Over time, Mandy and others began to suspect that autism looks different in girls. When they interviewed girls or women on the spectrum, they couldn’t always see signs of their autism but got glimmers of a phenomenon they call “camouflaging” or “masking.” In a few small studies starting in 2016, the researchers confirmed that, at least among women with high intelligence quotients, camouflaging is common. They also noted possible gender differences that help girls escape clinicians’ notice: Whereas boys with autism might be overactive or appear to misbehave, girls more often seem anxious or depressed.

Last year, a team of researchers in the United States extended that work. They visited several school yards during recess and observed interactions among 48 boys and 48 girls, aged 7 or 8 on average, half of each group diagnosed with autism. They discovered that girls with autism tend to stay close to the other girls, weaving in and out of their activities. By contrast, boys with autism tend to play by themselves, off to the side. Clinicians and teachers look for social isolation, among other things, to spot children on the spectrum. But this study revealed that by using that criterion alone, they would miss many girls with autism.

Typical girls and boys play differently, says Connie Kasari, a researcher at UCLA who coled the study. While many boys are playing a sport, she says, girls are often talking and gossiping, and involved in intimate relationships. The typical girls in the study would flit from group to group, she says. The girls with autism appeared to be doing the same thing, but what was actually happening, the investigators learned, was different: The girls with autism were rejected repeatedly from the groups, but would persist or try to join another one. The scientists say these girls may be more motivated to fit in than the boys are, so they work harder at it.

Delaine Swearman, 38, says she wanted badly to fit in when she was about 10 or 11, but felt she was too different from the other girls in her school. She studied the girls she liked and concluded, “If I pretended to like everything they liked and to go along with everything, that maybe they would accept me,” she says. Her schoolmates were avid fans of the band New Kids on the Block. So Swearman, who says she had zero interest in the band, feigned a passion she did not feel. She made a few more friends, but felt she was never being herself. Swearman, like Jennifer, was not diagnosed until adulthood, when she was 30.

Even when teachers do flag girls for an autism evaluation, standard diagnostic measures may fail to pick up on their autism. For example, in a study last year, researchers looked at 114 boys and 114 girls with autism. They analyzed the children’s scores on the Autism Diagnostic Observation Schedule (ADOS) and on parent reports of autism traits and daily living skills, such as getting dressed. They found that even when the girls have ADOS scores similar to those of boys, they tend to be more severely impaired: The parents of girls included in the study had rated their daughters lower than the boys in terms of living skills and higher in terms of difficulties with social awareness and restricted interests or repetitive behaviors. The researchers say girls with less severe traits, especially those with high IQs, may not have scored high enough on the ADOS to be included in their sample in the first place.