the illusory truth effect

The illusory truth effect (also known as the illusion of truth effect, validity effect, truth effect, or the reiteration effect) is the tendency to believe false information to be correct after repeated exposures.

We will all believe something
if it is repeated enough.

This phenomenon was first identified in a 1977 study at Villanova University and Temple University. When truth is assessed, people rely on whether the information is in line with their understanding or if it feels familiar. The first condition is logical, as people compare new information with what they already know to be true. Repetition makes statements easier to process relative to new, unrepeated statements, leading people to believe that the repeated conclusion is more truthful. The illusory truth effect has also been linked to hindsight bias, in which the recollection of confidence is skewed after the truth has been received.

In a 2015 study, researchers discovered that familiarity can overpower rationality and that repetitively hearing that a certain statement is wrong can paradoxically cause it to feel right. Researchers attributed the illusory truth effect’s impact even on participants who knew the correct answer to begin with, but were persuaded to believe otherwise through the repetition of a falsehood, to “processing fluency”.

The illusory truth effect plays a significant role in fields such as advertising, news media, and political propaganda.

Wikipedia
(but there are many other references)