WALNUT CREEK, Calif. — Researchers have discovered 10 autism clusters in California, mainly in areas where parents have higher-than-average education levels.

The findings, released this week by the University of California, Davis, provide another important clue for solving the autism mystery, but leave many questions unanswered.

The clusters have autism rates nearly double those of surrounding areas.

The clusters are in the San Francisco Bay Area, Southern California and the Central Valley.

A San Diego-area cluster has autism rates of 61.2 per 10,000 births, compared with 27.1 per 10,000 births in the surrounding region.

Parents in the cluster neighborhoods are more likely to be white, have higher education levels and be slightly older, said Irva Hertz-Picciotto, a study author and leading autism researcher at the UC Davis MIND Institute.

These are all factors that other studies have shown put children at higher risk of being diagnosed with autism.

Because the clusters have similar demographics, “what we can say is that it’s unlikely that these clusters are due to some environmental contamination in these neighborhoods,” Hertz-Picciotto said.

It is probably not a nearby toxic waste site or polluting manufacturer that is to blame, she said. Instead, it may be a more widespread environmental factor, such as household products that are used by families with higher education levels, or something else that is a part of their lifestyles.

Or it may be that such families have better access to specialists and know how to navigate the system to get their child diagnosed with autism, Hertz-Picciotto said. If that is the case, surrounding areas could have more undiagnosed cases of autism.

Several of the clusters are near major autism treatment centers.

A Kaiser researcher who was not involved with the study called the findings interesting and said this should help shape future research. She noted that several studies have now shown a link with higher education levels and older parents.

“Is it a biologic factor, or certain exposures that they may have more of?” asked Lisa Croen, director of the autism research program at the Kaiser Permanente Division of Research in Oakland. “That’s the big unknown.”

Autism is a neurodevelopmental disorder often marked by an inability to communicate with others and repetitive behaviors such as hand-flapping. It is considered a lifelong condition that typically develops by the time a child is 3.

UC Davis researchers examined the 2.5 million births in California from 1996 to 2000.