Using federal special education data to judge autism's true prevalence is problematic, however. Schools and districts are not staffed by medical professionals. While one district may evaluate a child as having an autism spectrum disorder, another district could see the same child, with the same set of behaviors, and come up with a different disability—or find no disability at all.
However, those special education figures were the best we had until 2000, when the CDC started its monitoring program and attempted to bring some rigor to the diagnosis.
The time it takes to examine all this data is the reason the latest report was released last year but uses information gathered in 2010, said Jon Baio, an epidemiologist for the CDC and principal investigator for the Autism and Developmental Disabilities Monitoring Network. The next report will use data collected in 2012, he said.
Baio agreed that relabeling could have played a role in some of the autism rate increases. But CDC data show that the fastest-growing group of children with autism are those with normal to above-average intelligence. These are children who presumably would not have been misidentified as having an intellectual disability, he said.
"When you peel away all that has changed over time, what has changed to put children today at an increased risk of having autism? We really don't know what that may be at this point," Baio said.