The Book of why gives the example of Cornfield's refutation of R. A. Fisher's claim that a "Smoking Gene" could be the true cause of lung cancer deaths.

Given that studies showed that smokers were nine times more likely to develop lung cancer than non smokers, it would follow that if 11 percent of non-smokers have the “smoking gene,” then 99 percent of the smokers would have to have the smoking gene to hold the relationship. Further, if 12% of non-smokers had the gene, it would be mathematically impossible for the smoking gene to account for the association.