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Kevin McDermott is a member of the Post-Dispatch Editorial Board.

Sociologists have long understood that when people hear a statement again and again, they’re more likely to believe it’s true, regardless of whether it is. It’s called the “illusory truth effect,” and it’s been documented in studies going back decades.

But never before have we seen that principle employed by an American president in the ways we’re seeing today.

Consider some of the biggest of the “big lies” that President Donald Trump and his supporters have managed to pass off to too many Americans as truth, just by saying them repeatedly:

Trump turned around a failing Obama economy.

Trumpists repeat this myth like a religious chant. Maybe they’re confusing it with what happened eight years earlier.

When President Barack Obama took office in 2009, America was mired in the Great Recession, brought on largely by the deregulation policies of President George W. Bush. Over Republican opposition, Obama responded with fiscal stimulus, the automobile bailout, re-regulation and other policies that stopped the bleeding.

Look at any employment or stock market chart spanning 2008 to the present. You’ll see numbers plunging shortly before Obama’s election, bottoming out shortly after he took office, then rising steadily ever since. By those trend lines alone, you wouldn’t know a new president took office in January 2017.

As those pinko liberals at Forbes put it in a headline last fall, “Trump’s Job Gains Are Just A Continuation From Obama’s Presidency.”

Imposing tariffs forces trading partners to give the U.S. money.

Trump expresses this little falsity routinely in service to his trade wars. “We’ll be taking in … hundreds of billions of dollars in tariffs and charges to China and our farmers will be greatly helped,” he said last month.

But tariffs aren’t a charge to China; they’re taxes on Chinese imports that the importers (generally, U.S. companies) have to pay. Typically, the costs are passed onto American consumers in the form of higher prices — a recent 12% jump in the price of washing machines, for example.

That’s not to say China isn’t hurt by Trump’s tariffs. It makes their goods more expensive in America, which drives down sales.

But the notion that tariffs force China to hand money to the U.S. Treasury is nonsense. When Trump says things like, “This support for farmers will be paid for by the billions of dollars our Treasury takes in,” he’s neglecting to mention (or perhaps doesn’t understand) that those dollars are coming primarily from American pockets.

The Mueller report “exonerated” Trump’s campaign of collusion and reached no conclusion on obstruction.

Special counsel Robert Mueller’s report didn’t “exonerate” the campaign of cooperation with the Russians; it merely didn’t find conclusive proof within the significant circumstantial evidence — like the unchallenged fact that the campaign met with a Kremlin-linked lawyer offering dirt on Hillary Clinton, after which Trump himself concocted a public lie to cover it up.

As for obstruction: The report cites numerous instances in which Trump personally tried to thwart the investigation, including firing or attempting to fire people and pressuring others to alter its scope. The report makes clear Mueller viewed these as potentially illegal acts.

But the Justice Department says you can’t indict sitting presidents; if they break the law, impeachment is the remedy — which is why Mueller tossed the ball to the House, which promptly dropped it. “If we had had confidence that the President clearly did not commit a crime, we would have said so” aren’t the words of a man who is doing any “exonerating.”

As a matter of law, you can’t obstruct justice if no underlying crime has been proven.

This isn’t true and there isn’t any debate about that; as a legal assertion, it’s the equivalent of claiming the capital of Missouri is Ladue. Yet Trump and his minions — including lawyers who presumably know better, like Rudy Giuliani — keep saying it.

Legally, obstruction is its own crime, apart from whether other crimes have been proven. There’s a reason for that. As Rep. Justin Amash, the Michigan Republican who has called for Trump’s impeachment, tweeted recently: “Prosecutors might not charge a crime precisely because obstruction of justice denied them timely access to evidence that could lead to a prosecution.”

We have a real-life example: Trump fired FBI Director James Comey in 2017, and subsequently admitted he did it to shut down the Russia investigation — a textbook definition of obstruction. Who knows what Comey might have turned up had he not been fired?

There are other examples these days of Trumpist mythology elevated by repetition: No, Bush and Obama didn’t employ the same family-separation policy as Trump has done at the southern border. No, the GOP’s outrageous refusal to grant a hearing to Obama’s final Supreme Court nominee wasn’t supported by precedent. No, migrants aren’t more likely to commit crimes than natural-born Americans.

These are all lies — and they’re lies no matter how many times they say them.