Illusory truth effect
Also known as: Truth effect
A statement feels truer simply because we have heard it before.
What it means
The illusory truth effect is the increase in perceived truth of a statement that results from prior exposure to it, even when the statement is false and even among people who know better. Repetition boosts processing fluency, and that fluency is misread as a signal of accuracy, so familiar claims are judged more valid than novel ones. The effect operates with a single repetition, survives across days, and can occur despite contradicting stored knowledge when people rely on the feeling of familiarity rather than retrieving what they know. It is a central mechanism behind the persistence of misinformation, propaganda, and advertising slogans. Countermeasures include prompting people to consult their knowledge, source scrutiny, and reducing sheer repetition of falsehoods.
Examples
After seeing the claim 'the Atlantic is the largest ocean' several times, people rate it as more likely true, despite many of them actually knowing it is the Pacific.
A slogan heard in every ad break for a decade — a toothpaste 'recommended by dentists' — starts to feel like established fact long before anyone asks which dentists, or how many.
A rumour that the office is relocating gets repeated in three separate corridors, and by Friday everyone believes it. Nothing new was learned; it simply became familiar.
First described in Hasher, Goldstein & Toppino (1977).