Currently, researchers don’t know the exact number of people with an ASD in the United States. 

Researchers use different ways to determine prevalence that often give different results.

Some estimates of prevalence rely on previously published studies.  Researchers review all the published data on a topic and take the averages of these calculations to determine prevalence.  Independent researchers³ recently conducted two such reviews.  Based on these studies, the best conservative estimate⁴ of the prevalence of ASDs in the United States is that one child in 1,000 children has an ASD. 

Is autism more common now than it was in the past?

Researchers are not certain whether autism is more prevalent now than in the past for a number of reasons.  Although more cases of autism are being identified, it is not clear why.  Some of the increase may result from better education about the symptoms of autism or from more accurate diagnoses of autism. 

The new definition of autism as a spectrum disorder means that even people with mild symptoms can be classified as having an ASD, which could also account for the increase in identified cases.  As research moves forward using the current definition of ASDs, more definite numbers may be available to answer this question.