The Moderator-Mediator Variable Distinction in Social Psychological
Research: Conceptual, Strategic, and Statistical Considerations
Reuben M. Baron and David A. Kenny
University of Connecticut
In this article, we attempt to distinguish between the properties of moderator and mediator variables
at a number of levels. First, we seek to make theorists and researchers aware of the importance of
not using the terms moderator and mediator interchangeably by carefully elaborating, both conceptually
and strategically, the many ways in which moderators and mediators differ. We then go beyond
this largely pedagogical function and delineate the conceptual and strategic implications of making
use of such distinctions with regard to a wide range of phenomena, including control and stress,
attitudes, and personality traits. We also provide a specific compendium of analytic procedures appropriate
for making the most effective use of the moderator and mediator distinction, both separately
and in terms of a broader causal system that includes both moderators and mediators.
The purpose of this analysis is to distinguish between the
properties of moderator and mediator variables in such a way
as to clarify the different ways in which conceptual variables
may account for differences in peoples' behavior. Specifically,
we differentiate between two often-confused functions of third
variables: (a) the moderator function of third variables, which
partitions a focal independent variable into subgroups that establish
its domains of maximal effectiveness in regard to a given
dependent variable, and (b) the mediator function of a third
variable, which represents the generative mechanism through
which the focal independent variable is able to influence the
dependent variable of interest.
Although these two functions of third variables have a relatively
long tradition in the social sciences, it is not at all uncommon
for social psychological researchers to u, the terms moderator
and mediator interchangeably. For example, Harkins,
Latan6, and Williams 0980) first summarized the impact of
identifiability on social loafing by observing that it "moderates
social loafing" (p. 303) and then within the same paragraph
proposed "that identifiability is an important mediator of social
loafing:' Similarly, Findley and Cooper (1983), intending a
moderator interpretation, labeled gender, age, race, and socioeconomic
level as mediators of the relation between locus of
control and academic achievement. Thus, one largely pedagogiThis
research was supported in part by National Science Foundation
Grant BNS-8210137 and National Institute of Mental Health Grant
R01 MH-40295-01 to the second author. Support was also given to him
during his sabbatical year (1982-83) by the MacArthur Foundation at
the Center for Advanced Studies in the Behavioral Sciences, Stanford,
California.
Thanks are due to Judith Harackiewicz, Charles Judd, Stephen West,
and Harris Cooper, who provided comments on an earlier version of
this article. Stephen P. Needel was instrumental in the beginning stages
of this work.
Correspondence concerning this article should be addressed to Reuben
M. Baron, Department of Psychology U-20, University of Connecticut,
Storrs, Connecticut 06268.
cal function of this article is to clarify for experimental researchers
the importance of respecting these distinctions.
This is not, however, the central thrust of our analysis. Rather,
our major emphasis is on contrasting the moderator-mediator
functions in ways that delineate the implications of this distinction
for theory and research. We focus particularly on the
differential implications for choice of experimental design, research
operations, and plan of statistical analysis.
We also claim that there are conceptual implications of the
failure to appreciate the moderator-mediator distinction.
Among the issues we will discuss in this regard are missed opportunities
to probe more deeply into the nature of causal
mechanisms and integrate seemingly irreconcilable theoretical
positions. For example, it is possible that in some problem areas
disagreements about mediators can be resolved by treating certain
variables as moderators.
The moderator and mediator functions will be discussed at
three levels: conceptual, strategic, and statistical. To avoid any
misunderstanding of the moderator-mediator distinction by erroneously
equating it with the difference between experimental
manipulations and measured variables, between situational and
person variables, or between manipulations and verbal self-reports,
we will describe both actual and hypothetical examples
involving a wide range of variables and operations. That is,
moderators may involve either manipulations or assessments
and either situational or person variables. Moreover, mediators
are in no way restricted to verbal reports or, for that matter, to
individual-level variables.
Finally, for expository reasons, our analysis will initially
stress the need to make clear whether one is testing a moderator
or a mediator type of model. In the second half of the article,
we provide a design that allows one to test within the structure
of the same study whether a mediator or moderator interpretation
is more appropriate.