In South Korea one in 38 children may have autism, a surprisingly high number based on a new research approach that suggests autism is a global problem that is significantly underdiagnosed.
The estimate, which translates into 2.64% of children, is far higher than the estimated 1% rate seen in studies in the United States and Europe.
The study is the first to estimate autism in South Korea, and while the results needs to be confirmed, it suggests autism may be more common than previously thought.
"Are we surprised? Yes," said Dr Young-Shin Kim of Yale University, whose study was funded by the advocacy group Autism Speaks and published online in the American Journal of Psychiatry.
Dr Kim's team used a painstaking research method that involved screening 55,000 children aged 7 to 12 in the South Korean city of Goyang. The team surveyed parents about their children's behaviour, then followed up with evaluations of at-risk children to confirm their diagnosis.
This population-based approach was designed to capture cases that might not be detected with methods that use school or medical records to identify autistic children.
"The high prevalence comes from this new population we included in our study - the kids without any previous developmental delays or mental health issues," Dr Kim said.
The autism rate among children from special schools was 0.75%, compared with 1.89% in regular school classrooms.
In Asia, parents are largely ignorant about the disorder, which has no cure, and are reluctant to face it.
"A lot of parents in Korea do not recognise autism symptoms. We are not sure if the figure is correct, but if it is, then numbers of autistic children may be underdiagnosed," said a spokeswoman working at a specialist clinic.
"One problem that seems to pop up frequently is that parents do not want to acknowledge that their child/children may be autistic," said the woman, who asked not to be named.
"Many do not recognise the need for clinical care, which is essential," said a doctor at another such clinic. "Korean culture may make it more difficult to (accept) autism."
The highly structured nature of South Korean schools, in which the school day can exceed 12 hours, may also contribute to the high numbers of children with undetected autism.
"For quiet, high-functioning children with autism spectrum disorders, this environment may reduce the likelihood of referrals to special education programs," the team wrote.
Geraldine Dawson of Autism Speaks said the study "confirms that autism is a significant global public health concern."
But it also suggests current research methods are underestimating autism in the United States and elsewhere.
The US Centres for Disease Control and Prevention estimates autism affects nearly one in 110 children.
Dr Marshalyn Yeargin-Allsopp, an epidemiologist at CDC, said it is likely the agency's method of estimating prevalence, focusing on probable cases, misses some children.
"We know that we are not capturing all of the cases," she said.
As for the South Korean study, she said, "We are concerned that this prevalence is so high, but we have to bear in mind they are using different methodology. Using different methodology gives you different estimates," she said.
But Craig Newschaffer, an autism researcher at Drexel University in Philadelphia who has seen the study, said there may be other reasons the South Korean estimates are higher.
"Most of the cases came from the general population sample," he said, in which only about 60% of parents participated. He said parents in regular schools who had concerns about their child's development might have been more likely to participate than those who had no concerns.
But to Newschaffer, the numbers are less important than the finding that autism is so widespread.
"Autism is a major public health problem in the United States, Europe and most likely worldwide. I don't think this is a game changer, but I think we need to be paying more attention to figuring out why."
(Reuters Health, Julie Steenhuysen and Dan-bee Moon, May 2011)