Illusory Truth Effect: Why a Repeated Claim Starts to Feel True

A statement that has been met before is easier to read, easier to understand and quicker to make sense of than one that is new, and that ease is felt as a quality of the statement rather than of the reader. The standard account is that people use this processing fluency as a cue to truth. True statements are, on the whole, encountered more often than false ones, so the cue is not foolish; but it fires for anything that has been repeated, and a reader who does not remember the earlier encounter cannot discount it. The feeling that a claim rings true is often the feeling of having heard it before.

The effect does not depend on believing the source, remembering the source, or even noticing the repetition. It has been found when statements are attributed to untrustworthy speakers, when the earlier exposure happened in a different task, and when participants are warned that some of what they will read is false. Dechêne, Stahl, Hansen and Wänke's 2010 meta-analysis of 51 studies found the effect reliably whether measured as the rise in a statement's rating from one encounter to the next or as the gap between repeated and new statements at test, with the second kind of comparison, the one the demo above uses, giving the larger effect.

Until 2015 the usual assumption was that knowledge protects: repeating a statement you know to be false, the thinking went, cannot make you believe it. Fazio, Brashier, Payne and Marsh tested that directly and found that it is not so. Repetition raised truth ratings for statements that contradicted well-known facts, such as the Atlantic being the largest ocean, even among people who could answer the underlying question correctly afterwards. Knowledge reduces the ratings overall, so known falsehoods are still rated as less true than unknown ones, but repetition shifts both by about the same amount. The authors called this knowledge neglect: when a statement feels fluent, people often do not consult what they know.

Two limits matter for reading the demo above. The effect is an average shift in judged likelihood, not a conversion from disbelief to belief, and a few points of difference between your two averages is the kind of shift the studies measure. And the demo rates each statement once, so your gap cannot separate repetition from any difference between the two sets of statements, which are twelve different statements rather than one statement in two forms; the evidence that repetition is the cause comes from the studies, which counterbalanced which statements were repeated.

Six of the twelve statements in the rating round are false, and a page about repetition installing a falsehood owes its readers the corrections. Ottawa, not Toronto, is the capital of Canada; Canberra, not Sydney, is the capital of Australia; peanuts grow underground and cashews grow on trees; bats can see; and a goldfish remembers for far longer than a few seconds.

Lynn Hasher, David Goldstein and Thomas Toppino published "Frequency and the conference of referential validity" in the Journal of Verbal Learning and Verbal Behavior in 1977. Forty college students rated how certain they were that each of 60 statements was true or false, on a seven-point scale, on three occasions two weeks apart. The statements were plausible items of general knowledge, some true and some false. Twenty of the statements appeared in all three sessions; the other 40 in each session were new.

The repeated statements were rated more valid at each successive session, and the new statements were not. The rise appeared for true and false statements alike. The authors concluded that the frequency with which a statement is encountered confers a kind of validity on it, independent of its content.

Fazio, Brashier, Payne and Marsh's 2015 experiment, published in the Journal of Experimental Psychology: General, is the design the demo on this page follows. Forty Duke University students first rated 88 statements for how interesting they were, with truth not mentioned as a task, and then rated 176 statements, the 88 repeats and 88 new ones, for truth on a six-point scale. The set mixed true and false statements, and the false ones, whose ratings are the figures reported here, were divided between statements that contradicted facts most people know and statements that contradicted obscure facts. Repeated falsehoods received a mean truth rating of 3.53 against 3.26 for new falsehoods, and the gap was about the same for statements whose underlying fact the student answered correctly in a knowledge check afterwards (2.93 against 2.59) as for statements they did not know (4.13 against 3.92). A second experiment used multinomial modeling to show that participants sometimes relied on fluency even when the relevant knowledge was available to them.

Repetition is the cheapest tool of persuasion, which is why the effect is everywhere. The rows below are illustrations, not findings from the studies cited on this page.

Fluency cannot be switched off, so the counter-measures work by adding a second check that fluency does not supply.

• Ask where you first heard it. If the answer is nowhere in particular, the feeling of truth is probably the feeling of familiarity.
• Count sources, not repetitions. Three outlets repeating one press release are one source.
• Check against what you know, on purpose. The 2015 experiment showed that knowledge does not protect automatically, but Brashier, Eliseev and Marsh found in 2020 that asking people to rate truth at first exposure prevented the effect for statements whose facts they knew, because it made them consult their knowledge while the statement was still new.
• When correcting a false claim, repeat it as little as possible. State the true fact first and most often; the correction's job is to make the truth the familiar version.
• Be slowest with the claims that feel most obvious. That feeling is what the effect produces.

The illusory truth effect is one of several effects of repetition and familiarity, and it is often confused with its neighbors.

• It is not the mere exposure effect, in which repeated contact with something makes it better liked. Both run on familiarity, but mere exposure changes liking and illusory truth changes judged truth; a repeated jingle can be annoying and still make its claim feel true.
• It is not the continued influence effect, in which a piece of misinformation keeps shaping reasoning after it has been corrected. That effect is about corrections failing to remove a claim; illusory truth is about repetition installing one.
• It is not the availability heuristic, which judges how common something is by how easily examples come to mind. Repetition makes examples available, so the two travel together, but availability is a judgment of frequency and illusory truth a judgment of fact.