The Pervasive Problem With Placebos in Psychology

TLDR

Researchers must compare treatment to a control that accounts for non‑treatment factors, yet most psychological studies fail to match expectations between groups, creating a pervasive design flaw that undermines causal inference across mental health, cognition, and education. The authors propose that measuring expectations and adopting alternative experimental designs can control placebo effects and strengthen causal claims in psychological interventions. They illustrate the problem with a video‑game‑training example and recommend measuring expectations and using alternative designs to eliminate expectation differences. The failure to match expectations is a fundamental design flaw that can undermine causal inference in psychological studies.

Abstract

To draw causal conclusions about the efficacy of a psychological intervention, researchers must compare the treatment condition with a control group that accounts for improvements caused by factors other than the treatment. Using an active control helps to control for the possibility that improvement by the experimental group resulted from a placebo effect. Although active control groups are superior to “no-contact” controls, only when the active control group has the same expectation of improvement as the experimental group can we attribute differential improvements to the potency of the treatment. Despite the need to match expectations between treatment and control groups, almost no psychological interventions do so. This failure to control for expectations is not a minor omission—it is a fundamental design flaw that potentially undermines any causal inference. We illustrate these principles with a detailed example from the video-game-training literature showing how the use of an active control group does not eliminate expectation differences. The problem permeates other interventions as well, including those targeting mental health, cognition, and educational achievement. Fortunately, measuring expectations and adopting alternative experimental designs makes it possible to control for placebo effects, thereby increasing confidence in the causal efficacy of psychological interventions.