Causal processes in psychology are heterogeneous

Niall Bolger*, Katherine S. Zee, Maya Rossignac-Milon, Ran R. Hassin

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

76 Scopus citations


All experimenters know that human and animal subjects do not respond uniformly to experimental treatments. Yet theories and findings in experimental psychology either ignore this causal effect heterogeneity or treat it as uninteresting error. This is the case even when data are available to examine effect heterogeneity directly, in within-subjects designs where experimental effects can be examined subject by subject. Using data from four repeated-measures experiments, we show that effect heterogeneity can be modeled readily, that its discovery presents exciting opportunities for theory and methods, and that allowing for it in study designs is good research practice. This evidence suggests that experimenters should work from the assumption that causal effects are heterogeneous. Such a working assumption will be of particular benefit, given the increasing diversity of subject populations in psychology.

Original languageAmerican English
Pages (from-to)601-618
Number of pages18
JournalJournal of Experimental Psychology: General
Issue number4
StatePublished - Apr 2019

Bibliographical note

Publisher Copyright:
© 2019 American Psychological Association.


  • Causal effect heterogeneity
  • Causal processes
  • Mixed models
  • Repeated measures
  • Theory development


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