Mediation: How Does a Cause Reach an Outcome?

Mediation asks not merely whether a cause is related to an outcome, but how its influence travels through an intermediate process. If residents sleep better after a neighbourhood introduces quiet hours, reduced night-time noise may be the mediator: the rule changes the noise first, and the change in noise then affects sleep. The question moves from “does it work?” to “through what pathway?”

A mediator is different from a moderator. Mediation describes the pathway that transmits an effect; moderation describes the conditions under which an effect becomes stronger, weaker or changes direction. Identifying a mediator can help place an intervention at the mechanism carrying the effect, rather than only chasing the final outcome.

However, finding associations among a cause, mediator and outcome does not by itself prove that causal chain. Timing, unmeasured variables and measurement error can create the same pattern. Mediation is a structural hypothesis about mechanism, and it still requires an appropriate design and supporting evidence.

https://www.annualreviews.org/content/journals/10.1146/annurev.psych.58.110405.085542