Chapter 1:

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.