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Can a Variable be both a Mediator and a Moderator? Dominique Muller (University of Grenoble/University Institute of France) Charles M. Judd (University of Colorado) Vincent Yzerbyt (Catholic University of Louvain)

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2 Mediation ( Baron & Kenny, 1986; Judd & Kenny, 1981 ) X is a treatment and units are randomly assigned to conditions Me is a mediator measured after X (and before Y) Y is the dependent variable

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3 X Y Me Mediation X Y

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4 X Y Me Mediation Without a causal argument, these are nothing else than regression equations, not a strong case for mediation Social psychologists have been very concerned with causality in mediation models (e.g., Spencer, Zanna, & Fong, 2005)

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5 Mediation in the MacArthur approach The MacArthur approach (see Kraemer, Kiernan, Essex, & Kupfer, 2008) takes its root in clinical interventions and randomized clinical trials (RCT) c, a, and b conjointly significant or c, a, and d conjointly significant Mediation

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6 Moderation in social psychology S Y P

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7 P Y S

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8 S Y P P Y S Social psychologists are fine with both models Only preferences dictate which one will be used Moderation in social psychology Moderation is equated with simply a statistical interaction

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9 X is a treatment and units are randomly assigned to conditions Mo, the moderator, is measured PRIOR to the treatment And therefore, the Treatment and the moderator are uncorrelated Moderation in the Mac Arthur approach Moderation is not the same as simply a statistical interaction

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10 In social psychology: Definition of moderation In Mac Arthur Approach: A moderator P is a variable that affects a S – Y relationship or a moderator S is a variable that affects a P – Y relationship A moderator is a variable that affects a causal effect of X on Y

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11 Can a variable be both a mediator and a moderator? From a social psychology perspective: -> Me is a mediator (c, a, and b are conjointly significant) -> Me is a moderator (the d interaction is significant) For the MacArthur approach: -> Me is only a mediator (c, a, and d are conjointly significant)

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12 XY Me For social psychologists

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13 XY Me This explains why it has been argued that this is a case of moderated mediation (Preacher, Rucker, & Hayes, 2007) For social psychologists

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14 XY Me XMe For the MacArthur approach

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15 XY Me XY XY XMe Can a variable be both a mediator and a moderator? Social psychology perspective MacArthur Approach

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16 Conclusion Can a variable be both a mediator and a moderator? Yes, it can be both if moderation and statistical interactions are equated No, if moderation and statistical interactions are not equated How should we define moderation?

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