Researchers in California have identified 10 regional clusters where autism rates are roughly double that of surrounding areas. Most of the clusters are in places where parents have higher than average levels of education and researchers found no evidence of environmental risk factors.
Clusters of children diagnosed with autism tend to occur in places where parents are older, more educated, and white, according to a study by researchers at the University of California, Davis.
The study found no link to local pollution or chemical exposures — which some consumer groups have cited as possible causes of autism clusters.
The results suggest that areas in California with apparently high rates of autism spectrum disorders are probably just places where parents are more likely to obtain a diagnosis for their child, the researchers say.
"It doesn't necessarily mean that higher education causes autism," says Irva Hertz-Picciotto, one of the study's authors and a researcher at the UC Davis MIND Institute. "It gets you the diagnosis more frequently."
The UC Davis study looked at the geographic distribution of about 10,000 children who were born in California from 1996 through 2000 and later diagnosed with an autism spectrum disorder.
A cluster was defined as a community in which the proportion of children diagnosed with autism was a least 70 percent higher than in surrounding areas.
The study found that differences in parents' age, education and ethnicity explained the cluster most of the time.
Higher Education More Diagnosis
For example, it found that children of parents who finished college were at least four times more likely to be diagnosed than children of parents who didn't finish high school.
Children were also more likely to be diagnosed if they were born in a community near a regional service center for people with autism.
Hispanic parents were under-represented in all 10 of the clusters, according to the study. That could be because some parents are reluctant to seek help from a state agency if they have a member of the family who is undocumented, Hertz-Picciotto says.
No Evidence Of Environmental Risk
The study may be most interesting because it did not find any environmental explanation for higher autism rates, says Steven Novella, a neurologist at Yale University.
"You can't prove a negative," he says. But the results of this and other studies suggest that "If there are environmental factors, they're small," Novella says.
The California results also show how widely autism diagnosis rates can vary from place to place, Novella says. In some areas of the state, children were four times as likely to be diagnosed as in other areas.
That suggests that in many areas there are still a huge number of children with autism spectrum disorders who are slipping through the cracks, Novella says. Copyright 2010 National Public Radio. To see more, visit http://www.npr.org/.