A reproducible systematic map of research on the illusory truth effect

People believe information more if they have encountered it before, a finding known as the illusory truth effect. But what is the evidence for the generality and pervasiveness of the illusory truth effect? Our preregistered systematic map describes the existing knowledge base and objectively assesses the quality, completeness and interpretability of the evidence provided by empirical studies in the literature. A systematic search of 16 bibliographic and grey literature databases identified 93 reports with a total of 181 eligible studies. All studies were conducted at Western universities, and most used convenience samples. Most studies used verbatim repetition of trivia statements in a single testing session with a minimal delay between exposure and test. The exposure tasks, filler tasks and truth measures varied substantially across studies, with no standardisation of materials or procedures. Many reports lacked transparency, both in terms of open science practices and reporting of descriptive statistics and exclusions. Systematic mapping resulted in a searchable database of illusory truth effect studies (https://osf.io/37xma/). Key limitations of the current literature include the need for greater diversity of materials as stimuli (e.g., political or health contents), more participants from non-Western countries, studies examining effects of multiple repetitions and longer intersession intervals, and closer examination of the dependency of effects on the choice of exposure task and truth measure. These gaps could be investigated using carefully designed multi-lab studies. With a lack of external replications, preregistrations, data and code, verifying replicability and robustness is only possible for a small number of studies.

Illusory Truth Occurs Even with Incentives for Accuracy

Fake news sites, politicians, and advertisers often make false claims believable by repeating them. Repeated statements feel easier to process, and thus truer, than new ones. In two large experiments (N = 1188), we investigated whether monetary incentives for accuracy reduce this 'illusory truth effect.' Repetition misled people regardless of whether they could earn money for correct answers. The illusion occurred even when participants received item-by-item reminders about possible rewards. Our findings suggest that motivation is not always enough to disengage people from using heuristics to evaluate truth, with implications for a “post-truth world.”

The effects of repetition frequency on the illusory truth effect

Repeated information is often perceived as more truthful than new information. This finding is known as the illusory truth effect, and it is typically thought to occur because repetition increases processing fluency. Because fluency and truth are frequently correlated in the real world, people learn to use processing fluency as a marker for truthfulness. Although the illusory truth effect is a robust phenomenon, almost all studies examining it have used three or fewer repetitions. To address this limitation, we conducted two experiments using a larger number of repetitions. In Experiment 1, we showed participants trivia statements up to 9 times and in Experiment 2 statements were shown up to 27 times. Later, participants rated the truthfulness of the previously seen statements and of new statements. In both experiments, we found that perceived truthfulness increased as the number of repetitions increased. However, these truth rating increases were logarithmic in shape. The largest increase in perceived truth came from encountering a statement for the second time, and beyond this were incrementally smaller increases in perceived truth for each additional repetition. These findings add to our theoretical understanding of the illusory truth effect and have applications for advertising, politics, and the propagation of “fake news.”

How Repetition Affects What Kids and Adults Believe

How do you know that something is true? Is it because you learned it in school? Is it because you heard it before? Our brains can remember a lot of information, but we do not always use this information when deciding what is true. Sometimes we use shortcuts like, “Have I heard this before?” rather than thinking about what we know. These shortcuts can lead us to make mistakes—like thinking that a false statement is true just because you have heard it many times. Researchers call this the illusory truth effect. People are more likely to believe information they have heard multiple times. This happens even when people should know that what they are hearing is wrong! They already have the correct information stored in memory, but repetition makes them think that false information is true. In this article, we discuss why this happens, and how this might affect kids like you!

Distinguishing the binary of news – fake and real: The illusory truth effect

This study seeks to uncover the effects of source and repetition on the illusory truth effect and the dissemination of fake news on social media with an online experiment. This study found that in a personalized source system where trustworthy traditional news sources and personal contacts converged on social media, repetition has a big influence on the trustworthiness of news source and balance of news story. Although most people intend to share real news stories with balance, the illusory truth effect causes mis-judgement, which makes fake news more likely to go viral than real news. The multi-group SEM analysis of the two groups – without source and with source – showed that readers in the no source group rated the effect of repetition on news evaluation as more significant than the with source group. The findings suggest that the effect of source has diminished in the evaluation of news quality. However, sharers on social media are becoming more influential.

An initial accuracy focus reduces the effect of prior exposure on perceived accuracy of news headlines

The illusory truth effect occurs when the repetition of a claim increases its perceived truth. Previous studies have demonstrated the illusory truth effect with true and false news headlines. The present study examined the effects that different ratings made during initial exposure have on the illusory truth effect with news headlines. In two experiments, participants (total N = 575) rated a set of news headlines in one of two conditions. Some participants rated how interesting they were, and others rated how truthful they were. Participants later rated the perceived accuracy of a larger set of headlines that included previously rated and new headlines. In both experiments, prior exposure increased perceived accuracy for participants who made initial interest ratings, but not for participants who made initial truthfulness ratings. The increase in perceived accuracy that accompanies repeated exposure was attenuated when participants considered the accuracy of the headlines at initial exposure. Experiment 2 also found evidence for a political bias: participants rated politically concordant headlines as more accurate than politically discordant headlines. The magnitude of this bias was related to performance on a cognitive reflection test; more analytic participants demonstrated greater political bias. These results highlight challenges that fake news presents and suggest that initially encoding headlines’ perceived truth can serve to combat the illusion that a familiar headline is a truthful one.

The Effect of Repetition on Truth Judgments Across Development

According to numerous research studies, when adults hear a statement twice, they are more likely to think it is true compared with when they have heard it only once. Multiple theoretical explanations exist for this illusory-truth effect. However, none of the current theories fully explains how or why people begin to use repetition as a cue for truth. In this preregistered study, we investigated those developmental origins in twenty-four 5-year-olds, twenty-four 10-year-olds, and 32 adults. If the link between repetition and truth is learned implicitly, then even 5-year-olds should show the effect. Alternatively, realizing this connection may require metacognition and intentional reflection, skills acquired later in development. Repetition increased truth judgments for all three age groups, and prior knowledge did not protect participants from the effects of repetition. These results suggest that the illusory-truth effect is a universal effect learned at a young age.

Repetition increases Perceived Truth even for Known Falsehoods

Repetition increases belief in false statements. This illusory truth effect occurs with many different types of statements (e.g., trivia facts, news headlines, advertisements), and even occurs when the false statement contradicts participants’ prior knowledge. However, existing studies of the effect of prior knowledge on the illusory truth effect share a common flaw; they measure participants’ knowledge after the experimental manipulation and thus conditionalize responses on posttreatment variables. In the current study, we measure prior knowledge prior to the experimental manipulation and thus provide a cleaner measurement of the causal effect of repetition on belief. We again find that prior knowledge does not protect against the illusory truth effect. Repeated false statements were given higher truth ratings than novel statements, even when they contradicted participants’ prior knowledge.