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 fact is wrong can affect the hearer's beliefs. Researchers attributed the illusory truth effect's impact 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".
Although the truth effect has been demonstrated scientifically only in recent years, it is a phenomenon with which people have been familiar for millennia. One study notes that the Roman statesman Cato closed each of his speeches with a call to destroy Carthage ("Ceterum censeo Carthaginem esse delendam"), knowing that the repetition would breed agreement, and that Napoleon reportedly "said that there is only one figure in rhetoric of serious importance, namely, repetition", whereby a repeated affirmation fixes itself in the mind "in such a way that it is accepted in the end as a demonstrated truth".
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