Most people believe that they judge the truth of a statement by its content:…

Most people believe that they judge the truth of a statement by its content: whether it fits with what they know, whether it comes from a reliable source, whether it makes sense. Decades of research suggest that something much simpler also plays a powerful role: whether we have heard it before.

The effect was first demonstrated in 1977 by Lynn Hasher, David Goldstein and Thomas Toppino. Over several sessions, participants rated how true a series of statements seemed. Some statements appeared more than once. Even though the participants were not told which statements were accurate, they rated repeated statements as more likely to be true than new ones. Psychologists now call this the illusory truth effect.

The main explanation involves a concept called processing fluency. When we encounter information we have seen before, our brains process it more easily, and we tend to interpret that ease as a signal of truth. In most everyday situations, this is a reasonable shortcut, since things we hear repeatedly are often true. The problem is that repetition makes false statements feel familiar too.

More worrying still, knowledge does not provide complete protection. In a 2015 study, Lisa Fazio and colleagues found that repetition increased belief even in statements that contradicted facts the participants already knew. Someone who knows perfectly well that the Atlantic is the ocean between Europe and North America may nonetheless find a repeated false claim about it slightly more convincing the second time.

The implications for public life are obvious. Advertisers have long understood that repeating a slogan makes it feel credible, and false rumours that circulate widely on social media may become more believable simply because people encounter them again and again. Even well-meaning corrections can carry a risk: if a myth is repeated prominently in order to deny it, readers may later remember the myth but forget the denial.

For this reason, many experts recommend that corrections lead with the accurate information, mention the false claim briefly and only once, and then repeat the truth. They also suggest a simple habit for individuals: when a claim feels familiar, pause and ask not "Have I heard this before?" but "Where did I hear it, and do I have any good reason to believe it?" Familiarity, after all, is not evidence.