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Tuesday, December 13, 2022

The Trajectory of Truth: A Longitudinal Study of the Illusory Truth Effect

Henderson, E. L., Simons, D. J., & Barr, D. J.
(2021). Journal of Cognition, 4(1), 29.
DOI: http://doi.org/10.5334/joc.161


Repeated statements are rated as subjectively truer than comparable new statements, even though repetition alone provides no new, probative information (the illusory truth effect). Contrary to some theoretical predictions, the illusory truth effect seems to be similar in magnitude for repetitions occurring after minutes or weeks. This Registered Report describes a longitudinal investigation of the illusory truth effect (n = 608, n = 567 analysed) in which we systematically manipulated intersession interval (immediately, one day, one week, and one month) in order to test whether the illusory truth effect is immune to time. Both our hypotheses were supported: We observed an illusory truth effect at all four intervals (overall effect: χ2(1) = 169.91; M(repeated) = 4.52, M(new) = 4.14; H1), with the effect diminishing as delay increased (H2). False information repeated over short timescales might have a greater effect on truth judgements than repetitions over longer timescales. Researchers should consider the implications of the choice of intersession interval when designing future illusory truth effect research.


We used a repeated measures, longitudinal design to investigate the trajectory of the illusory truth effect over time: immediately, one day, one week, and one month. Both of our hypotheses were supported: We observed a main effect of the illusory truth effect when averaging across all four delay conditions (H1). The illusory truth effect was present at all four intervals, but the size of the effect diminished as the interval duration increased (H2). The repeated-minus-new difference was largest when tested immediately (0.67) and shrank after one day (0.39), one week (0.27), and one month (0.14). This reduction in the illusory truth effect over time is inconsistent with an earlier meta-analysis that found no relationship between the size of the effect and intersession interval across studies (Dechêne et al., 2010), but it is consistent with one between-subjects study showing a smaller effect after one week than after a few minutes (Silva et al., 2017, Experiment 1).

The reduced effect after a delay is consistent with the recognition, familiarity, and processing fluency explanations of the illusory truth effect. All three explanations predict larger effects for recently repeated items and smaller effects as feelings of recognition, familiarity or fluency fade with time.

A caveat to the processing fluency account occurs when the source of fluency is obvious (e.g., when participants recognise that statements have been recently repeated). In such cases, participants might not use processing fluency to make their judgments of truth, thereby eliminating the effect (Alter & Oppenheimer, 2009; Nadarevic & Erdfelder, 2014; Oppenheimer, 2004). Our results challenge this fluency discounting explanation because the size of the illusory truth effect was greatest when tested immediately, when participants should be most aware that some statements had been repeated. Similarly, the source disassociation hypothesis predicts that the illusory truth effect should increase with time as people forget that they saw the statements during the experiment, remembering only the semantic content and attributing it to a source outside the experiment. Here we find the opposite.