This book concerns making inferences about causal effects based on observational data using genetic instrumental variables, a concept known as Mendelian randomization. In this chapter, we introduce the basic idea of Mendelian randomization, giving examples of when the approach can be used and why it may be useful.


Distinguishing between a factor which is merely associated with an outcome and one which has a causal effect on the outcome is problematic outside of the context of a randomized controlled trial. Instrumental variables provide a way of assessing causal relationships in observational data, and Mendelian randomization is the use of genetic variants as instrumental variables.