“Fake news” is Donald Trump’s favorite catchphrase. Since the election, it has appeared in some 180 tweets by the President, decrying everything from accusations of sexual assault against him to the Russian collusion investigation to reports that he watches up to eight hours of television a day. Trump may just use “fake news” as a rhetorical device to discredit stories he doesn’t like, but there is evidence that real fake news is a serious problem. As one alarming example, an analysis by the internet media company Buzzfeed revealed that during the final three months of the 2016 U.S. presidential campaign, the 20 most popular false election stories generated around 1.3 million more Facebook engagements—shares, reactions, and comments—than did the 20 most popular legitimate stories. The most popular fake story was “Pope Francis Shocks World, Endorses Donald Trump for President.”

Fake news can distort people’s beliefs even after being debunked. For example, repeated over and over, a story such as the one about the Pope endorsing Trump can create a glow around a political candidate that persists long after the story is exposed as fake. A study recently published in the journal Intelligence suggests that some people may have an especially difficult time rejecting misinformation. Asked to rate a fictitious person on a range of character traits, people who scored low on a test of cognitive ability continued to be influenced by damaging information about the person even after they were explicitly told the information was false. The study is significant because it identifies what may be a major risk factor for vulnerability to fake news.

Ghent University researchers Jonas De keersmaecker and Arne Roets first had over 400 subjects take a personality test. They then randomly assigned each subject to one of two conditions. In the experimental condition, the subjects read a biographical description of a young woman named Nathalie. The bio explained that Nathalie, a nurse at a local hospital, “was arrested for stealing drugs from the hospital; she has been stealing drugs for 2 years and selling them on the street in order to buy designer clothes.” The subjects then rated Nathalie on traits such as trustworthiness and sincerity, after which they took a test of cognitive ability. Finally, the subjects saw a message on their computer screen explicitly stating that the information about Nathalie stealing drugs and getting arrested was not true, and then rated her again on the same traits. The control condition was identical, except that subjects were not given the paragraph with the false information and rated Nathalie only once.

The subjects in the experimental condition initially rated Nathalie much more negatively than did the subjects in the control condition. This was not surprising, considering that they had just learned she was a thief and a drug dealer. The interesting question was whether cognitive ability would predict attitude adjustment—that is, the degree to which the subjects in the experimental condition would rate Nathalie more favorably after being told that this information was false. It did: subjects high in cognitive ability adjusted their ratings more than did those lower in cognitive ability. The subjects with lower cognitive ability had more trouble shaking their negative first impression of Nathalie. This was true even after the researchers statistically controlled for the subjects’ level of open-mindedness (their willingness to change their mind when wrong) and right-wing authoritarianism (their intolerance toward others), as assessed by the personality test. Thus, even if a person was open-minded and tolerant, a low level of cognitive ability put them at risk for being unjustifiably harsh in their second evaluation of Nathalie.

One possible explanation for this finding is based on the theory that a person’s cognitive ability reflects how well they can regulate the contents of working memory—their “mental workspace” for processing information. First proposed by the cognitive psychologists Lynn Hasher and Rose Zacks, this theory holds that some people are more prone to “mental clutter” than other people. In other words, some people are less able to discard (or “inhibit”) information from their working memory that is no longer relevant to the task at hand—or, as in the case of Nathalie, information that has been discredited. Research on cognitive aging indicates that, in adulthood, this ability declines considerably with advancing age, suggesting that older adults may also be especially vulnerable to fake news. Another reason why cognitive ability may predict vulnerability to fake news is that it correlates highly with education. Through education, people may develop meta-cognitive skills—strategies for monitoring and regulating one’s own thinking—that can be used to combat the effects of misinformation.

Meanwhile, other research is shedding light on the mechanisms underlying the effects of misinformation. Repeating a false claim increases its believability, giving it an air of what Stephen Colbert famously called “truthiness.” Known as the illusion of truth effect, this phenomenon was first demonstrated in the laboratory by Hasher and her colleagues. On each of three days, subjects listened to plausible-sounding statements and rated each on whether they thought it was true. Half of the statements were in fact true, such as Australia is approximately equal in area to the continental United States, whereas the other half were false, such as Zachary Taylor was the first president to die in office (it was William Henry Harrison). Some of the statements were repeated across days, whereas others were presented only once. The results showed that the average truth rating increased from day to day for the repeated statements, but remained constant for the non-repeated statements, indicating that subjects mistook familiarity for verity.

More recent research reveals that even knowledge of the truth doesn’t necessarily protect against the illusion of truth. In a 2015 study published in the Journal of Experimental Psychology: General, Lisa Fazio and her colleagues asked subjects to rate a set of statements on how interesting they found them. Following Hasher and colleagues’ procedure, some of the statements were true, whereas others were false. The subjects then rated a second set of statements for truthfulness on a six-point scale, from definitely false to definitely true. Some of the statements were repeated from the first set, whereas others were new. Finally, the subjects took a knowledge test that included questions based on the statements. The results revealed that repetition increased the subjects’ perception of the truthfulness of false statements, even for statements they knew to be false. For example, even if a subject correctly answered Pacific Ocean to the question What is the largest ocean on Earth? on the knowledge test, they still tended to give the false statement The Atlantic Ocean is the largest ocean on Earth a higher truth rating if it was repeated. When a claim was made to feel familiar through repetition, subjects neglected to consult their own knowledge base in rating the claim’s truthfulness.

These studies add to scientific understanding of the fake news problem, which is providing a foundation for an evidenced-based approach to addressing the problem. A recommendation that follows from research on the illusion of truth effect is to serve as your own fact checker. If you are convinced that some claim is true, ask yourself why. Is it because you have credible evidence that the claim is true, or is it just because you’ve encountered the claim over and over? Also ask yourself if you know of any evidence that refutes the claim. (You just might be surprised to find that you do.) This type of recommendation could be promoted through public service announcements, which have been shown to be effective for things like getting people to litter less and recycle more. For its part, research on individual differences in susceptibility to fake news, such as the study by De keersmaecker and Roets, can help to identify people who are particularly important to reach through this type of informational campaign.

At a more general level, this research underscores the threat that fake news poses to democratic society. The aim of using fake news as propaganda is to make people think and behave in ways they wouldn’t otherwise—for example, hold a view that is contradicted by overwhelming scientific consensus. When this nefarious aim is achieved, citizens no longer have the ability to act in their own self-interest. In the logic of democracy, this isn’t just bad for that citizen—it’s bad for society.