The Illusory Truth Effect


Want to convince someone of something entirely untrue? Just keep repeating it. Over and over. If a statement is repeated often enough, it has a funny way of starting to seem true. This is something researchers call the illusory truth effect, first identified in 1977 by scientists from Villanova University and Temple University.

It had been largely assumed, however, that this would really only work for subjects in which people had no prior knowledge — that is, you can’t trick someone into believing, say, the largest ocean on Earth is the Atlantic when they know it’s actually the Pacific. But, to the contrary, a recent paper published in the Journal of Experimental Psychology found that the illusory truth effect even applies to those who really should know better.


In one experiment, researchers — led by Lisa Fazio of Vanderbilt University — asked a group of undergrads to read through a list of sentences, some of which were obviously true, some of which were obviously false, and others that were trickier. For example: “The Pacific Ocean is the largest ocean on Earth.” (True.) “A date is a dried plum.” (False.) “Oslo is the capital of Finland.” (Also false, but depending on your knowledge of world capitals, perhaps not obviously so.) Next, the students were given another set of statements, but this time, they were to rank each one on a scale of 1 to 6, with 1 meaning definitely false and 6 meaning definitely true. And last, they answered multiple-choice questions that corresponded to the statements they’d just read.

The researchers found that repeated falsehoods were more likely to be accepted as accurate, regardless of whether stored knowledge could have been used to direct a contradiction. To put it more bluntly: Repetition increased perceived truthfulness, even for contradictions of well-known facts. … Reading a statement like ‘A sari is the name of the short plaid skirt worn by Scots’ increased participants’ later belief that it was true,” Fazio and her colleagues write, “even if they could correctly answer the question ‘What is the name of the short pleated skirt worn by Scots?’

Fazio and her colleagues replicated this finding in a second experiment, and in their paper, they nod to research done by others that has pointed at a potential reason why. “Recent work suggests that the ease with which people comprehend statements (i.e. processing fluency) underlies the illusory truth effect,” they write. “Repetition makes statements easier to process (i.e. fluent) relative to new statements, leading people to the (sometimes) false conclusion that they are more truthful.” Fazio said that these results may very well apply to statements made by politicians, who, after a while, may even begin to believe themselves.


Without a doubt, there are many other areas in life where we mistake repetition for truth. Advertisers, politicians, and the media all take advantage of this effect. As any advertiser, politician or member of the media knows, it’s far easier to sell the illusion of truth rather than the actual truth. We live in an incredibly complex world and simple explanations are easier to repeat and easier to understand. If something is difficult to think about and thus hard to repeat, people will unfortunately tend to believe it less. Sadly, this does not bode well for those who are trying to convince people with complex explanations for complex phenomena in a complex world.

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