We all like to think of ourselves as being impervious to misinformation, but even the most well-informed individuals are still prone to the illusory truth effect. We may be skeptical of a false claim the first time it floats through our Twitter timeline, but the more we are exposed to it, the more we start to feel like it’s true—and our pre-existing knowledge does little to prevent this.

In fact, all it takes is one repeated exposure of a given statement to increase our perception of truth.²⁰ Subsequent repetitions of the statement make us even more likely to believe it, but the largest effect occurs right after we encounter a statement for the second time. This shows how very quickly our beliefs can be manipulated. Here’s how this can affect us:

We become vulnerable to misinformation

You might think you can easily spot misinformation when it comes from an unreliable source (or if there’s no clear source at all), but this might not help you avoid the illusory truth effect. While it’s true that we often consider the source's trustworthiness when evaluating a statement’s truthfulness, research suggests that the illusory truth effect persists regardless of the source’s credibility.²¹ In other words, repeatedly encountering misinformation from unreputable sources can still influence judgments of truth, even when people are well aware that the source cannot be trusted. 

It’s not surprising that these effects can influence our decisions. Perhaps we end up making unsound financial moves because we have been led to believe inaccurate—but often repeated—financial advice. Or maybe we commit to a fad diet after encountering several false health claims from online fitness “gurus.” The effect also plays a role in materialistic behavior. We end up buying things we don’t need because we start to believe repetitive marketing statements like “This product is the best in the world” or “This product will solve all your problems.” The more we’re exposed to repeated misinformation, the more likely we are to accept it as truth—leaving us susceptible to poor decisions.

We share misinformation without realizing it

After believing the information we see repeatedly, we often start sharing these inaccurate details ourselves. If you’ve heard the common myth that we “only use 10 percent of our brains,” have you ever repeated it to others? Even if you had a hunch that this “fact” is not so scientifically accurate, it’s easy to repeat to others because its familiarity makes it feel true. This shows how even well-meaning individuals can become unknowingly complicit in the spread of myths and misconceptions. 

Our tendency to present opinions as facts adds another layer to the problem. According to a recent study, information presented as fact (e.g., “The diameter of the moon is 2,160 miles”) is more likely to produce the illusory truth effect than information presented as opinion (e.g., “Election Day should be a national holiday”).22 Unfortunately, people often share their personal opinions as if they are facts. Think of common claims like “vaccines cause autism,” “global warming is a hoax,” or “MSG is harmful.” This tendency can make us even more susceptible to believing (and repeating) political opinion statements or misleading claims frequently appearing in common discourse.

In our age of social media, it’s incredibly easy for misinformation to spread quickly to huge numbers of people. This can have a significant impact on society, impacting everything from people’s political leanings to their health behavior.

Fake news and social polarization

The evidence suggests that global politics have already been strongly influenced by online propaganda campaigns run by bad actors who understand that all they need to do to help a lie gain traction is to repeat it again and again. These people knowingly conceal the actual truth, instead sharing inaccurate social-political statements to mislead the public and push their own agendas.

But it’s not just bad actors with malicious intent that leverage the illusory truth effect to sway public opinion. As mentioned above, well-meaning individuals can also spread misinformation and cause people to encounter the same false statements again and again, contributing to the widespread acceptance of falsehoods. 

Here’s another concerning finding: it doesn’t seem to matter whether sharers of “fake news” take the time to carefully evaluate the content they forward along. Previous studies have found that misinformation spreads not because people lack critical thinking skills but because sharing information online (whether fictional information or actual truths) becomes a rewarding habit.²³ After all, social media platforms reward accounts that are active and engaged, encouraging users to habitually share information they come across to garner more attention.

This pervasive spread of misinformation contributes to social polarization, especially as echo chambers eliminate differing viewpoints. We all know what comes of this: groups of people with deeply entrenched beliefs and resistance to opposing perspectives. This enhances societal division as people become more isolated in their ideological bubbles, eroding our collective trust in truth and making it much more difficult to have productive debates and dialogues.

The dangers of health misinformation

The illusory truth effect can be downright dangerous to society. Not only can it contribute to division and its harmful ramifications, but it can also cause people to believe recklessly inaccurate health misinformation. 

