Illusory Truth Effect: Why We Believe “Fake Facts”

by | Sep 30, 2019


An IFOD a few years ago concerned ten common myths that are untrue:

  • We only use 10% of our brain.
  • Most heat escapes from our head.  
  • Swallowed Gum Stays in your Stomach for Seven Years. 
  • Lightning Doesn’t Strike the Same Place Twice. 
  • A Penny Dropped from the Empire State Building Would Kill Someone if it Hit Them on the Head. 
  • The Hair and Nails Continue to Grow for Months After Death.
  • Alcohol Kills Brain Cells. 
  • Cracking Your Knuckles Causes Arthritis. 
  • If you Swim Soon After You Eat You’ll Get a Cramp and Drown. 
  • The Great Wall Of China is the ONLY Man Made Object Visible From Space.

All ten myths are totally false.

Why We Believe Untrue Things

Why do we believe things that are not true? Research has found that when we decide whether something is true or not we mainly look to two factors: (1) does the information fit with our view of the world, and (2) does the information feel familiar?

Our Worldview is a Fact Filter

The first factor, information fitting our worldview was the topic of an IFOD last week: our beliefs are considered true or false based on the beliefs of the groups to which we belong. Our social groups shape and reinforce our beliefs and define how we view the world.

Our identities are tangled up in how we see the world and we filter information based on our worldview. If something fits, we allow it to reinforce our beliefs. Information that is contrary hits a wall of disbelief.

Why Repeated Information is Believable

Another aspect of why we believe what we believe is even more basic: repetition. “Repeated statements receive higher truth ratings than new statements, a phenomenon called the illusory truth effect.” Source.

Use of repetition to create a sense of truth is a phenomenon long exploited by the advertising industry and politicians, among others. Repeating the same incorrect assertions over and over and over lead to us believe those assertions.

Why does this occur?

Our brains have evolved to be selectively lazy. Thinking, reasoning and analysis requires cognitive energy, and our brains, which already use about 20% of our body’s energy even though the brain is only about 2% of our body’s mass, seeks for ways to conserve energy. Thinking hard actually burns more calories than vegging out! If we stopped and thought long and hard about every situation or piece of information we encountered we’d quickly be mentally exhausted and never get anything done. Its just not possible. Instead, our brains rely on mental shortcuts to operate efficiently and prefer to conserve cognitive resources where possible. Nobel Prize winner Daniel Kahneman refers to this as “thinking fast” (mental shortcuts) and “thinking slow” (reasoning and analysis). Our brains prefer using mental shortcuts and thinking fast because thinking slow requires energy and effort and uses limited cognitive resources.

Being exposed to new information requires more brain power than experiencing familiar information. With new information, the brain must think and use its resources to consider it, categorize it, store it in memory and the like. Receiving old or repeated information is easier – it takes much less brain power to add repeated information to already existing information. Information that is familiar is called “fluent” information and is easier for our brains to process than new information.

Research has found that “the fact that we find it easier to process information that we’ve encountered many times before . . . creates a sense of fluency which we then (mis)interpret as a signal that the content is true.” Source

Believing that repeated information is true is a mental shortcut that is wired into our brains.

Fluency can trump both reasoning and actual contrary knowledge of the truth in determining whether something is true. This illusory truth effect does not vary in strength based on intelligence or analytic thinking style. Thus, regardless of how smart we are, all of us at times fall victim to this effect of thinking that repeated information is true.

From an evolutionary standpoint, thinking that familiar information is true makes sense. Usually it is. Things that are true are more often repeated than things that are not true.

Dangers of Repetition in the Connected Age

We are now increasingly consuming information through social media and other outlets that don’t employ fact-checkers or comply with ethical reporting standards. Untrue information is easily repeated and spreads like wildfire when it fits particular worldviews. Repetition of untruths over these channels can lead to large swaths of the population believing things that just are not true.

What can we do to fight the illusory truth effect?

We can’t be hyper-diligent all the time about believing repeated statements as this is how our brains have evolved. However, we can force our brains to think and not rely on the fluency mental shortcut for important matters. The key is to ask “what is the source of this information?” If the source is biased, such as an advertisement, a politician or political pundit, we must force ourselves to be skeptical – even if the information fits our worldview.


  1. John- great post. On a separate note, I just researched the meaning of my name and its corresponding numerological qualities and it is spot on!! Tell me if this isn’t me in a nutshell:

    “Name Number 5 QualitiesThe adventurer number 5 is about humanity, happiness from what life has to offer and freedom of expression. The pulsation of the number 5 is a brave, unconventional one, that is drawn to the unpredictable and life’s indulgences. On a higher plane of life, the number 5 is targeted at combating temptation and supporting others in adapting to life’s unpredictability.”

  2. I am a recent troke victim and have found that It’s very hard work trying to reconnect the brain to the muscles they are supposed to control. I often am very tird after doing a sudoku puzzle.

  3. This is very interesting and makes total sense from an evolutionary biology perspective–doing or thinking “new” things was highly risky out on the tundra where we all used to live–very much in the spirit if the ‘it ain’t broke, don’t fix it” mantra. Deviating from the ‘perceived wisdom’ is how progress is made but comes with tremendous risk. Our species can only afford to have a small number of us, relatively speaking, taking those risks.

    • Spot on remarks, as usual. You are the best Luv2nap

  4. John thank you for sharing


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