Cognitive biases
The illusory truth effect: why what we have heard many times seems true
Leo is watching a nature show with his son. The boy asks: “Dad, is it true that an ostrich buries its head in the sand when it’s scared?” Leo answers right away: “True, everybody knows that.” Then they check together on a zoo’s website. It turns out to be a myth. A frightened ostrich that cannot run away lies down and presses its head and neck to the ground. From far away, it looks as if its head is missing. Leo is surprised: where did he “know” this from? He doesn’t remember where he read it. He had simply heard it many times: in cartoons, in jokes, in the phrase “bury your head in the sand.”
This is the illusory truth effect: a statement we have heard or read several times seems truer to us than one we meet for the first time. Repetition proves nothing. But a familiar phrase sounds more convincing than an unfamiliar one.
Below, we look at what research has shown, why repetition works this way, and how not to confuse “familiar” with “true.” In the middle of the article you will scroll through Leo’s feed yourself, and then play “true or not.”
Contents
In short: how the illusory truth effect works
What the illusory truth effect is
The classic experiment was run by psychologists (a 1977 paper). People rated how true statements were about facts that few people know exactly. Some of the statements were shown to them again. The repeated statements seemed truer to people than the new ones.
Since then the experiment has been repeated many times. In a combined analysis of 51 studies, the effect held up. One way to measure it is the way our game does: compare how true already familiar statements seem against completely new ones. With this kind of comparison, the effect came out even stronger.
For a long time it was believed that knowledge protects you. If you know that the largest ocean is the Pacific, hearing the phrase “The Atlantic Ocean is the largest on Earth” repeated will not fool you. Psychologists tested this and saw that the effect appeared even when people knew the right answer. The person has the knowledge, but at the moment of judging, they don’t rely on it and trust the feeling of “sounds familiar.”
One repetition is enough. When statements were shown up to 27 times, perceived truth grew with each repetition. But it grew most at the second encounter, and after that each addition got smaller. And repetition works not only right away. In a long-term test, the effect was found both right after repetition and after a day, a week, and a month, though it weakened with time.
Try it yourself: Leo’s feed
Now you are in Leo’s place. The game takes a couple of minutes. Here is what will happen.
Step 1. Leo scrolls through a feed of short notes on his phone. You will read six notes and mark each one: interesting or so-so. Nothing else is needed.
Step 2. Leo’s son sets up a game of “true or not.” Twelve statements, one per screen. For each, you move a slider: from “definitely false” to “definitely true.”
Step 3. At the end we will show what came out and reveal the right answers.
Why is the game set up this way? Half of the statements in step 2 are true and half are false. Which of them end up in the feed is chosen by the computer, and each player gets a different set. That way, familiar and new statements differ in only one thing: you have already seen some of them, and not the others. The slider is empty at first, so that it does not hint at an answer.
What usually happens. In studies, people on average rate familiar statements as truer than new ones. Though the familiar ones are no more reliable: among them there is as much truth and fiction as among the new ones.
If you did not get a difference, that is not a failure of the experiment. Twelve statements are too few to judge one person: a single hard statement can shift the whole average. Besides, you just read what the trick is, and could have deliberately distrusted the familiar. In studies, the effect shows up when the answers of many people are compared.
When it is not a mistake. In life we usually hear the true more often than the made-up. So relying on the familiar is often sensible. Researchers believe that this is exactly how people learn it: the familiar usually turns out to be right. It becomes a mistake when something false is repeated: in ads, rumors, the news.
Why the repeated seems true
Picture a path through the grass. The more people walk it, the easier it is to walk. But the ease of the path says nothing about whether it leads where you need to go. This is an image, not an exact description of how the brain works. But it conveys the main point.
The familiar is easier to understand. The brain processes a phrase we have already met faster and more easily. Psychologists call this fluency. A person does not notice where the ease comes from and takes it for a sign of truth. This is how the effect is usually explained. There are also other explanations: for example, that a familiar phrase links more easily with what we already know.
The source is forgotten faster than the phrase itself. Leo remembers the thing about the ostrich but doesn’t remember where he heard it: on a nature show or in a cartoon. All that is left is the feeling “I know this.” This is an everyday observation, but it shows well why repetition is so easily passed off as knowledge.
There is a limit. In one study, repetition did not make completely absurd phrases more believable, for example “The Earth is a perfect square.” In another, repetition added belief to any statements, even quite implausible ones, and most of all to those people were unsure about. Scientists argue about the details, but they agree on the main thing: if a falsehood is at least somewhat like the truth, repetition makes it more convincing.
Where it gets in the way
News and social media. Researchers showed people headlines of made-up news the way they appear on social media. Even one encounter made a headline more believable, both right away and a week later. This worked even when the headline was labeled as disputed by fact-checkers. A repost you see for the third time seems more right, though no new arguments have been added.
Ads and slogans. We hear a phrase from an ad dozens of times. A month later, “the best choice” no longer sounds like a promise but like something everyone knows.
Work and family. “Everyone says the new boss is strict.” If you look closely, “everyone” is often one and the same person who was retold several times. A rumor repeated in three conversations seems verified.
The availability heuristic works in a similar way: what comes to mind easily seems frequent. Here, what is easy to read seems right. Confirmation bias adds its own part: what is familiar and matches our opinion is what we check least of all.
How not to confuse “familiar” with “true”
The illusory truth effect is one of the biases on the cognitive bias map, in the section on what we notice. Also there is the negativity bias: why bad news grabs attention harder than good.
Frequently asked questions
If I know about the effect, does it stop working?
There is no tested evidence that knowing about the effect alone protects you. Researchers tested something else: what a person does at the moment of meeting a statement. So it is more useful not to remember the effect “in general” but to get used to one question: “How do I know this?”
Is it always a mistake to trust what is familiar?
No. In life we usually hear the true more often than the made-up, so relying on the familiar is often sensible. It becomes a mistake when something false is repeated: in ads, rumors, the news.
