MONDAY, May 9 (HealthDay News) -- An estimated one in 38 South Korean children -- or 2.6 percent -- has an autism spectrum disorder, a new study says -- figures that experts believe could be similar in the United States.

Estimates of autism prevalence in the United States generally currently put the number at about 1 in 110 children, or less than 1 percent.

But those numbers may be underestimates because they rely on information about children who have been diagnosed with autism or who have autism symptoms noted in their medical records, explained Geraldine Dawson, chief science officer for Autism Speaks, a New York City-based advocacy and research organization, which funded the research.

The new South Korean study used a more comprehensive, population-based approach that looked for autism among all children, including those in mainstream schools who had never been identified as having problems.

The researchers said it's likely that using a similar approach in the United States would also spot more kids with autism spectrum disorders.

"We expect the prevalence of autism spectrum disorders in the United States and in other countries, if we used the same method, will be in the range of 2 to 3 percent," said Dr. Young Shin Kim, an assistant professor at the Child Study Center at the Yale University School of Medicine and lead author of the study.

However, Kim stressed that the study does not provide evidence that more children are developing autism nowadays vs. before. Rather, she said, "it means they have been there all along but have not been counted in previous prevalence studies."

Kim also said that she and her team were "surprised by the magnitude" of the difference from earlier estimates.

Their findings were published online May 9 online in the American Journal of Psychiatry.

For their study, researchers from Yale and George Washington University assessed more than 55,000 children, 7 to 12 years old, who lived in and around Seoul, South Korea, for an autism spectrum disorder. Kim said that South Korea was chosen because it is modern, politically stable, has national health insurance and has a high literacy rate.

Collecting the data took five years. Researchers had the parents or teachers of children, gathered from the rolls of regular and special education schools and a disability registry, answer an autism spectrum screening questionnaire. Children who screened positive for possible autism were offered more comprehensive assessments.

They found that 2.6 percent of the children had an autism spectrum disorder. Among children in special education with an autism spectrum disorder, boys outnumbered girls five to one. In the general population, the ratio was 2.5 to one, the study found.

The reason for the differences, according to the researchers, may be that children who are receiving special services are more likely to have intellectual disability, and intellectual disability is more common among males than females.

In the special education group, about three-fourths of the children had classic autism and one-fourth had a milder form, such as Asperger's syndrome or Pervasive Developmental Disorder-Not Otherwise Specified (PDD-NOS).

In the other children, three-fourths had PDD-NOS or Asperger's, and one-fourth had classic autism.

The average IQ of children in the general population suspected to have autism was higher than it was for children in the special education group (98 vs. 75). About 59 percent of children with autism in the special education group had an intellectual disability, but just 16 percent of those with autism in the general school population did, and even then it tended to be mild.

Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at the Steven and Alexandra Cohen Children's Medical Center of New York, said that "one of the striking findings is that there were significant number of children who seem reliably identified as having an autism spectrum disorder who were in general education population and who were not previously identified as having any problem and who were receiving no services."

Though many of those cases of autism were on the mild end of the spectrum, he noted, some were more severe.

But Adesman said it seems likely that there are also children in U.S. schools who have an autism spectrum disorder that hasn't been identified.

"There are children who come into my practice at 7 or 9 years old who haven't yet been diagnosed," Adesman said.

The researchers said that replicating the results in other populations of children -- including in the United States and worldwide -- is essential.

More information

The U.S. National Institute of Mental Health has more on autism.