THE president seldom expresses contrition. One exception came after a vulgar video, showing Donald Trump boasting of grabbing women by their nether-parts, nearly torpedoed his presidential campaign. “I said it. I was wrong and I apologise,” he acknowledged in a video statement. The next year, as president, he allegedly claimed that the tape had been faked. Mr Trump has certainly exhibited a remarkable factual flexibility that his supporters have usually indulged. But given the incontrovertible video evidence at hand—both the original tape and Mr Trump’s apology—surely this fiction would not stand? To test this, YouGov, our pollster, put the question to 532 Americans who had voted for Mr Trump. The results: 49% of Trump supporters were unsure whether it was their man on the tape and 9% said that it was not. As a freshly tortured Winston Smith eventually agreed in the final pages of "1984", two and two make five.

Optimists have long clung to the hope that what the RAND Corporation, in a newly published study, calls “truth decay” can be vanquished with clearer thinking. Without bad opinions, wrote John Stuart Mill in "On Liberty", people “lose, what is almost as great a benefit, the clearer perception and livelier impression of truth, produced by its collision with error”. Though America has gone through periods of truth decay in the past, write RAND’s authors—the heyday of yellow journalism during the Gilded Age or the turmoil of the Vietnam war and Watergate—it has not been accompanied by such stark disagreement over objective facts and scientific truths.

Confirmation bias and motivated reasoning are not confined to one party. When YouGov polled Americans on whether they believed government employment data, 45% of Democrats thought that there were more unemployed people than the Bureau of Labour Statistics was letting on, compared with 25% of Republicans. When Barack Obama was president, Republicans were much more sceptical of the numbers.

Social media, though widely blamed for inflaming partisan tensions, cannot explain the decades-long uptick in political polarisation nor the especially pronounced rise among elderly Americans who are not glued to Twitter. Teasing cause from effect is, with the current data, well-nigh impossible. “As passions rise, post hoc reasoning rises,” says Jonathan Haidt, a professor of psychology at New York University. “Technology is a big part of the story, but rising hatred makes people much more willing to believe any outlandish thing.”