Psychologists call it the illusory truth effect: a statement you have encountered before feels more true than one you have not, regardless of whether it is actually true.
The mechanism is mundane. Your brain tracks how easily a thought comes to mind — what researchers call fluency — and fluency is a decent everyday proxy for truth, because true things do tend to come up more often. So the shortcut usually works.
Repetition hijacks it. A false claim heard five times becomes fluent, and fluency gets misread as familiarity with a fact.
The genuinely uncomfortable finding is that knowing the truth does not fully protect you. In studies, people rated repeated falsehoods as more likely true even when they could correctly state the accurate answer when asked directly. Knowledge and gut feeling are stored differently, and the gut is not consulting the file.
This is why forwarded messages work, why advertising slogans work, and why correcting a rumour by repeating it often spreads it further.
The only real defence is a slightly annoying habit: when something feels obviously true, ask whether you have ever actually checked it, or merely heard it a lot.
