The number of autistic children enrolled in special school programs nearly tripled from 2000 to 2010. However, researchers at Pennsylvania State University said the ratio of autistic children has increased because of a new classification system aimed at categorizing individuals with intellectual disability as autistic.

Researchers noticed the total number of children diagnosed in a broad category of brain development disorders, which includes autism, remained the same from 2000 and 2010, a study published in the American Journal of Medical Genetics showed. However, the number of autism cases spiked from 93,624 in 2000 to 419,647 in 2010.

Upon investigation, the team concluded children who were earlier diagnosed with intellectual disability are now being classified under the autism category. The researchers also noticed the fall in the number of children who were labeled as intellectually disabled during the study period -- to 457,478 from 637,270.

“If you asked me, ‘Is there a real increase in the prevalence of autism?’ maybe there is, but probably much lower than the reported magnitude,” lead researcher and geneticist Santhosh Girirajan said in a statement.

Girirajan said the increase in the number of autistic children potentially was due to the movement of the children from one category of disorder to another. The research team explained since autism is a combination of a number of symptoms and can also co-occur with a number of other disorders, it is hard to measure its actual prevalence, resulting in shifting diagnoses.

"For quite some time, researchers have been struggling to sort disorders into categories based on observable clinical features, but it gets complicated with autism because every individual can show a different combination of features," said Girirajan.