Causal processes in psychology are heterogeneous.

Area of Science:

• Psychology
• Experimental Psychology
• Research Methods

Background:

• Individual differences in response to experimental treatments are widely acknowledged but often overlooked in psychological theories and findings.
• Existing research frequently treats subject-specific variations as error rather than a key aspect of causal effects.
• Within-subjects designs offer a direct way to examine effect heterogeneity, yet this potential is often underutilized.

Purpose of the Study:

• To demonstrate that causal effect heterogeneity can be readily modeled in psychological experiments.
• To highlight the theoretical and methodological opportunities presented by the discovery of effect heterogeneity.
• To advocate for incorporating the assumption and analysis of effect heterogeneity into standard research practices.

Main Methods:

• Analysis of data from four repeated-measures experiments.
• Application of statistical modeling techniques to examine subject-by-subject effect variations.
• Comparison of findings from designs that allow for individual-level effect analysis.

Main Results:

• Effect heterogeneity is demonstrable and modelable using existing data.
• Identifying and analyzing individual differences in treatment effects offers significant insights.
• Integrating effect heterogeneity into study designs is a feasible and beneficial practice.

Conclusions:

• Experimenters should adopt a working assumption that causal effects are heterogeneous.
• Acknowledging and modeling effect heterogeneity enhances the robustness of psychological findings.
• This approach is particularly relevant given the increasing diversity of participant populations in psychological studies.