The Southern California catchment area that includes Dana Point and Mission Viejo.
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."