Illusory Truth Effect

Illusory Truth Effect is the finding that repeated exposure to a claim increases how true it feels, entirely apart from whether any evidence for it has accumulated. The mechanism runs through processing fluency: a statement you've encountered before is easier to process the second time, that ease registers as a subtle feeling of rightness, and the brain reads that feeling as evidence — even though "I've heard this before" and "this is accurate" are completely different properties of a claim.

The effect was first demonstrated in the 1970s and has since been shown to survive things that should defeat it. Repeating a claim increases its felt truth even for statements people know are false at the time they first rate them, and even when the claim directly contradicts something the person has stated they know to be true — familiarity keeps nudging the feeling of truth upward regardless. It also isn't limited to full repetition of the same sentence; hearing a rumor from three different people who all heard it from the same original source feels like independent corroboration, when it's the same one data point repeated through different mouths — a specific trap in incident channels and rumor mills alike, where a claim gains apparent weight purely from circulating rather than from anyone having actually re-checked it.

This is the mechanism that makes disinformation and internal misinformation both effective through sheer repetition rather than argument — a false claim repeated confidently and often can out-compete a true one stated once, carefully, with citations. It's also a quieter trap inside a codebase or a team: a comment or a Slack message asserting "this endpoint is idempotent" gets copy-pasted into three other places, and by the fourth repetition it reads as established fact that nobody has actually verified against Ground Truth.

The defense is to track a claim's actual evidentiary history rather than the feeling it produces — ask when and how many independent times it was checked, not how many times it's been said, and treat "I've seen this stated several times" as a fact about circulation, not about truth.