Researchers have found clusters of autism in 10 areas around California -- but with no suggestion of a link to local pollution or other environmental exposures, they said.
Instead, the only consistent factor among the areas -- identified largely in Southern California and the San Francisco Bay region -- was a population of well-educated parents, Karla Van Meter of the University of California Davis and colleagues found.
In six of the clusters, a college-educated parent conferred a risk of autism that was more than four times as great as a parent who didn't graduate from high school, they reported online in Autism Research.
"At this point, we don't think that's due to differences in the actual rates," coauthor Irva Hertz-Picciotto, also of UC Davis, told MedPage Today.
The observational study determined autism cases through a database of state-sponsored developmental services, a system which relies on parents to voluntarily seek services.
Parents with higher education are likely to be better insured and more attuned to seek diagnosis and services for their autistic children than less-educated parents -- an effect seen in prior studies, the researchers said.
The clear-cut link with mom and dad's education should be reassuring to parents living in the cluster areas, Hertz-Picciotto said.
"There was such a uniform pattern across all of these clusters [linking higher education to autism rates]," she said. "This is really not something people should be worried about who are living in those areas."
The real concern was that local pollution sources, such as dumps or factories, would be associated with increased autism incidence, the researchers noted.
While autism clearly has a genetic component, the search for its cause has also turned up plenty of clues involving prenatal and early childhood environments, Hertz-Picciotto explained.
So her group applied a new cluster-detection technique to the addresses on birth certificates -- a location where late gestational or early neonatal exposures were likely to have occurred.
Among all 2.4 million children born from 1996 through 2000 in California, 9,900 had autism as recorded in the California Department of Developmental Services database through February 2006. The database is estimated to include 75 to 80 percent of all California children with the condition.
When cases and controls were analyzed by geographic location, clusters of elevated incidence emerged in eight of the 21 catchment areas of the state's developmental services regional centers.
Ten met stringent criteria, each with at least 70 percent greater risk than surrounding areas in seven different tests for clustering. Another two areas were potential clusters, according to lower criteria, with a positive result in at least four but not all seven tests.
These areas were:
Overall, these clusters have a parent population with higher educational levels. In six of the ten clusters, the likelihood of autism when a parent didn't finish high school was one-quarter of the likelihood when a parent graduated college.
For three clusters, autism was less than half as likely when a parent had only some college education as when at least one parent had graduated from college.
Older parental age also had a small association, as did birth to Hispanic parents.
The researchers cautioned that they could not rule out environmental exposures related to higher parental education, such as genetic susceptibility or access to assisted reproduction or induced labor.
Nor did the study rule out environmental triggers entirely, Hertz-Picciotto noted.
"We'll continue to be searching for the environmental factors, but I don't think they're going to be sources of pollution in specific neighborhoods," she said in an interview. "We think that the kinds of environmental exposures that we need to look for are probably pretty broad."