UChicago researchers recently found that nearly half of the health-related TikTok videos they analyzed contained false information.²⁴ Not only that, but health information is often attention-grabbing and emotionally charged, so it’s well-suited for going viral, spreading rapidly within a short period of time. When people see the same misleading health information over and over, the illusory truth effect posits that they are more likely to believe these falsities. In 2021, for example, a viral TikTok trend encouraged people to put cloves of garlic up their noses, claiming it could relieve congestion, and many people really tried it. In actuality, this strange “health hack” does nothing for congestion but causes irritation and risks damaging nasal tissue. Our advice: save your garlic for the chicken noodle soup.

Now more than ever, it is important to be wary of the fact that the way we assess the accuracy of information is biased. These issues also highlight the responsibility of social media platforms to curb misinformation or at least arm users with the tools they need to better identify and evaluate potentially inaccurate content.

How do we gauge whether a claim is true or false? Naturally, you might assume we use our existing base of knowledge, and maybe a couple of well-placed Google searches, to compare a claim to available evidence. No rational person would accept a statement as true without first holding it up to the light and critically examining it, right?

Unfortunately, humans are rarely rational beings. Every single day, we make an average of 35,000 decisions.¹⁹ With all of those choices to make, and the huge volume of information coming at us every second, we can’t possibly hope to process everything as deeply as we might like.

To conserve our limited mental energy, we rely on countless shortcuts, known as heuristics, to make sense of the world. This strategy can often lead us to make errors in our judgment. Here are a few fundamental heuristics and biases that underlie the illusory truth effect.

We are often cognitively lazy

According to the renowned behavioral economist Daniel Kahneman, there are two thinking systems in our brains: System 1 and System 2. System 1 is fast and automatic, working without our conscious awareness; meanwhile, System 2 handles deeper, more effortful processing, and is under our conscious control.¹ Since it’s doing the harder work, System 2 drains more of our cognitive resources; it’s effortful and straining to engage, which we don’t like. So, whenever possible, we prefer to rely on System 1 (even if we don’t realize that’s what we’re doing).

This preference for easy processing (also known as processing fluency) is more deeply rooted than many of us realize. In one experiment, participants were shown images on a screen while researchers measured the movements of their facial muscles. Some of the images were made easier to process by having their outlines appear before the rest of the picture—only by a fraction of a second, so briefly that participants didn’t consciously realize it was happening. Still, when processing was made easier in this way, people’s brows relaxed, and they even smiled slightly.¹ Processing fluency even has implications in the business world: stocks with pronounceable trading names (for example, KAR) consistently do better than unpronounceable ones (such as PXG).

The problem with processing fluency is that it can influence our judgments about the accuracy of a claim. If it’s relatively effortless to process a piece of information, it makes us feel like it must be accurate. Consider another experiment, where participants were given problems that were deliberately designed to trip people up. For example:

If it takes 5 machines 5 minutes to make 5 widgets, how long would it take 100 machines to make 100 widgets?

100 minutes     OR     5 minutes

Most of us, when we read this problem, intuitively want to say 100 minutes—but it’s actually 5. For some of the participants in this study, researchers made the questions more difficult to read, by presenting them in a barely-legible, small, gray font. When they did this, participants actually performed better—because they had to engage their more effortful System 2 thinking to parse the question. Meanwhile, when the problems were written in a normal, easy-to-read font, people were more likely to go with their (incorrect) intuition.¹

Familiarity making processing easy

What does processing fluency have to do with the illusory truth effect? The answer lies with familiarity. When we’re repeatedly exposed to the same information—even if it’s meaningless, or if we aren’t consciously aware that we’ve seen it before—it gradually becomes easier for us to process. And as we’ve seen, the less effort we have to expend to process something, the more positively we feel about that thing. This gives rise to the mere exposure effect, which describes how people feel more positively about things they’ve encountered before, even very briefly.

In a classic experiment illustrating the mere exposure effect, Robert Zajonc took out ads in student newspapers at two Michigan universities over a period of a few weeks. Every day, the front page of each paper featured one or more Turkish words. Some words appeared more frequently than others, and the frequency of each word was also reversed between the two papers, so that the most-frequently appearing word in one paper would be the least-frequently appearing one in the other.

