ABSTRACT
In the previous chapters, we repeatedly used the word ‘causal’ to describe the inferences obtained by Mendelian randomization. In this chapter, we clarify what is meant by the causal effect of an exposure on an outcome. We give a more detailed explanation of the theory of instrumental variables, and explain in biological terms various situations that may lead to violations of the instrumental variable assumptions and thus misleading causal inferences. We conclude by discussing the difference between testing for the presence of a causal relationship and estimating a causal effect, and the additional assumptions necessary for causal effect estimation.
