Illusory Truth Effect
Believing a claim more because you have heard it before
What is it?
The illusory truth effect is the tendency to rate a statement as more likely to be true simply because you have encountered it before. It was first shown by Lynn Hasher and colleagues, whose participants rated repeated trivia statements as more valid than new ones, whether the statements were actually true or false. The mechanism is fluency: a familiar sentence is easier to process, and that ease is mistaken for a sign of accuracy. Later work by Lisa Fazio and colleagues found the effect can occur even when people know the correct answer. In decisions, it lets slogans, rumours, sales pitches and oft-repeated "everyone knows" claims pass as established facts. Unlike a preference for what is familiar, it changes what you believe, not just what you like. It tends to get worse when a claim is plausible, when you meet it in many places without checking its source, and when you are busy or distracted.
Example
Accepting that a competitor is struggling because the rumour came up in several meetings. Believing a nutrition claim you have seen in many posts without ever checking a study. Treating a manager's oft-repeated line about what customers want as proven fact.
References
Hasher, L., Goldstein, D., & Toppino, T. (1977). Frequency and the Conference of Referential Validity. Journal of Verbal Learning and Verbal Behavior, 16(1), 107-112.
Dechêne, A., Stahl, C., Hansen, J., & Wänke, M. (2010). The Truth About the Truth: A Meta-Analytic Review of the Truth Effect. Personality and Social Psychology Review, 14(2), 238-257.
Fazio, L. K., Brashier, N. M., Payne, B. K., & Marsh, E. J. (2015). Knowledge Does Not Protect Against Illusory Truth. Journal of Experimental Psychology: General, 144(5), 993-1002.
How to Prevent It
Doxa uses AI and can make mistakes. How it's built
Do I believe this because of evidence, or because I keep hearing it?
Where did I first come across this claim, and who was behind it?
Are these many sources independent, or repeating one original?
Would this sound as convincing if I were hearing it for the first time?
What do I already know that contradicts this?
Trace a key claim back to its original source before relying on it.
When you first meet a claim, rate how true it seems before it becomes familiar.
Keep a list of "things everyone says" in your field and check them one by one.
In meetings, ask "how do we know that?" when a claim is repeated.
When correcting a false claim, state the accurate version rather than repeating the myth.
Related Decisions
Choosing a supplier or vendor
Sales claims heard often may start to feel like facts
Choosing a technology or tool
Hype repeated across blogs and talks may feel like proof
Making a major lifestyle change
Health claims heard again and again may feel proven