The increase in autism in California since the 1990s is real, and not just the result of changes in how children are diagnosed, new research suggests.
Many doctors had argued that earlier diagnosis and the inclusion of milder cases were the main reasons for the rising numbers. But when Dr Irva Hertz-Picciotto of the University of California tested this claim, she found that among children under five, new diagnoses rose from 0.8 per 10,000 children a year in 1990 to 11.2 in 2006, and the number of children with autism rose from 6.2 per 10,000 in 1990 to 42.5 in 2006.
Looking back at cases from 1990 to 1996, the study found that the inclusion of milder cases explained about 56% of the increase, and earlier age at diagnosis about 12%. So the researchers stress that the rest of the increase may be real. "It's time to start looking for the environmental factors responsible for the large rise in autism in California," the lead researcher said.
The study was published in the journal Epidemiology in 2009.
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