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And I'm Michele Norris.

For more than a decade, scientists have been trying to understand why certain communities report having so many kids with autism. They've looked at air pollution, contaminants in drinking water and pesticide exposure without much success. Now, a team of scientists in California has found a different explanation.

NPR's Jon Hamilton reports.

JON HAMILTON: The team looked at 10 California communities with autism rates at least 70 percent higher than those in surrounding areas.

Irva Hertz-Picciotto of the University of California, Davis, says they didn't find any local environmental problems. But she says they did identify two major risk factors. One was having a certain type of parent.

Dr. IRVA HERTZ-PICCIOTTO (Department of Public Health Sciences, University of California, Davis): These would be parents with higher levels of education, more likely to be white and more likely to be older, within the framework of people who can reproduce.

HAMILTON: Hertz-Picciotto says parents' age and race had a relatively small effect, but the effect of education was huge. In most areas, a child whose parents completed college was four times as likely to be diagnosed with autism as a child whose parents did not finish high school.

Dr. HERTZ-PICCIOTTO: It doesn't necessarily mean that higher education causes autism, but that it gets you the diagnosis more frequently.

HAMILTON: Hertz-Picciotto says that's partly because educated parents are more likely to request services for children with autism.

Dr. HERTZ-PICCIOTTO: It may also have to do with people with more education are just better able to navigate a very complicated and possibly arcane health system.

HAMILTON: But she says parents weren't the only factor affecting diagnosis rates.

Dr. HERTZ-PICCIOTTO: A couple of the clusters were found actually fairly close to a treatment center in California.

HAMILTON: She says parents near state-run treatment centers are probably more aware of autism and autism services.

So you might wonder if the clusters reflect over-diagnosis. Hertz-Picciotto says no, the rates were still lower than those found in communities studied by the Centers for Disease Control and Prevention, which means lots of children are probably slipping through the cracks. Hertz-Picciotto says her study found that children of Hispanic parents, for example, were less likely than other children to receive an autism diagnosis.

Dr. HERTZ-PICCIOTTO: It's entirely possible that families are hesitant to go to a state-funded agency if they have a member of the family who might be undocumented, for instance.

HAMILTON: The study has implications for apparent autism clusters in other parts of the U.S.

Steven Novella, a neurologist at Yale University, says that when researchers encounter a cluster, the first thing they should ask is whether it can be explained by demographic factors.

Dr. STEVEN NOVELLA (Neurologist, Yale University): Only once you rule all those things out, you say, okay, is there any environmental, biological cause? Are we looking at a genetic cluster or an environmental trigger cluster?

HAMILTON: Demographics might have explained an apparent cluster from the 1990s in Brick Township, New Jersey. Many parents in this predominantly white, middle-class community were convinced their kids were being harmed by something in the water. But a federal investigation found no problem.

Novella says that hasn't stopped people from trying to link other autism clusters to local chemicals or pollution.

Dr. NOVELLA: There are proponents of the environmental paradigm of autism who have, I think, jumped the gun on preliminary data and say, aha, see, this is evidence of an environmental trigger, when really that data does not exist.

HAMILTON: Novella says if something in the environment does trigger autism, it's probably something found everywhere, not just in some communities. The new study appears in the journal Autism Research.

Jon Hamilton, NPR News.

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