South Korean researchers say they found that one in every 38 children showed some signs of autism. That's significantly higher than the rate in the U.S. of one in every 100 children.
Researchers questioned parents instead of basing their study on education and medical records like the U.S.
They say the findings don't mean there are suddenly more children with autism spectrum disorders. Instead, they believe many cases weren't counted in previous studies.
Two-thirds of the children with autism traits in the study were in the mainstream school population, hadn't been diagnosed before and weren't getting any special services. Many of those undiagnosed children likely have mild social impairments, rather than more severe autism.
The CDC wasn't involved in the new study, although another federal agency, the National Institute of Mental Health, provided some funding. The group, Autism Speaks, which advocates for more aggressive autism screening, also helped pay for the study. Autism Speaks had no role in the study's design.
The research, published Monday in the American Journal of Psychiatry, attempted to screen all 55,000 schoolchildren, ages 7 to 12, in a district of Goyang City, near Seoul.
However, only about two-thirds of mainstream children participated. About 63 percent of their parents filled out a survey. The researchers acknowledged that parents of affected children might be more likely to fill out the survey.
The Associated Press contributed to this story.