The Not So Illusory Truth Effect: A Rational Foundation for Repetition Effects

The illusory truth effect - the finding that repeated statements are believed more - is understood as a cognitive bias at the core of the psychology of beliefs. Here, we propose that the effect, rather than representing a flaw in human cognition, is a rational adaptation to generally high-quality information environments. Using a formal model, we show that increasing belief in repeated statements improves belief accuracy when a source is credible (i.e., likely to tell the truth) but sometimes makes errors. The theory unifies four key findings in the literature while predicting a testable edge case for the illusory truth effect: when a source is likely to convey falsehoods. Using a large (N = 4,947) pre-registered online experiment, we show that the illusory truth effect is substantially smaller in a low-quality (mostly false) relative to a high-quality (mostly true) information environment. In fact, a majority of participants in the low-quality condition do not demonstrate any illusory truth effect. We identify the deployment of an alternative strategy in the low-quality condition where participants decrease their belief given repetition. Three process-level indicators - response times, cognitive reflection, and the prior plausibility of items - confirm an adaptively rational interpretation. In sum, we suggest the illusory truth effect may not be purely illusory, highlighting its adaptive foundations and the ability of people to efficiently navigate complex environments.