You would think that greater primary documentation would lead to a better cultural agreement about the “truth.” In fact, the opposite has happened.

Consider the difference in the examples of the John F. Kennedy assassination and 9/11. While you’ve probably seen only a single film clip of the scene from Dealey Plaza in 1963 when President Kennedy was shot, hundreds of television and amateur cameras were pointed at the scene on 9/11. Yet neither issue is settled for Americans; in one recent survey, about as many people said the government was concealing the truth about 9/11 as those who said the same about the Kennedy assassination.

Documentary proof seems to have lost its power. If the Kennedy conspiracies were rooted in an absence of documentary evidence, the 9/11 theories benefited from a surfeit of it. So many pictures from 9/11 flooded the internet, often without much context about what was being shown, that conspiracy theorists could pick and choose among them to show off exactly the narrative they preferred. There is also the looming specter of Photoshop: Now, because any digital image can be doctored, people can freely dismiss any bit of inconvenient documentary evidence as having been somehow altered.

This gets to the deeper problem: We all tend to filter documentary evidence through our own biases. Researchers have shown that two people with differing points of view can look at the same picture, video or document and come away with strikingly different ideas about what it shows.

That dynamic has played out repeatedly this year. Some people look at the WikiLeaks revelations about Mrs. Clinton’s campaign and see a smoking gun, while others say it’s no big deal, and that besides, it’s been doctored or stolen or taken out of context. Surveys show that people who liked Mr. Trump saw the Access Hollywood tape where he casually referenced groping women as mere “locker room talk”; those who didn’t like him considered it the worst thing in the world.

Lies as an Institution

One of the apparent advantages of online news is persistent fact-checking. Now when someone says something false, journalists can show they’re lying. And if the fact-checking sites do their jobs well, they’re likely to show up in online searches and social networks, providing a ready reference for people who want to correct the record.

But that hasn’t quite happened. Today dozens of news outlets routinely fact-check the candidates and much else online, but the endeavor has proved largely ineffective against a tide of fakery.