptoms, could boys do so as well? "Studying how girls with ASD camouflage their difficulties will certainly teach us about boys too. Lots of boys are camouflaging too. Are they doing it differently? Less well, or as well? Less often? All these questions remain to be answered scientifically," Dr. Mandy said.

Is there a cost to all the pretending? A number of studies show that girls with ASD tend to experience depression and anxiety more often than either boys with ASD10-11 or typically-developing girls,12 although no one knows why.

Dr. Mandy's research points out that some of the differences between girls and boys with autism – such as those related to depression and anxiety – mirror gender differences in the general population.4 Few studies have teased out the role gender plays among the population with, and without, autism.

"When we see gender differences on the autism spectrum, are these similar to gender differences in the rest of the population? To answer this we need to include male and female, non-ASD control groups in our studies of ASD gender differences," he said.

And that is a question for future research.

Thank you to the thousands of families registered with IAN Research. Their willingness to share information about their experiences with autism made the IAN research study mentioned in this article possible.