Abstract
The illusory truth effect, in which repetition increases belief, is often seen as a cognitive bias. Here, we argue that it may reflect a rational adaptation to generally high-quality information environments. A formal model shows that increasing belief in repeated statements improves accuracy when a source usually tells the truth but sometimes errs. The model captures previous findings and, unlike existing accounts, predicts that repetition should do little to increase belief when the source is not credible. In a large (N = 4,947) preregistered experiment with a quota-matched U.S. sample, the effect was substantially smaller when headlines appeared in a mostly false rather than mostly true newsfeed. In a second preregistered experiment (N = 2,484), repetition increased partisans' belief only in claims from the 2024 U.S. presidential candidate they trusted. Repeated misinformation may therefore be most persuasive when it comes from sources people trust.