When researchers scoured California birth records hoping to uncover an environmental cause behind autism, they came up short. Instead they found that autism cases hover or cluster in areas near treatment centers and in neighborhoods with highly educated residents.

Researchers mapped the locations of more than 2 million births in California occurring between 1996 and 2000. They then identified approximately 10,000 children within the group who were later diagnosed with autism.

What emerged were 10 autism clusters, or areas within the state where the rate of autism appeared to be higher than in other parts of the same region. But the clusters did not correspond with any large-scale environmental exposures such as pollutants. Instead the areas centered around the locations of major autism treatment centers and neighborhoods of well-educated people, primarily in southern California and in the San Francisco Bay area.

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The findings reported this week online in the journal Autism Research don’t so much indicate a higher rate of autism in the 10 cluster areas, but rather a higher rate of diagnosis, according to senior study author Irva Hertz-Picciotto, a professor of public health sciences at the M.I.N.D. Institute at the University of California, Davis.

“In the U.S., the children of older, white and highly educated parents are more likely to receive a diagnosis of autism or autism spectrum disorder. For this reason, the clusters we found are probably not a result of a common environmental exposure. Instead, the differences in education, age and ethnicity of parents comparing births in the cluster versus those outside the cluster were striking enough to explain the clusters of autism cases,” she says.

Meanwhile, the researchers say their findings indicate that if there is an environmental factor behindĀ autism, it is more likely something extremely close to home or a substance that’s not confined to a particular geographic area.