Are Mendelian randomization investigations immune from bias due to reverse causation?
Publication: European Journal of Epidemiology
Stephen Burgess, Sonja A Swanson & Jeremy A Labrecque
21 February 2021
Mendelian randomization uses genetic variants as instrumental variables to make causal inferences about the effect of a risk factor on an outcome. If a genetic variant satisfies the instrumental variable assumptions for the given risk factor and outcome, then an association between the genetic variant and the outcome implies the risk factor affects the outcome in some individuals at some point in the life-course.
Combining the instrumental variable assumptions with further assumptions and precise specification of the outcome (including specifying a time period for the outcome) allows valid testing of a more specific causal hypothesis and/or valid estimation of global or local, and point or period average causal effects.
In this short manuscript, the researchers discuss three ways in which Mendelian randomization analyses may be susceptible to bias due to reverse causation. They conclude that while the analyses offer some protection against biases, they are not totally immune from the phenomenon; and that researchers should consider carefully whether their findings could be explained by genetic variants having a primary association with the outcome, and how previous versions of an outcome can impact the stated risk factor.