A Robust Example of Collider Bias in a Genetic Association Study

To the Editor: “Collider bias” (also referred to as the “reversal paradox”)1 describes the artificial association created between two exposures (A and B) when a shared outcome (X) is included in the model as a covariate (Figure 1). A recent paper by Aschard et al. described the potential for collider bias when adjusting for heritable covariates in genetic association studies.2 However, in their examples, the authors acknowledged that they could not exclude the possibility of a true biological explanation for the genetic association seen only in the adjusted model. Furthermore, the extent to which this bias could create a completely spurious genetic association, rather than just modify the magnitude of the effect,3 remains unclear