Irva Hertz-Picciotto of the University of Davis M.I.N.D. Institute says many have viewed with skepticism the apparent increase in autism by California-born children aged 6 from fewer than 9 in 10,000 for those born in 1990 to more than 44 in 10,000 for those born in 2000.

Some argue the increase is due to migration into California of families with autistic children, inclusion of children with milder forms of autism and earlier ages of diagnosis because of improved surveillance or greater awareness.

Hertz-Picciotto and co-author Lora Delwiche analyzed data collected by the state of California and the U.S. Census Bureau.

The methodology eliminated migration as a potential cause of the increase in the number of autism cases. It also revealed that less than one-tenth of the increased number of reported autism cases could be attributed to the inclusion of milder cases of autism. Twenty-four percent of the increase could be attributed to earlier age at diagnosis.

"These are fairly small percentages compared to the size of the increase that we've seen in the state," Hertz-Picciotto says in a statement.

The study, published in the journal Epidemiology, suggests that research should shift from genetics to the host of chemicals and infectious microbes that are likely at the root of neurodevelopment of California's children.