By Clayton Simmons, published on May 1, 2009 - last reviewed on June 19, 2009
Autism rates worldwide are soaring, and no one knows why. Some have suggested that changes in how doctors diagnose the syndrome are driving the numbers, but astudy in Epidemiology finds that this explanation accounts for only a fraction of the jump, leaving the lion's share a mystery.
The researchers found a 600 percent surge in kids under 5 diagnosed with autism in California from 1995 to 2006. They also found partial justification for two theories about the rise. First, physicians are indeed diagnosing kids earlier, so more children in younger age cohorts seem to have the disorder today. Second, physicians have switched to a broader set of diagnostic criteria. But, together, these explain only a third of the rise. The researchers also eliminated migration to California as a potential cause.
Co-authors Irva Hertz-Picciotto and Lora Delwiche of UC Davis acknowledge that some potentially important diagnostic issues weren't measured. Physicians may be more aware of the disorder, making them more likely to test kids for autism in the first place. Parents, for their part, are more willing to have their kids evaluated. But "the possibility of a true increase in incidence [of autism] deserves serious consideration," they write.
One thing that's clear: Autism is not purely genetic. Studies of risk factors are ongoing, and some areas hold promise; research implicates higher pollutant levels and infections in pregnant women, for example. Older fathers and increased stress in mothers may also play small roles. Whatever the causes, the trend in California shows no sign of slowing.