Carlos Barria / Reuters

As Special Counsel Robert Mueller’s investigation into Russian interference in the 2016 election has heated up, so too have Donald Trump’s Twitter fingers. The president is fond of repeating certain disparaging phrases about Mueller’s investigation on the social media platform, repeatedly referring to it as a “hoax” and a “scam.” But his favorite moniker by far is “Witch Hunt”—embellished, in recent weeks, to “Rigged Witch Hunt”—which Trump has used a whopping 84 times this year alone in reference to Mueller’s investigation.

This repeated public condemnation is almost certainly having a psychological effect on how Americans view the investigation. Multiple studies have shown that when something is repeated often enough, people start to think of it as true, whether it actually is—a concept known as illusory truth. “When a statement is repeated, it starts to feel more familiar,” said Keith Payne, a psychology and neuroscience professor at the University of North Carolina at Chapel Hill. “That feeling of familiarity is easily interpreted as the feeling of truth.” Payne is the co-author of a study that found that even when people know a claim is false, just a few repetitions can make them more likely to think it’s true.

Trump’s consistent tweeting—and the constant media coverage of those tweets—makes his favorite phrases familiar to the American public. And that familiarity could be key to making his claims seem plausible, even believable. By consistently referring to the investigation as a “hoax” and a “witch hunt,” Trump is framing the debate on his terms.

“If you start arguing about whether something is or isn’t a witch hunt, you’ve basically conceded that it’s plausible that it’s a witch hunt, or that that’s the right term to use,” says Tom Stafford, a cognitive scientist at the University of Sheffield in Britain. So when Trump says, over and over, that the Mueller investigation is a “rigged witch hunt,” people subconsciously begin to think the idea that it is a witch hunt could be plausible—even when there’s no evidence to substantiate that claim.

Public opinion polls, at least in part, bear out the theory that repetition leads to belief. A June poll from Politico and Morning Consult showed that 53 percent of Republicans have an unfavorable impression of Mueller, up 26 percent from July 2017. A July Ipsos poll showed that 75 percent of Republicans believe that the Mueller investigation is politically motivated against Trump, a number substantiated by a CBS News/YouGov Battleground Tracker poll that found that 70 percent of Republicans believe the investigation to be a “political witch hunt.” These opinion polls appear to show that a substantial portion of  Americans not only believe the president’s claims about the investigation, but also that they’re comfortable adopting the specific language he uses to characterize it.

It’s not just the president’s tweets that are constantly repeating his “truths.” Media outlets like Fox News frequently broadcast his claims as trustworthy, something that is also almost certainly contributing to their plausibility among the segments of the population that trust the network. Eighty-five percent of poll respondents whose most trusted network was Fox News believed that the Mueller investigation was a witch hunt as early as June 2017, according to a Suffolk University poll—more than seven times the respondents for any other network. In their March 2018 poll, Suffolk found that just 35 percent of viewers who trusted Fox also trusted the Russia investigation to be fair and accurate.

These poll numbers all center around people whose political affiliations make them more likely to side with the president in the first place. Payne says that there are always at least two psychological forces at work: the force of familiarity, which is behind the illusory-truth effect, and the force of motivated reasoning, which is when people try to make new information fit in with their previously held beliefs. “In the case of President Trump making false statements about the Mueller investigation, the familiarity of repetition is working in the same direction for Republicans as their motivated reasoning is, so you have two things pointing in the same direction,” he said.

Trump’s repeated charges of “witch hunts,” “hoaxes,” and “scams” don’t seem to be making a difference among Democrats and independents’ opinions of the investigation, at least in terms of poll numbers. The recent CBS News/YouGov Battleground Tracker poll found that 37 percent of independents and just 8 percent of Democrats believed investigations into Russian meddling were a witch hunt, numbers not significantly different from the June 2017 Suffolk/USA Today poll, which had independents at 35 percent and Democrats at 11 percent.

In the same way that Republicans are motivated to believe the president, Democrats are motivated to find reasons not to. And, Stafford adds, repetition doesn’t work in isolation from other psychological effects like political affiliation, ideology, worldview, or recent experience. “Lots of other things affect plausibility about whether something is actually true that are more important than repetition,” he said. “It’s not like a magic spell.”

The force of illusory truth, though, is probably still having some effect on people who disagree with the president, including people who watch networks that are more critical of the president, or air his tweets in order to criticize them. Seeing a phrase or hearing a claim over and over and over again, regardless of whether you agree with it, makes it familiar to you. And familiarity is key—for people who are attuned to political news as much as for people who aren’t. “The more a false statement is repeated, our research suggests that for really familiar, often-repeated statements it makes people less likely to even check their knowledge,” Payne said.

Every time a Trump tweet calling the investigation a “witch hunt” flashes up on people’s Twitter feeds or television screens—regardless of the context—it’s becoming more and more familiar to them. They’re becoming increasingly fluent in the language of Trump’s claimed innocence. Even if they don’t think Mueller’s investigation is a rigged witch hunt, they are becoming more and more familiar with the idea that it could be.

Stafford says the way to combat this effect would be for people to stop responding directly to the president’s charges. “It’s actually very ineffective to say, ‘Oh, this isn’t a witch hunt,’ because what you’re doing is reinforcing the ‘witch hunt’ frame,” he says. “You can’t just get people to stop believing something by contradicting it.” Stafford suggests creating a positive, alternative story, such as “Mueller’s investigation is aimed at safeguarding America’s elections.” But that’s not always a natural position for the president’s opponents, or the media, to take. What effect all of this repeated language could have on public opinion of the investigation itself is unclear. It probably won’t turn public opinion in the president’s favor anytime soon, but it might mean that people take the Russia investigation less seriously—the most recent Suffolk poll is already indicating that it’s a relatively unimportant issue for voters in the midterms. It’s almost certainly succeeding in framing the debate around whether it’s a witch hunt. And maybe, for the president’s purposes, that’s enough.

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