Study co-author Peter S. Bearman of Columbia University and the director of the Paul F. Lazarsfeld Center for the Social Sciences, and colleagues examined birth and diagnostic records for all children born in California from 1992 to 2000. All children were tracked until June 2006 to allow ample time for diagnosis of autism.
As the disorder became better known, the average age of autism diagnosis fell from 5.9 in 1992 to 3.8 of those born in 2000.
"As knowledge has spread about autism, information is now more evenly distributed across different kinds of communities," Bearman said in a statement. "It is also easier to find someone who can diagnose autism, so we no longer see these huge differences in rates of diagnosis. However, it appears that poor kids living in poor neighborhoods still are not being diagnosed."
Overall, of the 4,906,926 million children born in California from 1992 to 2000, 18,731 or .38 percent were diagnosed with autism jumping from 29 per 10,000 in 1992 to 49 per 10,000 in 2000, the study said.
The study is published in the American Sociological Review.