Researchers have reported that countries that have a higher precipitation level also have higher autism rates. The reasons could be increased pollutants carried by precipitation, increased television viewing due to bad weather, or a lack of vitamin D due to staying indoors.

Sean Nicholson, a Cornell University policy analyst and an author of the study said, "If it rains a lot, children spend more times inside. It could be that there was something positive outside that they're getting less of, or there's something in the indoor environment that's harmful."

The study published in Archives of Pediatrics & Adolescent Medicine said that the assumption that wet weather might trigger the disorder has not been clinically proven and would need further testing. Dr. Noel Weiss, a University of Washington epidemiologist who wrote the accompanying editorial said, "This is iffy, tentative stuff, not because the researchers aren't competent but because it's a difficult problem to study. The hope is that by reading this article, other researchers will look into this question."

Environmental factors are said to play an important role in autism and lead author Michael Waldman, a Cornell University economist said that his son was diagnosed with an autism spectrum disorder at age 3 but has recovered and is now a normal third-grader. "If you look at the autism literature now, they're much more open to an environmental trigger," he added.

Waldman said the new study published by the American Medical Association is a "more refined" version of a statistical analysis published online two years ago as a National Bureau of Economic Research "working paper."

The new study analyzed data from three states and studied children born between 1987 and 1999. It examined the number of autism cases and precipitation rates in each county as well as the amount of precipitation children born in different years were exposed to in their first three years. Factors such as income, race and ethnic differences were kept in mind during the study. The researchers from a 2003 U.S. Department of Education survey that collected state autism rates data noted that the lowest rates of autism were in New Mexico, Mississippi, Colorado, Oklahoma and Tennessee, while those with the highest rates were the more northern states of Minnesota, Oregon, Indiana, Maine and Massachusetts.

Lee Grossman, president of the Autism Society of America was not convinced. He said, "It just does not seem plausible. It does not match up with any of the demographics that we follow." He added that his organization has 170 chapters and "what's striking is the similarity … in terms of the prevalence and incidence of autism." Environmental factors playing a role did make sense Grossman added but no one could determine exactly what they were.