Four times as many children have been diagnosed with autism in the past two decades, amid improvements in awareness and screening and evolving definitions. A new study suggests that diagnoses have increased more rapidly among younger adults over the past decade.

Autism spectrum disorder has increased 175% among people in the US, from 2.3 per 1,000 in 2011 to 6.3 per 1,000 in 2022, the researchers found. Diagnosis rates rose at a faster rate among adults in their 20s and 30s during that time, according to a study published Wednesday in JAMA Network Open.

The researchers involved in the study aimed to assess how many adults have been diagnosed with autism because previous studies have mostly focused on school-age children, said Luke Grosvenor, the study’s lead author and a postdoctoral fellow in the research division of Kaiser Permanente of Northern California.

“There has been a lack of research focused on adults with autism,” Grosvenor said.

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

Study tracks rising rates of autism in adults

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

Rising rates of autism in adults show that “we need to improve transition services for people with autism and their families” when they become adults, Grosvenor said.

Although children may have access to services such as speech therapy and other therapies in school, they often lose access to such care as they become adults. But the need isn’t going away, Grosvenor said.

“This is the bedrock of autism services,” Grosvenor said, adding that it was “important” that adults with autism get the care they need.

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

Dr. Alycia Halladay, chief scientific officer of the Autism Science Foundation, said the numbers from the study showed similar results to those reported by the Centers for Disease Control and Prevention for younger children. But she said the study’s estimates for adults are “enormously important” and call attention to the need to follow autistic patients over time. She said the study also shows the value of collecting information from medical records so researchers can provide a more detailed look at people with autism.

Still, the study doesn’t provide a more detailed look at how older adults with autism fare, 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 gender

The JAMA Network study also reported differences in autism rates by race, ethnicity, and gender. Rates of diagnosis 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 to 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 among boys during the study period.

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

“Boys and girls’ brains are simply different and develop at different rates,” Halladay said. “Girls have better social skills earlier and are therefore thought to be able to mask autistic traits better than boys.”

This article originally appeared on USA TODAY: Autism is becoming more prevalent in young adults