Mediators, Moderators, and Mechanisms

Recently Marc Bellemare shared a post highlighting an article in American Political Science Review: Explaining Causal Findings Without Bias: Detecting and Assessing Direct Effects.  He does an awesome job giving an overview of the article. If you read his post, you will see that the paper emphasizes causal mechanisms and introduces this through controlled direct effects:

 "their method not only tells you whether M  is a mechanism through which D causes y, it can also tell you whether there is any significant amount of statistical variation left in the causal relationship flowing from D through y after M is accounted for"

Previously, I have been working on a post related to mediators and moderators, and his post motivated me to wrap it up today.

In the article Mediators and Mechanisms of Change in Psychotherapy Research, Kazdin provides some clarity about the differences and relationships between mediators, moderators, and mechanisms:

Mediator: an intervening variable that may account (statistically) for the relationship between the
independent and dependent variable. Something that mediates change may not necessarily explain the processes of how change came about. Also, the mediator could be a proxy for one or more other variables or be a general construct that is not necessarily intended to explain the mechanisms of change. A mediator may be a guide that points to possible mechanisms but is not necessarily a mechanism.

Mechanism: the basis for the effect, i.e., the processes or events that are responsible for the change; the reasons why change occurred or how change came about.

Moderator: a characteristic that influences the direction or magnitude of the relationship between and independent and dependent variable. If the relationship between variable x and y varies is different for males and females, sex is a moderator of the relation. Moderators are related to mediators and mechanisms because they suggest that different processes might be involved (e.g., for males or females).

Reference:

Mediators and Mechanisms of Change in Psychotherapy Research
Alan E. Kazdin Annu Rev Clin Psychol. 2007;3:1-27