Scientists Explain How Repeating A Lie Creates “The Illusion Of Truth”


Repeating a lie frequently enough makes it seem true, at least it’s how most propagandists make us believe what they want. Psychologist Tom Stafford argues that if we understand how this effect works, we can avoid falling for it. Here’s how.


“Repeat a lie often enough and it becomes the truth,” was a propaganda law attributed to the Nazi Joseph Goebbels. In psychology, this phenomenon is 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, but sometimes participants see a parallel version which isn't true (something like "A date is a dried plum").

After a break – of minutes or even weeks – the participants do the procedure again, but this time some of the items they rate are new, and some they saw before in the first phase. The key finding is that people tend to rate items they've seen before as more likely to be true, regardless of whether they are true or not, and seemingly for the sole reason that they are more familiar.

Advertisers and politicians know very well how to use this phenomenon to their own advantage.

You can simply see how clear the results are in a laboratory environment. But there’s no need for an experiment, actually. If you look around a bit more carefully, you’ll see that many others are aware of this fact and they’re using it to persuade other people. But of course, it’s not always as simple as that. If it was, there would be no need for all the other techniques of persuasion.

One obstacle is what you already know.

Even if a lie sounds plausible, why would you set what you know aside just because you heard the lie repeatedly?

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? Their results show that the illusion of truth effect worked just as strongly for known as for unknown items, suggesting that prior knowledge won’t prevent repetition from swaying our judgments of plausibility.

At first, this looks like bad news for human rationality, but when interpreting psychological science, you have to look at the actual numbers. What Fazio and colleagues actually found, is that the biggest influence on whether a statement was judged to be true was... whether it actually was true. The repetition effect couldn’t mask the truth. With or without repetition, people were still more likely to believe the actual facts as opposed to the lies.

When we need to make quick judgements, we adopt shortcuts – heuristics which are right more often than wrong.

Relying on how often you've heard something to judge how truthful something feels is just one strategy. If repetition was the only thing that influenced what we believed we'd be in trouble, but it isn't.

When there is limited sourcing and time, we use short-cuts in judging how plausible something is. Often this works. Sometimes it is misleading.

Double-checking is necessary.

If something sounds plausible is it because it really is true, or have we just been told that repeatedly? This is why scholars are so mad about providing references - so we can track the origin on any claim, rather than having to take it on faith.

In short, think before you repeat!
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