The Illusory Truth Effect

“Frequency information is automatically registered in memory, and it influences judgments of validity even when subjects are explicitly warned about the manipulation.”Hasher Goldstein & Toppino, Journal of Verbal Learning and Verbal Behavior 1977

In 1977 Lynn Hasher, David Goldstein, and Thomas Toppino showed subjects sixty plausible trivia statements across three sessions two weeks apart. Forty of the statements changed every session; twenty repeated unchanged. After each statement subjects rated how true it felt on a 1-7 scale. The mean truth rating for repeated statements rose from 4.2 in session one to 4.6 in session two. The mean rating for new statements stayed flat at 4.0. The effect was independent of whether the statements were actually true. Repeated false statements gained as much truth-credit as repeated true ones. Repetition was being metabolized as evidence.

Dechêne Stahl Hansen and Wänke (2010) ran a meta-analysis of 51 illusory-truth studies and reported a mean effect size of g = 0.47, with the effect surviving warnings, surviving incentives for accuracy, and surviving in subjects who explicitly recognized that a statement was a repeat. Fazio Brashier Payne and Marsh (2015) showed that the lift even survives knowing the correct answer: subjects who knew that the largest ocean is the Pacific still rated the repeated false claim “the largest ocean is the Atlantic” as more true on second exposure.

The mechanism is processing fluency. A statement that is easier to process feels more true, and a repeated statement is easier to process than a new one because the encoding pathway has already been walked. The familiarity signal is read as truth-evidence before the knowledge check fires, and the knowledge check often does not fire at all because the familiarity signal precludes the system-two override (Reber & Schwarz 1999).

You are about to do a two-round version of the paradigm. Round one: twelve statements, one truth slider each (0 = obviously false, 100 = obviously true). Round two: eighteen statements — the twelve from round one, plus six new ones, in shuffled order. Same slider. I never reveal during either round which ones are actually true or false, so the only thing changing between round one and round two is your own familiarity. At the end I show you the lift, the gaps, and the answers.

Familiarity and truth are supposed to be two separate things. The illusory truth effect makes them one.