When the same statement appears a second time, people often rate it as truer than when they first encountered it. This is called the illusory truth effect, or repetition-induced truth effect. A meta-analysis combining 51 studies found that it is not merely a quirk of one small experiment: on average, repeated statements received higher subjective truth ratings than new ones.
One important explanation is processing fluency. Words and sentences encountered before are easier to recognise and understand. That ease really indicates only “I have processed this before”, but it can be misread as “This is more credible”. Familiarity can also lose its source label: people remember seeing a claim without remembering whether it came from a reliable report, an advertisement or an unsupported retelling.
Imagine an unverified workplace rumour appearing first in a group chat, then on a meeting slide, and later in a colleague's retelling. Three encounters may feel like three pieces of support, even though all of them trace back to one person. Repetition has added no independent observation and repaired none of the original evidential gap, but the sentence has become easier to process.
This effect is not a law of mind control. Prior knowledge, plausibility, instructions to check accuracy, repetition frequency and spacing can all alter the result, and no individual must show it on every judgement. The useful response is not to distrust everything familiar, but to separate “How often have I encountered this?” from “How many independent sources support it?” and then inspect the original evidence. Familiarity is a memory cue, not proof.
https://pubmed.ncbi.nlm.nih.gov/20023210/
https://pmc.ncbi.nlm.nih.gov/articles/PMC8116821/