Mediation: Exploring Relevant Mechanisms

Chapter 8 — Mediation: Exploring Relevant Mechanisms Knowing that a treatment works is useful. Knowing why it works is transformative. This chapter develops mediation analysis as the structured approach to uncovering causal pathways between treatment and outcome. It introduces the identification assumptions required to recover direct and indirect effects, the pitfalls of ex post mediation, and experimental designs—including paired and crossover designs—that allow researchers to intervene on mediators. Understanding mediation is essential for both EP1 (mechanism knowledge) and EP2 (predicting what transfers across environments).

1. Estimating causal impacts yields effects of causes, but without knowledge of mechanisms, the analyst has an incomplete understanding of the causal pathways.
2. Mediation analysis is a structured approach to understand the relevant causal pathways, but new identification assumptions must be invoked to recover parameters of interest.
3. The limits of ex post mediation analysis highlight the value of considering theory and identifying potential causal pathways during the design stage.
4. Pairing lessons on mediators and moderators provides key insights into Experimental Problems 1 and 2, allowing us to understand both the effects of causes and the causes of effects.