Why False Claims Seem Truer the More You Hear Them

People only use about 10% of their brain. It’s a fact we’ve all heard—except it isn’t a fact at all. It’s a myth.

While activity in some brain regions fluctuates depending on what you’re doing, you use pretty much all of your brain , all of the time. And yet according to surveys of teachers conducted around the world, about half said they believed this 10% myth to be true.

How do such falsehoods make their way into our (fully utilized) brains? One way, it turns out, is through repeated encounters: The more we hear a particular claim or piece of information, the more we tend to give it credence—regardless of whether it’s true.

“When we're exposed to a piece of information repeatedly, it seems truer than the first time we heard it,” says Sarah Barber, an associate professor of psychology at Georgia State University who studies human memory and truth estimations.

People in Barber’s field refer to this as the “illusory truth effect.” In our era of online misinformation and conspiracy mongering , research into the illusory truth effect has both ramped up and taken on greater significance. That work has consistently shown that the more we’re exposed to the same falsehoods or fabrications—in our social media feeds, for example, but also on TV, in conversation with friends, or anywhere else—the truer they seem.

For a 2021 study in Cognition Research , Barber and a colleague found that the greatest leap in perceived truth occurs when we encounter a claim for the second time. Although the size of the illusory truth effect diminishes thereafter, each additional encounter further strengthens our perception of a claim’s legitimacy, that study found.

This doesn’t mean that, with enough repeat exposures, our perception of a claim will flip from false to true. “There’s a difference between perceived truth and belief,” Barber says. “It's not that you will suddenly start believing something just because you keep hearing it, but repetition can make information feel less false.”

Imagine a continuum with total belief at one end and total disbelief at the other. The more you encounter a statement or claim, the more it incrementally shifts toward the “belief” end of that continuum—even if it never crosses the midline.

Read More: Forget the Midlife Crisis. It May Be Your Happiest Chapter Yet

Barber highlighted health-related misinformation and mendacious political propaganda as examples of falsehoods that are frequently repeated online. The more we encounter the same bogus claims—perhaps especially if those claims are mixed into our social media feeds alongside posts from our friends, stories from legitimate news sources, and other trustworthy information—the less outlandish they seem and the more validity we ascribe to them, she says.

There are several possible explanations for the illusory truth effect. The most frequently cited revolves around something cognitive psychologists call “processing fluency.” The idea here, supported by a lot of research , is that information we’ve encountered before is easier for the brain to process, and the brain equates this ease with truthfulness.

“If you think about it from the perspective of information being passed on via word of mouth, which is a form of evidence collection we’ve used since time immemorial, then it makes sense we’d assign more truth value to things we hear repeatedly,” says Shauna Bowes, an assistant professor of psychology at the University of Alabama in Huntsville. “Unfortunately, this doesn’t work as well for us in the context of how we consume information today.”

Bowes, along with Vanderbilt University’s Lisa Fazio, published a study in January that found the illusory truth effect influences our belief in conspiracy theories just as it does for trivia tidbits.

Bowes’ and Fazio’s study also found that neither a person’s openness to conspiracy thinking nor the amount of time they spend reflecting on a claim’s validity diminish the effect of repeated exposures. In other words, even people who aren’t prone to conspiracy thinking and who take time to consider the truth of the claims they encounter are still susceptible to the illusory truth effect.

“I study individual differences—so how people differ in ways that may reduce the influence of false information—but with the illusory truth effect, those differences don’t seem to matter,” Bowes says. “If you’re more rational or prone to reflection, that doesn’t move the needle.”

“It’s not like if you keep hearing that the Earth is a perfect square, you’ll start thinking it's true,” she adds. “But you will see it as slightly less implausible than you did before.”

Unfortunately, knowing that the illusory truth effect exists doesn’t seem to offer much protection from it, Bowes says. The only real safeguard is to reduce your exposure to false claims and conspiracy theories.

That could mean limiting the time you spend on sites and platforms that traffic in lies and misinformation. A better solution, she says, would involve social media platforms and other information sources working harder to promote factual information and minimize the spread of falsehoods and fake news.

“As long as algorithms are flooding our social media feeds with false content, we’re all vulnerable,” she says.