Prevention Science

, Volume 20, Issue 3, pp 419–430 | Cite as

Testing the Causal Direction of Mediation Effects in Randomized Intervention Studies

  • Wolfgang WiedermannEmail author
  • Xintong Li
  • Alexander von Eye


In a recent update of the standards for evidence in research on prevention interventions, the Society of Prevention Research emphasizes the importance of evaluating and testing the causal mechanism through which an intervention is expected to have an effect on an outcome. Mediation analysis is commonly applied to study such causal processes. However, these analytic tools are limited in their potential to fully understand the role of theorized mediators. For example, in a design where the treatment x is randomized and the mediator (m) and the outcome (y) are measured cross-sectionally, the causal direction of the hypothesized mediator-outcome relation is not uniquely identified. That is, both mediation models, x → m → y or x → y → m, may be plausible candidates to describe the underlying intervention theory. As a third explanation, unobserved confounders can still be responsible for the mediator-outcome association. The present study introduces principles of direction dependence which can be used to empirically evaluate these competing explanatory theories. We show that, under certain conditions, third higher moments of variables (i.e., skewness and co-skewness) can be used to uniquely identify the direction of a mediator-outcome relation. Significance procedures compatible with direction dependence are introduced and results of a simulation study are reported that demonstrate the performance of the tests. An empirical example is given for illustrative purposes and a software implementation of the proposed method is provided in SPSS.


Mediation analysis Randomized design Direction of effects Direction dependence Non-normality 



No funding was received for this work.

Compliance with Ethical Standards

Conflict of Interest

The authors declare that they have no conflict of interest.

Ethical Approval

This article does not contain any studies with human participants or animals performed by any of the authors.

Informed Consent

Informed consent was not required for this study.

Supplementary material

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Copyright information

© Society for Prevention Research 2018

Authors and Affiliations

  • Wolfgang Wiedermann
    • 1
    Email author
  • Xintong Li
    • 1
  • Alexander von Eye
    • 2
  1. 1.Statistics, Measurement, and Evaluation in Education, Department of Educational, School, and Counseling Psychology, College of EducationUniversity of MissouriColumbiaUSA
  2. 2.Michigan State UniversityEast LansingUSA

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