Uncertainty over what causes the syndrome has become a trigger for wild speculation, says Mark Henderson

Autism is a highly heritable disorder, which is strongly influenced by genes. But it is not a genetic condition in the same way as Huntington's disease or cystic fibrosis, in which mutated DNA is the one and only cause. It is influenced by environmental factors, but what they are we do not yet know.

This uncertainty about environmental triggers has itself become a trigger for wild speculation. All sorts of hypothetical causes have been advanced and found wanting, from cold-hearted “refrigerator mothers” to vaccines.

This week a new contender has emerged. In a paper published in Archives of Paediatrics & Adolescent Medicine, Michael Waldman, of Cornell University, has suggested a link to rainfall. He noted that autism prevalence seemed to be lower in the south and west of the United States than in northern states, and wondered whether climate might be responsible. When he studied counties in Oregon, California and Washington, a trend emerged. Autism was more likely to be diagnosed in children living in wetter counties.

This supposed link 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.

That, to be fair, is not Waldman's interpretation. He thinks it more likely that wet weather means children spend more time indoors, which may have knock-on effects. They may watch more TV. They get less sunlight and thus produce less vitamin D. They may be more exposed to household chemicals.

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