The question
Through what pathway, under what conditions, and by what process does an effect occur?
Definition
Mediation concerns intermediate pathways, moderation concerns conditional variation, and mechanism concerns the process producing an effect. None should be upgraded into another without evidence appropriate to that inference.
Suppose an intervention changes how students study, and their exam performance later improves.
Researchers might ask three different questions.
Did the intervention increase active retrieval, which in turn contributed to better performance?
Did the intervention work differently depending on students' baseline knowledge or study conditions?
What process actually produced the improvement?
These are questions about mediation, moderation and mechanism.
They concern the same effect, but they ask different things.
Mediation asks about an intermediate pathway
A mediator is an intermediate variable through which part of the relation or causal effect between a candidate cause and an outcome may operate.
The simplest schematic is X → M → Y, where X is the candidate cause or intervention, M is the mediator, and Y is the outcome.
In the study example, the intervention might increase active retrieval, which is then related to improved exam performance.
The mediation question is: through what intermediate variable or pathway might part of the effect operate?
Mediation can narrow the search for how an effect occurs by identifying candidate intermediate pathways.
But the arrows in a mediation model do not establish their own causal interpretation.
A mediation analysis may reveal an indirect statistical relationship consistent with a proposed pathway. Whether that pathway can be interpreted causally depends on the design, measurement and assumptions supporting the inference.
Even if the intervention itself is experimentally manipulated, the mediator often is not. Other factors may influence both the mediator and the outcome.
So a statistical indirect effect is not automatically a causal indirect effect.
Or more generally:
A statistical pathway is not automatically a causal process.
This does not make statistical mediation uninformative. It means the strength of the claim should match the strength of the evidence.
A mediator is not automatically the mechanism
Suppose there is good evidence that the intervention increases active retrieval and that this increase contributes causally to improved performance.
We have learned something important about the pathway through which the intervention works.
We still may not have identified the complete mechanism.
The broader process could involve changes in attention, memory, feedback, practice and other interacting components.
A mediator can be part of a mechanism without being the mechanism.
A mediator may capture one component of a process, one pathway among several, or an imperfect proxy for a broader process.
The simple structure X → M → Y should therefore not be mistaken for a complete map of causal reality.
The same caution applies to time.
If X occurs before M and M before Y, that ordering may support a proposed causal pathway. But temporal sequence alone does not establish that X caused M, that M caused Y, or that M constitutes the mechanism.
A sequence in time is not yet an explanation of process.
Moderation asks when or for whom an effect differs
Now suppose the same study intervention produces larger improvements for students with higher baseline knowledge, or works differently under different study conditions.
The question has changed: when, where or for whom does the effect differ?
This is moderation.
A moderator is a variable or condition across which the size, direction or presence of an effect differs.
Moderation therefore concerns conditional effects and can reveal effect heterogeneity: an effect need not be uniform across all people, contexts or conditions.
That variation can matter scientifically and practically even before its cause is understood.
A moderator does not automatically explain the difference
Suppose the intervention works better for students with higher baseline knowledge.
That tells us something important: the effect differs according to baseline knowledge.
It does not yet tell us why.
Baseline knowledge might itself play a causal role. But it could also mark differences in previous experience, learning strategies, opportunity, motivation or other relevant conditions.
A moderator can identify where an effect differs without identifying what causes that difference.
Moderation can therefore reveal a pattern that calls for further explanation without providing that explanation by itself.
A moderator can be causal.
But demonstrating moderation does not automatically establish that causal role.
Mechanism asks how the effect is produced
A mechanism is an organised process or set of causal relations through which an effect or phenomenon is produced.
Its question is: by what process is the effect produced?
This is broader than identifying one intermediate variable or one condition under which the effect changes.
A mechanism may contain several interacting components or pathways. It need not reduce to one measured mediator, and it need not exist at one particular level of explanation. Relevant mechanisms can be behavioural, cognitive, biological, interpersonal or ecological, or span several levels at once.
Mechanistic evidence must therefore bear on the process producing the effect, not merely on the existence of an intermediate association.
The three concepts can coexist
Distinguishing mediation, moderation and mechanism does not mean only one can apply at a time.
A mediator may participate in a mechanism. A mediated pathway may itself vary across people or conditions. A mechanism may operate differently depending on context.
Their distinction concerns the question each answers, not a competition among three explanations.
These are different explanatory achievements, not a ranking of scientific value.
Three explanatory questions
Through what intermediate variable or pathway might part of the effect operate? — Mediation.
When, where or for whom does the effect differ? — Moderation.
By what process is the effect produced? — Mechanism.
The distinction can be stated compactly: mediation concerns intermediate pathways, moderation concerns conditional variation, and mechanism concerns the process producing an effect.
Evidence about one can inform the others.
But none should simply be upgraded into another kind of explanation without evidence appropriate to that inference.
Ask which question the evidence actually answered
Research language can make these distinctions disappear.
A study identifies an indirect effect and calls it a mechanism.
Another finds that an effect differs across groups and says it has explained who responds.
Another observes the expected temporal sequence and treats the causal process as established.
In each case, the useful question is not how sophisticated the analysis sounds.
It is what the evidence actually established.
If M mediated the effect, did the analysis identify an indirect statistical relationship, or is its causal interpretation justified?
If Z moderated the effect, did the study identify where the effect differs, or establish why it differs?
If M is described as the mechanism, what evidence actually bears on the process producing the effect?
Mediators, moderators and mechanisms can all deepen our understanding of human change. Their value depends partly on keeping their contributions distinct.
Do not confuse an intermediate pathway, a condition of variation and a process of production.