“Repeat a lie often enough and it becomes the truth”, is a law of propaganda often attributed to the Nazi Joseph Goebbels. Among psychologists something like this known as the “illusion of truth” effect. Here’s how a typical experiment on the effect works: participants rate how true trivia items are, things like “A prune is a dried plum”. Sometimes these items are true (like that one), but sometimes participants see a parallel version which isn’t true (something like “A date is a dried plum”).
Recently, a team led by Lisa Fazio of Vanderbilt University set out to test how the illusion of truth effect interacts with our prior knowledge. Would it affect our existing knowledge? They used paired true and un-true statements, but also split their items according to how likely participants were to know the truth (so “The Pacific Ocean is the largest ocean on Earth” is an example of a “known” items, which also happens to be true, and “The Atlantic Ocean is the largest ocean on Earth” is an un-true item, for which people are likely to know the actual truth).
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