Why repeating lies makes them seem true

Vox talks about the "illusory truth effect", which surely has become something dangerous in the world of social media and other forms of echo chamber:

Psychological science consistently finds when a lie gets repeated, it’s slightly more likely to be misremembered as truth. It’s called the “illusory truth effect.” It’s a tendency the whole news media — as well as consumers of news — should be wary of. And it’s a reason not to give notorious bullshitters such a substantial spotlight. Especially bullshitters whose lies hurt others and whose lies have a track record for virality....

The illusory truth effect has been studied for decades — the first citations date back to the 1970s. Typically, experimenters in these studies ask participants to rate a series of trivia statements as true or false. Hours, weeks, or even months later, the experimenters bring the participants back again for a quiz. 

On that second visit, some of the statements are new, some are repeats. And it’s here that the effect shows itself: Participants are reliably more likely to rate statements they’ve seen before as being true — regardless as to whether they are or not. 

When you’re hearing something for the second or third time, your brain becomes faster to respond to it. “And your brain misattributes that fluency as a signal for it being true,” says Lisa Fazio, a psychologist who studies learning and memory at Vanderbilt University. The more you hear something, the more “you’ll have this gut-level feeling that maybe it’s true.” 

Most of the time this mental heuristic — a thinking shortcut — helps us. We don’t need to wrack our brains every time we hear “the Earth is round” to decide it’s true or not. Most of the things we hear repeated over and over again are, indeed, true. 

But falsehoods can hijack this mental tic as well.