06/08/2011 09:00 am ET | Updated Aug 08, 2011

Autism Prevalence: Are More Children Really Developing ASD Now Than Before?

The collective gasp you heard coming from the autism community last month was a response to the American Journal of Psychiatry's publication of a study of autism prevalence in South Korea. The study, which focused on the Ilsan district of Goyang City, South Korea, found an autism rate of 2.64 percent among 7- to 12-year-olds. That is more than two and a half times the U.S. rate of approximately 1 percent.

I interviewed one of the study's lead authors, Roy Richard Grinker, by email. Grinker is a Professor of Anthropology at George Washington University, the father of a daughter on the autism spectrum, and author of "Unstrange Minds," a history and cross-cultural examination of autism. (Full disclosure: Grinker also appears in my documentary, "Loving Lampposts: Living Autistic."

Grinker says that the study shows "that autism is more common than we think, and that claim is supported by the use of a different method of estimating prevalence."

If this is true, then it's very big news. After all, when prevalence rates rise in the U.S., people often respond, "Why are there so many more autistic people than there used to be?" This response is so common that many people now believe that there is an epidemic of autism.

But Grinker is suggesting that the real question we should ask when see a rate like 2.64 percent is, "How did the researchers look for autism?" In other words, increased prevalence does not necessarily mean that there is more autism than there used to be. Instead, it may mean that we are looking for autism more than we used to. To understand why this might be true, you have to understand what makes the Korea study different.

In the U.S., the Centers for Disease Control estimate the prevalence of autism spectrum disorders (ASD) by examining health and/or educational records. Grinker calls these studies "superb," but he also points out that the CDC's approach "meant that researchers were counting only those children who appeared in records" (the CDC itself acknowledges this point in a New York Times article about the Korea study). "What would we find," Grinker wondered, "if we screened all children in a population for ASD?"

Grinker worked with a team of autism experts that included lead researcher Dr. Young-Shin Kim, a child psychiatrist and epidemiologist at the Yale Child Study Center. Together, they designed a study that would measure the prevalence of autism among the entire elementary school population of the Ilsan district.

That total population turned out to include 36,592 children in participating mainstream schools and 294 listed in Islan's Disability Registry (the latter group was identified as having a high probability of having ASD). As a first step, all parents were asked to complete a preliminary 27-item questionnaire that assessed their children for possible symptoms of and behaviors associated with autism.

To lay the groundwork for this step, Grinker and the team engaged in "extensive community outreach, educational sessions with teachers, interviews and focus groups." Grinker says, "We learned how to talk about autism in a way that didn't frighten people away from our study." It may seem strange to have an anthropologist on a team of scientists performing epidemiological research, but Grinker argues, "This kind of approach, which considers culture as an important aspect of medical and psychological research, can be useful anywhere, including the U.S."

Ultimately, parents of 23,337 children completed the questionnaire. The team made extensive efforts to contact the parents of children who screened positive for ASD and to invite the children for multi-hour evaluations with the gold-standard diagnostic instruments, validated in the Korean language. Two clinical teams made final diagnoses, and each team included a Korean child psychiatrist trained in both Korea and the U.S. The final 2.64-percent prevalence estimate was made up of .75 percent from the high-probability group and 1.89 percent from regular schools. (The authors emphasize that their results have not been replicated, and certain factors may have caused them to overestimate or underestimate the prevalence rate.)

Grinker suggests that the surprisingly high percentage of children with ASD in mainstream classrooms may be partially explained by the culture of Korean schools. He notes, "The schools emphasize structure and routine within the classroom, learning by rote memory, and few transitions or alterations in daily schedules." You couldn't ask for a better classroom for an autistic child. Nevertheless, Grinker emphasizes, "The team would never have found these kids if they weren't suffering, or didn't have significant functional impairments." Given the rigor of the study and the expertise of the team, it seems likely that these children were accurately diagnosed.

Grinker points out that a total population study is more tedious, costly and time-consuming than the CDC's records-based approach. But the results are instructive.

The study reminds us that the rate of any condition depends in part on how we measure it. When we actively look for autism, we find more of it simply because we are looking.

Grinker has been running a small experiment when he tells people about the study. To some, he says that the rate is 2.64 percent, and they respond, "Shocking!" To others, he says, "'The prevalence is 26.4 per 1,000," which is of course 2.64 percent, and people respond by saying, "That doesn't sound high to me." Is a prevalence of 2.64 percent high? As the Korea study shows, it all depends on what rate you were expecting.