Repetitive behaviors have long been considered a hallmark of autism, but new research suggests that girls with the developmental disorder are less likely than boys to exhibit such symptoms.

In a study looking at over 700 kids on the spectrum, researchers found girls are far less likely than boys to display repetitive and restricted behaviors like limited flexibility with routines, hand-flapping or having narrow areas of interest. Moreover, researchers said they may be able to explain this gender disparity biologically.

The findings, published this month in the journal Molecular Autism, may offer clues to the lopsided gender breakdown seen in the autism population. According to the latest federal estimates, the developmental disorder is nearly five times more common in boys than girls.

“Autism has primarily been studied from the viewpoint of boys with the disorder,” said Vinod Menon of Stanford University School of Medicine, who worked on the study. “Understanding gender differences can help in identifying the behavioral skills that are most important to remediate in girls vis-a-vis boys.”

For the study, researchers looked at evaluations for 128 girls and 614 boys. The children scored similarly on assessments for social behavior and communication, but when it came to repetitive and restricted behaviors, girls had lower scores more in line with typically-developing individuals, the study found.

Meanwhile, the scientists sought to understand what might be driving these trends biologically. They looked at brain scans from 25 boys and 25 girls with autism as well as 19 typically-developing boys and 19 typically-developing girls, all of whom were matched for age and IQ.

Among children with autism, the researchers found differences between boys and girls affecting the portion of the brain responsible for motor activity, a component of many repetitive behaviors.

“This replication provides the strongest evidence to date for gender differences in a core phenotypic feature of autism,” Menon said.

The findings may point to a need for a different approach with girls, researchers said.

“The discovery of gender differences in both behavioral and brain measures suggests that clinicians may want to focus diagnosis and treatments for autistic girls differently than boys,” said Kaustubh Supekar of Stanford University School of Medicine who led the study.

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