No Evidence of Experimenter Demand Effects in Three Online Psychology Experiments

Experimenter demand effects occur when participants alter their behavior to align with perceived study hypotheses, threatening internal validity. Concern about demand effects is pervasive in psychology. Experimenter demand may be especially acute in studies relying on experienced participants recruited online (e.g., via Prolific), who may readily guess hypotheses, or using common paradigms (e.g., vignette studies and interventions) where study goals are transparent. We conducted three preregistered experiments (N = 2,254) examining whether explicit demand cues influence online participants’ behavior across three paradigms commonly used in psychology: a dictator game, replicating prior work on demand effects (Experiment 1); a moral dilemma vignette (Experiment 2); and an intervention on group attitudes (Experiment 3). We randomly assigned participants on Prolific to receive information about the study’s hypothesis or to a no-information control. As expected, we find that receiving such information significantly shifts participants’ beliefs about the study’s hypothesis, creating an experimenter demand. Yet we find no evidence that learning any study’s hypothesis alters participants’ behavior, judgments, or attitudes, suggesting that demand effects may be elusive in online samples. These findings offer important insights for the design and interpretation of modern psychology experiments.