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Hypotheses involving mediation are common in the behavioral sciences. Mediation exists when a predictor affects a dependent variable indirectly through at least one intervening variable, or mediator. Methods to assess mediation involving multiple simultaneous mediators have received little attention in the methodological literature despite a clear need. We provide an overview of simple and multiple mediation and explore three approaches that can be used to investigate indirect processes, as well as methods for contrasting two or more mediators within a single model. We present an illustrative example, assessing and contrasting potential mediators of the relationship between the helpfulness of socialization agents and job satisfaction. We also provide SAS and SPSS macros, as well as Mplus and LISREL syntax, to facilitate the use of these methods in applications.
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The behavioral science literature is replete with studies demonstrating that a particular independent variable explains variability in a dependent variable. Establishing relationships between variables is important, because correlation is a necessary (but not sufficient) condition for claiming that two variables are causally related. Of even greater scientific interest is explaining how or by what means a causal effect occurs. Questions about causeeffect relations invoke the idea of mediation, the process by which some variables exert influences on others through intervening or mediator variables. For example, evidence suggests that job autonomy, cognitive ability, and job-related skills all predict job performance. But it is even more informative to be able to claim that they exert their effects on job performance through role breadth-the variety of tasks a person performs on the job (Morgeson, Delaney-Klinger, & Hemingway, 2005). Assad, Donnellan, and Conger (2007) found that the effect of optimism on romantic relationship quality is mediated by cooperative problem solving. Kalyanaraman and Sundar (2006) showed that perceived interactivity of a Web portal functions as a mediator of the effect of customization on attitudes toward the portal. Such hypotheses go beyond mere description and help to explain process and causality.
There exists a large and growing literature on methods of testing simple mediation hypotheses-those in which the effect of some causal variable X on some proposed outcome Y is mediated by a single variable M. Our focus in this article is to discuss and illustrate the...