Interaction fallacy.

We define interaction fallacy as the situation in which heterogeneity of odds ratios suggests an interaction that does actually not exist among the corresponding risk ratios. We provide a hypothetical example of interaction fallacy between the presence of the germ-line BRCA1 mutation, age at first live birth, and breast cancer risk based on data from the literature. More generally, we present a set of hypothetical conditions under which interaction fallacy may occur.