The Illusory Truth Effect: How Repetition Shapes Belief

The illusory truth effect, also known as the illusion of truth effect, validity effect, or reiteration effect, refers to the tendency of individuals to believe false information to be correct after repeated exposure. This cognitive phenomenon was first identified in a 1977 study conducted at Villanova University and Temple University. When assessing the truth of information, people often rely on whether it aligns with their existing knowledge or feels familiar. Repetition of statements makes them easier to process compared to new, unrepeated statements, leading individuals to perceive the repeated information as more truthful. The illusory truth effect has been associated with hindsight bias, which distorts the recollection of confidence after receiving the truth.

Power of Familiarity Over Rationality

In a 2015 study, researchers discovered that familiarity can override rationality, causing individuals to paradoxically believe that a statement is correct even when they know it is wrong. This effect was attributed to "processing fluency," where the ease of processing repeated information influences belief formation, even when the information is incorrect.

Impact in Advertising, News Media, and Political Propaganda

The illusory truth effect plays a significant role in various fields, including advertising, news media, and political propaganda. In advertising, the repetition of unfounded claims about a product can boost sales because some viewers may perceive the claims as coming from an objective source, making them more believable. Similarly, news media and political propaganda use the illusory truth effect as a persuasive technique, as repeated statements can lead individuals to accept them as truth, regardless of their factual accuracy.

Examples from History and Politics

While the scientific understanding of the illusory truth effect is relatively recent, the phenomenon has been recognized and exploited by individuals throughout history. For instance, the Roman statesman Cato closed each of his speeches with the call to destroy Carthage, understanding that repetition would foster agreement among his audience. Figures like Napoleon, Quintilian, Ronald Reagan, Bill Clinton, George W. Bush, Donald Trump, and Marcus Antonius in Shakespeare's Julius Caesar have all used the truth effect to their advantage by employing repetition to solidify their messages in the minds of their audiences.


The illusory truth effect is a powerful cognitive bias that demonstrates how repetition can influence belief formation, even when the repeated information is false. This phenomenon has far-reaching implications in various domains, from advertising to politics, where repeated statements can shape individuals' perceptions and acceptance of information as truth, regardless of its veracity.

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