After the exposure period was over, Zajonc sent out a questionnaire to both communities, asking respondents to give their impressions of 12 “unfamiliar” words. Some of the words were the Turkish words that had run in the newspapers. Participants rated each word from 1 to 7 based on whether they thought the word meant something “good” or “bad.” The results showed that, the more frequently participants had been exposed to a given word, the more positively they felt about it.^1,2

For a long time, psychologists (reasonably) believed that ease of processing was only important in situations where we lack knowledge about something—that we use it as sort of a last-ditch attempt to come to a conclusion. Unfortunately, the evidence suggests that things might work the other way around: ease of processing is our go-to tool to judge whether something is true, and it’s only if that fails that we turn to our knowledge for help. One study found that college students fell for the illusory truth effect even when a subsequent knowledge test showed that they knew the correct answer.³ This phenomenon is known as knowledge neglect: even though people knew the right answers, they could still be led astray by the illusory truth effect.

Referential theory

Christian Unkelbach and Sarah C. Rom proposed another interesting basis for the illusory truth effect, called referential theory, in 2017.²⁵ Referential theory suggests that we judge the truthfulness of a statement based on coherent references in our memory. In other words, we judge truth based on the meanings of individual words or phrases with strong connections in our memory. Repeated statements have more coherent references in our memory, and therefore, we assume they are more likely to be truthful. 

Consider this statement:  “The pan-American highway is the longest road in the world.” It’s possible that “pan-American highway” and “road” are already linked in your memory, so you perceive these elements of the statement as coherent.²² But elements like “longest” and “in the world” may not have been previously associated with the “pan-American highway.” Once you see this statement another time, your brain will recognize the coherent connections between all those elements, leading to a higher subjective truth rating.

The referential theory differs from theories of processing ease or familiarity, instead focusing on how words or phrases are connected semantically in our memory. In this way, the theory attempts to explain why the illusory truth effect still occurs when we’re aware that we’ve heard a statement before, as being aware of a statement’s repetition should theoretically reduce the effect of processing fluency or familiarity.

By now, most of us are all too familiar with the phrase “fake news.” The internet is a breeding ground for false rumors, conspiracy theories, and outright lies, and none of us are immune. According to a study published in the journal Science, on average, false stories reach 1,500 people six times faster than true stories do.^4,5 They’re also 70 percent more likely to be retweeted than real stories. Our media ecosystem is so awash in falsehoods, it’s inevitable that all of us will encounter fake news at some point—and in fact, we probably do so on a very regular basis. This alone puts us at risk of the illusory truth effect.

The illusory truth effect doesn’t just affect us by accident, either. Propagandists understand that repetition is key to getting people to accept your message, even if they don’t believe it at first. Even Adolf Hitler wrote in Mein Kampf that “slogans should be persistently repeated until the very last individual has come to grasp the idea.”⁸ When politicians repeat obvious untruths again, and again, and again, we should not just roll our eyes and write it off as a blunder. We should recognize that this is a deliberate strategy, aiming to familiarize people with the lie being told until they accept that lie as truth.

This all might be sounding a little overly dramatic, but this worry is warranted. Misinformation poses a sinister threat to democracy and the functioning of civil society in general. Around the world, fake news has fueled acts of violence. For example, in 2018, rumors spread on WhatsApp sparked a mob killing in India.⁶ As the coronavirus pandemic spreads across the globe, conspiracy theories have driven large crowds of protesters to march in opposition to social distancing and mask regulations.⁹

While it’s true that bots are behind the spread of a lot of false stories, studies show the lion’s share of the blame lies with real humans.⁵ And yet, even as we propagate misinformation, most of us are anxious about the effects of fake news. A Pew Research poll found that 64% of American adults believe that fake news stories cause a “great deal of confusion.”⁷

Clearly, the problem is not a lack of awareness: people know that unreliable information circulates online. It seems that they just don’t think that they would ever fall for those stories. It’s important to be aware of the illusory truth effect and other biases that affect our judgment so that we are motivated to pause and think a bit more critically about information we might otherwise accept as true.

