On doing the "impossible": inferring that a putative causal relationship does not exist.

A simple heuristic is proposed for drawing a warranted inference that a causal relationship does not exist between two events. It is illustrated by a study of the putative indirect causal relationship between Autistic Disorder and the Fragile X syndrome. This study was designed to ensure that a failure to find such a relationship provided good reasons for concluding that it was unlikely there was a relationship between Autistic Disorder and the Fragile X syndrome.