The greatest increase in incidence among girls came from diagnoses of two subcategories of autism, Asperger syndrome and pervasive developmental disorder-not otherwise specified (PDD-NOS). Both categories are being subsumed into the autism diagnosis in the newest edition of the Diagnostic and Statistical Manual (DSM-5), the American Psychiatric Association's guidelines for diagnosis. And both are generally thought to represent the higher-functioning end of the autism spectrum.
"That is in a way interesting because it goes against the idea that girls are always more severely afflicted [than boys]," says Eric Fombonne, professor of psychiatry at Oregon Health and Science University, who was not involved in the research.
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Better diagnostic practices may explain these large hikes in incidence, says Maureen Durkin, professor of population health sciences and pediatrics at the University of Wisconsin in Madison.
"There's much more awareness of autism," say Durkin, who was not involved in the study. "There's much more screening going on. And the newer generation of clinicians are being trained in this so they are more likely to see it."
This increased attention to autism and its symptoms may also explain the rise in diagnoses of teenagers and adults.
An age-stratified analysis shows that children between the ages of 4 and 13 make up about 63 percent of the new autism cases. The fastest acceleration in new cases is in those diagnosed between 14 and 20 years of age.
Individuals diagnosed between 21 and 65 years of age account for about 9 percent of the new cases—but their proportion also significantly increased over the time frame of the study. Like girls, many of the adults are diagnosed with higher-functioning forms of autism, such as Asperger syndrome and PDD-NOS.
"If the incidence [in adults] is increasing, it just has to do with recognition of cases that have been missed up to that age," Fombonne says. "It cannot be that you develop autism at age 50." [emphasis added]
