Four times as many children have been diagnosed with autism in the past two decades amid improved awareness and screening and evolving definitions. A new study suggests diagnoses have increased at a faster clip among younger adults over the past decade.

Autism spectrum disorder spiked 175% among people in the U.S. from 2.3 per 1,000 in 2011 to 6.3 per 1,000 in 2022, researchers found. Diagnosis rates climbed at a faster rate among adults in their mid-20s to mid-30s in that period, according to a study published Wednesday in JAMA Network Open.

Researchers involved in the study set out to assess how many adults had been diagnosed with autism because earlier studies largely focused on school-age children, said Luke Grosvenor, the study’s lead author and a postdoctoral fellow at Kaiser Permanente’s Northern California research division.

“There has been a shortage of research focused on autistic adults,” Grosvenor said.

Children ages 5 to 8 had the highest autism rate at 30.3 diagnoses per 1,000 children in 2022. The second-highest rate was children 4 and younger at 28.8 per 1,000 children.

Study tracks growing autism rates in adults

The study found a far lower autism rate in younger adults than in children, but it also found that autism is increasing at a faster pace among those adults. The rate of autism for adults ages 26 to 34 increased more than 450% from 2011 to 2022, the study said.

The growing autism rates in adults show that “we need to improve transition services for autistic individuals and their families” when they become adults, Grosvenor said.

Though children might get access to services such as speech therapy and other therapy in school, they often lose access to such care when they become adults. But the need does not disappear, Grosvenor said.

“That is the services cliff in autism,” said Grosvenor, adding that it’s “critically important” that adults with autism get the care they need.

Experts not involved in the study said the research provides valuable insight into autism prevalence in adults, a group that federal researchers do not routinely monitor in autism surveillance.

Dr. Alycia Halladay, chief science officer for the Autism Science Foundation, said the figures in the study showed similar results to what the Centers for Disease Control and Prevention has reported for younger children. But she said the study’s estimates for adults are “enormously important” and draw attention to the need to track autism patients over time. She said the study also shows the value of harvesting information from medical records so researchers can provide a more detailed look at people with autism.

The study does not provide a more detailed look, however, at how older adults with autism are faring, Halladay said. She said she would like to see a study of adults with autism that addresses questions such as educational attainment and employment prospects.

Autism rates differ by race and sex

The JAMA network study also reported differences in autism rates by race, ethnicity and sex. The diagnosis rates were highest among American Indian or Alaska Native children and adults. The increase in autism was greater among Black, Asian, Hispanic and American Indian and Alaska Native children compared with white children.

Boys were still more likely to be diagnosed with autism than girls. But the report noted that autism diagnoses increased at a faster rate among girls than boys over the study period.

Halladay said biological factors contribute to higher autism rates among boys.

“The brains of boys and girls are just different, and they develop at different rates,” Halladay said. “Girls have better social skills earlier on, and so there is some thought that they could possibly be masking the features of autism better than boys.”