A new study claiming that counties with high levels of rainfall in the Pacific Northwest had higher autism rates generated a flood (sorry) of media coverage. As the Times of London notes, the study conducted by economist Michael Waldman of Cornell University and published in the Archives of Paediatrics & Adolescent Medicine
“has been seized on by those who claim, against the evidence, that mercury poisoning causes autism. Rain, they suggest, might be the conduit by which mercury pollution from industry is reaching the ground environment.”
Largely missing in a flurry of speculation over what accounted for the findings was that the study did not report a direct link between rainfall levels and autism. This could be because it tried to correlate the two and failed – or because it didn’t actually find a correlation. Instead, it found a link between a “relative preciptiation variable” and autism. Relative precipitation is a measurement of how far a county’s rain level is from the average level of precipitation. So the authors are arguing that when rain levels are “out of the ordinary,” autism is more likely to develop or be diagnosed.
As the Time’s science editor also points out,
Rainfall, too, is variable from year to year, but the trend of rising autism diagnoses goes in one direction. Prevalence is also similar in many countries with different climates. These inconvenient truths compromise the credibility of the link, yet they pass unexplained. There is every chance that this link is a statistical artifact – and one found in a single region. There is no reason to think it can be generalised beyond the US West Coast.
For an in-depth examination of the study, check out surgeon-blogger Orac’s post Rain man? Or does rainfall cause autism?