illusory truth effect / el efecto de la verdad ilusoria

Illusory truth effect

Repeated information is often perceived as more true than new information. This finding is called the illusory truth effect. Repetition influences beliefs about truth.

If I told you that people only use 10% of their brain, you would probably believe me if you had heard it before. You’re not alone.

Some research suggests that between 40% and 50% of people mistakenly believe this. However, the 10% myth was debunked in a 2009 study published in Frontiers in Human Neuroscience.

We saw a lot of misinformation circulating during the pandemic. People believed misinformation, fake news and conspiracy theories about coronavirus disease. It was a matter of how frequently this information was repeated.

The illusory truth effect explains why people believe fake news to be true and why advertising works.


We encounter many misleading statements in our daily lives, some of which can influence important decisions. The question is how such misconceptions enter our knowledge base and inform our choices.

In addition to repetition, some work suggests that the ease with which people comprehend statements, called processing fluency, underlies the illusory effect. Repetition simply makes statements easier to process than new statements, leading people to incorrectly conclude that they are more truthful.

Once we acquire a piece of knowledge, we tend to forget its source, even if we later realize that it came from an unreliable source. In other words, we tend to forget the original learning event.

If we don’t have any prior knowledge about the topic, if an idea is repeated enough and is easy to process, we are likely to believe it. Moreover, we can neglect the knowledge we already have if the new statement is easy to process and comes with repetition and familiarity.

Surprise or not, knowledge doesn’t protect us from the illusory truth effect unless we can retrieve that previous knowledge and judge new things on it.

The right question

Several years ago, I discovered the power of asking, “How do I know I’m right?”

This question confronts your confidence and overrides your ego. But more importantly, it makes us go back to the source of our knowledge and evaluate whether we’re missing some information or believed in an unreliable source.

By doing so, you can improve your decision-making process because you’re improving the quality of your knowledge. Errors in judgment come from both not knowing and confidently knowing the wrong thing, and evaluating facts is key when we need to make decisions.

Moreover, what we believe to be true shapes our perceptions, our biases, and changes not only our knowledge but also our behavior. The ability to question the source of knowledge is an essential way to develop our self-awareness.

Image by Kajetan Sumila at Unsplash