No one can ignore the rise in autism rates. A study revealed that numbers in California increased more than 250 percent in a decade, and studies in Atlanta and New York City suggest an even more dramatic jump. Whereas traditional estimates held that 1 in 2,000 children might show signs of autism, today many experts put the rate at 1 in 500—or even higher.

Researchers have attempted to link the soaring numbers to factors including prenatal exposure to certain medications, early-life infections, and inoculations for measles, mumps and rubella. But experts question whether the sharp rise in diagnoses is due to an authentic increase in incidence or to heightened awareness combined with evolving diagnostic standards.

While studies conducted before the 1990s typically used a narrow definition of autism, more recent research reflects a broad class of autism spectrum disorders. In previous surveys, mild diagnoses such as Asperger's syndrome were likely excluded. Proactive methods of case-finding used in these studies may also result in higher prevalence estimates.

Experts are therefore cautious about attributing the rise in autism cases to harmful environmental factors—though they haven't been ruled out. Most experts share the view of Geraldine Dawson, of the University of Washington Autism Center, who states that "there clearly is an increase in prevalence, and some, if not all, of this increase is related to expanding diagnostic classification. Whether or not this explains all of the increase, however, is unknown."