The illusory truth effect is tricky to avoid. Because it is driven by System 1, our unconscious and automatic processing system, we usually don’t realize when we’ve fallen prey to it. It’s also a very pervasive bias: research has shown that the effect is robust across individual differences in cognitive ability, and people are equally susceptible to the illusory truth effect regardless of their particular thinking style.¹⁰ That said, by making a deliberate and concerted effort to be critical of the claims we encounter, it is possible to get around this effect.

“Critical thinking” might seem like a boring, obvious answer to this problem, but it’s the best solution to avoid falling for the illusory truth effect. With such massive amounts of information filtering past our eyeballs every day, it’s easy to let suspicious claims slide and just move onto the next tweet or status update. But by neglecting to think critically the first time we encounter a false statement, we make ourselves more susceptible to the illusory truth effect.

In one study, participants were asked to read a list of widely-known facts, with a few falsehoods mixed in. One group of participants rated how true they thought each claim was, while the other rated how interesting they were. Later, both groups saw the same statements again, and rated how true they thought they were. The researchers found that, when participants had initially been asked to evaluate truthfulness instead of interestingness, they didn’t show signs of the illusory truth effect—but only when they had relevant knowledge about the statements.¹¹

In short, fact-checking claims the first time you hear them is important to reduce the power of the illusory truth effect. Google is your friend, and when it comes to lies that have political implications, websites like Politifact and Snopes are constantly working to fact-check and debunk fake news. Also try to train yourself to be aware of red flags that you might be reading bad intel. These include things like vague or untraceable sources, poor spelling and grammar, and stories that seem like they would be huge scoops yet aren’t being reported on by any mainstream sources.¹²

The illusory truth effect was first discussed in a 1977 paper by Lynn Hasher, David Goldstein, and Thomas Toppino. The three researchers had college students come into the lab on three separate occasions, each visit two weeks apart, to read a list of statements (both true and false) and rate how accurate they believed each of them was. Over the three sessions, participants rated both true and false statements as progressively more accurate.¹³

Hasher, Goldstien, and Toppino’s work was the first to demonstrate the power of repetition on belief. However, it’s important to note that, in this study, the statements being judged were “plausible but unlikely to be specifically known by most college students.” Most of the participants in this original experiment probably didn’t have any relevant knowledge, and were more or less flying blind when they were asked to judge the accuracy of each statement for the first time. Later studies would go on to demonstrate that the illusory truth effect occurred even when people did have knowledge about the claims they were evaluating.

Interest in the illusory truth effect grew in the late 2000s and 2010s, as the internet and social media became more and more important for disseminating information. It became a particularly popular research topic after the 2016 U.S. presidential election, when questions were swirling about the influence of foreign disinformation campaigns. Research showed that the illusory truth effect likely played a role in people’s acceptance of blatantly false stories on social media.¹⁴ Given its increasingly concerning implications in the digital age, future studies on the illusory truth effect could examine possible interventions to reduce the widespread acceptance of inaccurate information.

Advertisers might take advantage of the illusory truth effect by repeating messages across different platforms and mediums. Even if a consumer initially doubts the validity of a claim, repeated exposure can make it feel more truthful. For instance, if a fitness app repeatedly claims it's "the most effective workout app," users may start to believe it simply because they've seen the claim so many times before. Consider how often you hear certain brand slogans or taglines. Is Disneyland really “the happiest place on earth?” Is Gillette really “the best a man can get?” Whether or not these slogans are spouting truth, their frequent repetition can make consumers start to believe in their accuracy.

Companies that want to follow the lead of these major brands have to consider a few important caveats. First, brands have a responsibility to avoid presenting false or misleading information as fact—this can degrade consumer trust in your brand and is generally considered unethical. Second, if you want to make the most of the illusory truth effect, you’ll need to get your slogan in front of people repeatedly within a short timeframe. This is because the illusory truth effect diminishes with a delay between repetitions.²⁶ Fortunately, advanced advertising algorithms are making it easier and easier to retarget the same consumers with your ads, ensuring they see your repetitive messaging relatively frequently.

 Just like with any popular trend, if there's constant buzz around a particular AI model or technique and it's repeatedly presented as the "best" solution, people might default to using it—even in situations where it might not be the most appropriate choice.

