Seems that researchers at Cornell University and the University of Washington have found that the rate of autism is much higher in counties that have high precipitation. The lead researcher is Cornell's very prestigious economics professor Michael Waldman, who, according to his bio on Cornell's site is known more for his work in microeconomic theory, particularly industrial organization.
He is widely recognized as one of his field's top researchers in the area of applied microeconomic theory, where his main fields of interest are industrial organization and organizational economics. In these areas, he is best known for his work on learning and signaling in labor markets, the operation of durable goods markets, and the strategic use of tying and bundling in product markets.
Before doing his study on rainfall and autism, Waldman published an earlier study on autism and watching television.
His theories on the rainfall-autism link are fairly broad and encompassing — children may spend more time indoors watching TV or being subjected to toxic emanations in their homes; they may not receive enough Vitamin D; the rain itself may be subjecting the small children to toxic substances, such as mercury.
According to the LA Times, his coauthor, Noel S. Weiss suggested deficiencies in the data:
Weiss suggested that the data from which Waldman drew his autism statistics could be unreliable, as diagnoses and records of those diagnoses vary from state to state and county to county.
How about another reason — correlation doesn't mean causation. Could more rain mean that greater use of umbrellas may be the cause? Or the bonnet over the stroller? Or the toddlers don't get enough exercise when they're stuck indoors? Or they're not visiting the playground to see their friends? Or the toddlers are bored looking out the window at so much rain?
Autism is a serious issue, and the search for answers is an important one. Researchers have recently found a genetic link to autism. Those kinds of rigorous scientific studies seem to hold more promise for answers.