Although boys are four times as likely to be diagnosed with autism spectrum disorder than girls, a growing number of experts say autism in girls and women may be detected late or missed entirely because the disorder presents itself differently between genders.

And a new study by researchers at Children’s Hospital of Philadelphia has discovered yet another subtle but significant difference between school-age boys and girls with autism: The way they tell stories.

That finding points to the need for better research into gender differences in what has become the country’s fastest growing developmental disability.

The new research, published last month in the journal Molecular Autism, found that girls with autism use significantly more so-called cognitive process words – such as think or know or feel – than boys on the spectrum. The autism spectrum girls’ use of such words was closer to neurotypical girls’ and even neurotypical boys'. However, both girls and boys on the spectrum tended to use more nouns in their storytelling than their typically developing peers.

The study found that the storytelling of the girls with autism showed an overlap between neurotypical children – something that could delay an accurate diagnosis from clinicians expecting the same behaviors from girls and boys with the disorder.

It is not uncommon for women to go undiagnosed until well into adulthood, in some cases after a lifetime of social difficulties and no explanation.

“Autism is a social condition diagnosed using observable behavior, so we wanted to study an observable skill that relates to social ability,” said lead study author Julia Parish-Morris, a scientist with CHOP’s Center for Autism Research. “We chose storytelling because it involves much more than grammar and vocabulary. It relies on a sense of social appropriateness and sheds light on what speakers decide is important to convey.”

However, the greater use of cognitive process words by the girls with autism compared with their male peers didn’t mean the females’ behavior was more socially typical. The children who participated in the study were chosen because they had comparable severity of their autism symptoms, including equal socially adverse impacts, Parish-Morris said.

The new study wasn’t the first time the scientist encountered linguistic differences between boys and girls with autism. In an earlier CHOP study, Parish-Morris and other researchers looked at the use of filler words — such as uh and um — that are commonly used in many languages. Girls with autism tended to use um, the language space holder heard more in the speech of neurotypical children, than uh. Um is thought by some experts to signal more communication sophistication. Boys with autism used uh more.

One possible reason for the differences is that females on the spectrum have been known to exhibit behaviors — either intentionally or subconsciously — that appear at least superficially to be more like typical youngsters'. Those behaviors — making more eye contact, for example — can somewhat camouflage what experts expect from people on the spectrum.

Among the takeaways from this study, according to its authors, is the need to do more autism research that explores the differences between the genders and enables diagnoses that are informed by those differences. To do that, they say, there need to be more studies that include a greater proportion of females with autism.

Past studies of narrative abilities and speech patterns in people with autism have been done mostly with male subjects, assuming the results would hold true or females, according to Parish-Morris. Increasingly, autism experts say such assumptions can be erroneous.

In the CHOP study, the authors included a larger proportion of females than past comparable research — 21 girls and 41 boys with autism, and 19 girls and 21 boys who were neurotypical. The children were tasked with telling a story from illustrations that included a fisherman, a cat, and a fish.

“We just have such a poor understanding of the brains and the processes of verbally fluent [autistic] girls, it’s hard to make inferences about what all of this means on the inside,” Parish-Morris said. “Right now, we’re just observing what happens on the outside.”

With more gender-specific information, Parish-Morris said, fewer girls may fall through the diagnostic cracks and miss out on crucial early intervention services and other supports.

“I think every time we learn a little bit more about how these girls are presenting, we have that much of a chance of identifying them in childhood before they reach adolescence or adulthood,” said Parish-Morris.