On the flip side, if there are repeated negative claims about a new AI technology—whether they're true or not—people might become resistant to adopting it. For instance, if there's constant news about how a particular AI-driven innovation "isn't ready yet" or "has too many flaws," the broader public might start believing these statements and be hesitant to engage with the technology, even if it improves over time.

AI may also play a role in amplifying the illusory truth effect. Current natural language generation (NLG) tools often provide inaccurate information, and when users encounter the same misleading statements repeatedly, they may start to believe them as fact. Consider how Google’s AI Overview recently came under fire for giving dangerous answers to search queries, suggesting that running with scissors is a good form of cardio, that mixing glue into your pizza sauce can prevent the cheese from sliding off, and that geologists say you should eat one small rock a day to get important vitamins and minerals.²⁷

Now, AI is not simply making things up to mislead us. AI tools source information from human-made content, and we humans have a tendency to write humorous satire that may be obvious in the right context but concerning when individual statements are pulled out and presented as factual answers to search queries. AI doesn't understand what it’s saying, and it’s not yet very good at differentiating between information that’s true and information that’s intentionally misleading. While the above examples are just a few extreme and rare instances, AI also consistently shares outdated or biased information with subtle inaccuracies, which can alter our perception of truth similarly.

One famous example of the illusory truth effect shows that we only need to have encountered part of a statement in order for this bias to kick in. In Thinking, Fast and Slow, Kahneman writes that when people were repeatedly exposed to the phrase “the body temperature of a chicken”—an incomplete sentence that doesn’t actually make any claims—they were more likely to believe the statement that “the body temperature of a chicken is 144°” (or any other arbitrary number).¹ Apparently, partial familiarity is familiar enough.

This example could also demonstrate the referential theory of the illusory truth effect. If you recall, this theory suggests that repeated exposure to information strengthens the connections between words or statements in our memory, increasing our perception of truth. According to the referential theory, repeated exposure to the phrase “the body temperature of a chicken” could strengthen semantic connections between “chicken” and “body temperature.” As a result, our brains can process the whole phrase “the body temperature of a chicken is 144°F” more quickly, leading to a greater sense of truthfulness.

In the wake of the COVID-19 pandemic, the search for effective treatments and preventative measures was at the front of everyone’s mind, politicians and citizens alike. Given the considerable political benefits that would come with finding a cure, it’s not surprising that elected officials were especially keen to talk up promising new drugs.

But the strategy adopted by Donald Trump and his campaign—to repeatedly tout the benefits of a specific drug, hydroxychloroquine, before it was clinically proven—looked like an attempt to capitalize on the illusory truth effect. For months, Trump endlessly sung the praises of hydroxychloroquine, prompting tens of thousands of patients to request prescriptions from their doctors.¹⁵ Even now, with clinical trials showing that the drug is not effective to treat COVID-19,¹⁶ the belief that it works is still widespread. Trump’s claims were also repeated by public figures like Dr. Oz, increasing people’s exposure to them as well as giving them a veneer of legitimacy (Oz is an actual medical doctor).¹⁷

What is it

The illusory truth effect describes how, when we are repeatedly exposed to misinformation, we are more likely to believe that it’s true.

Why it happens

The main reason for the illusory truth effect is processing fluency: when something is easy to process (as familiar information is), we tend to assume that means it is accurate.

Example 1 – The body temperature of a chicken…

When people had previously been exposed to the incomplete sentence “the body temperature of a chicken,” they were more likely to endorse any completed version of this statement, filled in with an arbitrary number.

Example 2 – Illusory truth and hydroxychloroquine

In the early months of the global coronavirus pandemic, US president Donald Trump frequently talked about the drug hydroxychloroquine as a potential cure for COVID-19. These claims were repeated so often that many people continue to believe them, even after clinical evidence has contradicted them.

How to avoid it

Critical thinking and fact-checking are the best lines of defense against the illusory truth effect.

Fake News: Why Does it Persist and Who’s Sharing it? 

This article is an in-depth look at the phenomenon of fake news: where most of it is coming from, why it spreads so easily, and how the illusory truth effect plays a role.

How to Fight Fake News With Behavioral Science

This piece explores how we can leverage behavioral science to fight the spread of fake news, including some simple strategies that anybody can use to help protect themselves from misinformation.