A new U.S. research has found a higher rate of autism in neighborhoods with lots of college graduates.

    Researchers at the University of California in Davis (UCD) identified two groups that are more likely to have autistic children -- highly-educated and older parents.

    Using data on about 2.5 million births in California in a five-year period, researchers identified 10 autism clusters, or geographical areas in which there was a higher than usual incidence of children diagnosed with autism.

    The rate of autism in the clusters was about twice that of the surrounding areas, according to the study appearing in the Jan. 6 online issue of Autism Research.

    The parents' educational attainment was classified into one of four levels: less than 12 years of school, high school graduate, some college (13 to 15 years) or 16-plus years of school (college graduate or advanced degree).

    Parents who lived in the areas where autism was more prevalent were more likely to be college graduates or hold advanced degrees than parents who lived outside the clusters, said the study.

    Mothers and fathers who were older when their child was born were also more likely to have autistic children, but parental age was not linked as strongly with autism as educational attainment, said the study's lead author Karla Van Meter, who conducted the research when she was a doctoral student in public health sciences at UCA. Van Meter is now an epidemiologist with the Sonoma County Department of Public Health.

    "It confirms what we already knew, which is that highly educated parents are more likely to have children with autism," said Van Meter.

    The researchers identified 9,900 children born between 1996 and2000 who were diagnosed with autism by age 6.

    Just why there appears to be a link between parents' educational level and autism is unclear, but theories abound.

    "Nobody really knows for sure, but some think there may be something genetic going on, some believe better-educated parents are more successful in seeking services for their children or may have different expectations for the kids, some believe there could be some physical or chemical exposure in those households," Van Meter said. "It could be all of the above or some combination of factors. All are being studied."

    Though the data did not include information on household incomes, areas with highly-educated residents also tend to be higher-income areas, the researchers said.

    Andy Shih, vice president for scientific affairs at Autism Speaks, said that highly-educated parents tend to know more about autism, are more successful in getting their children treatment and are more likely to have the resources to move to be closer to treatment centers. The clusters tended to be near regional autistic treatment centers, the study found.

    "The unanswered question is, 'What about the parents who don't have the means to move to a place where there are better services, or who don't have the information on early signs of autism?'" Shih said. "This study points to a potential health disparity in our population."

    Autism is the neuro developmental disability that's marked by impaired social and communication skills, and repetitive behaviors.