The How to Search? team — content verified on the basis of research by Hasher et al. (1977), Merriam-Webster, and analyses of the Theranos case (full list of sources at the bottom of the article), current as of August 2026.
Short answer: “I feel it’s true” is the worst credibility test, because the human brain judges truth based on how easily it processes information (cognitive fluency), not on the basis of evidence. This phenomenon is called the illusory truth effect (Hasher et al., 1977), and it makes repeated, easy-to-absorb claims — true or not — start to “feel” like facts. In 2006, the phenomenon even got its own name: truthiness.
What the “feeling test” is and why it fails scientifically
When you hear a claim and your first reaction is “that sounds true” or “I feel that’s how it is” — your brain has just used a mental shortcut called the affect heuristic: it evaluated information based on an emotional reaction, not an analysis of the facts. The problem is that this feeling can be triggered artificially — without changing a single fact.
Cognitive psychologists call this processing fluency : the easier it is for the brain to process information — because it’s simple, it rhymes, you’ve heard it before, or it fits what you already think — the more “true” it seems. This mechanism is completely independent of whether the information is actually true.
The science behind the phenomenon: the illusory truth effect
The 1977 experiment that started it all
In 1977, researchers Lynn Hasher, David Goldstein, and Thomas Toppino of Villanova University and Temple University conducted an experiment that remains a foundation of this field of psychology to this day. Participants were shown a series of plausible-sounding sentences — some true, some false — and asked to rate their truthfulness in three separate sessions.
The result: sentences the participants had seen before were rated as more true at the second and third encounter — regardless of whether they were actually true. Repetition alone, without any new evidence, increased perceived credibility. The phenomenon was named the illusory truth effect , and since then it has been confirmed in more than 80 peer-reviewed scientific studies.
Why it works: the “easy = true” shortcut
The mechanism is explained by so-called cognitive fluency: the brain unconsciously mixes up two completely different questions — “is this easy for me to process?” and “is this true?”. Because in many everyday situations true information really is easier to understand (since it’s consistent with what we already know), the brain developed a shortcut: easy = true. This shortcut worked great for thousands of years of evolution. In a world flooded with content optimized for clicks — it fails catastrophically.
Truthiness: when “feeling the truth” got its own name
On October 17, 2005, in the first episode of his satirical show, American comedian Stephen Colbert coined the word “truthiness” — describing truth felt “from the gut, not from books”. The phenomenon caught on so well that in 2006 it was chosen as Word of the Year by the Merriam-Webster dictionary, winning the vote by a margin of 5 to 1 over… the word “google.”
“I don’t trust books. They’re all fact, no heart.” — Stephen Colbert, creator of the term “truthiness”
The official Merriam-Webster definition reads: “a truthful or seemingly truthful quality attributed to something not because of facts or evidence, but because of a feeling that it is true, or a desire for it to be so”. This is exactly the same mechanism we’re talking about in this article — only named and popularized culturally, not just scientifically.
True stories: when “I feel it’s true” cost millions
Theranos: 9 billion dollars built on charisma, not evidence
This is probably the most expensive lesson about the “feeling test” in business history. Elizabeth Holmes, founder of the medical startup Theranos, convinced investors that her device could perform hundreds of blood tests from a single finger prick. The company reached a valuation of 9 billion dollars, and Holmes became the youngest self-made female billionaire in the US — even though the technology never worked as promised.
Investors put in millions based on Holmes’s charisma and a “strong relationship” with her — as one investor, Chris Lucas of Black Diamond Ventures, who had “little insight” into how the company actually worked, testified in court.
Theranos never published a single peer-reviewed scientific publication confirming that its technology worked — a standard practice in the medical industry.
Alyse Killeen, a venture capital investor, put it bluntly, commenting on the collapse of Theranos:
“What led to the Theranos disaster comes down to an unreflective reliance on ‘gut feeling’ instead of thorough, in-depth verification.” — Alyse Killeen, Managing Director, StillMark
In January 2022, a jury found Holmes guilty of defrauding investors. Sentence: more than 11 years in prison. The company that “felt” like a medical breakthrough turned out to be entirely fiction.
Other high-profile cases of the same mechanism
Theranos isn’t an isolated case. The same pattern — confidence and charisma replacing evidence — repeats throughout the history of financial fraud (Bernard Madoff’s pyramid scheme, in which investors “felt” that Madoff’s reputation and confidence guaranteed safety) and health disinformation (Andrew Wakefield’s retracted, falsified 1998 publication linking vaccines to autism — a claim that “felt” true to many parents, despite the lack of any confirming scientific evidence, and despite the publication later being formally retracted by the journal The Lancet).
Why it works even more strongly in the AI era
Language models generate answers that are maximally fluent, grammatically coherent, and confident in tone — exactly the features our brain associates with truth. An AI answer that hallucinates a nonexistent fact sounds just as confident as one that gives verified information. This makes the “feeling the truth” mechanism more unreliable today than ever — because the fluency of a text has stopped being a signal that someone (or something) actually knows what they’re talking about.
Checklist: how to recognize when your “feeling” is fooling you
Ask: have I heard this before?
If a claim sounds familiar, it may be the illusory truth effect, not evidence. Repetition ≠ confirmation.
Check whether it fits “too well” with what you already think
We feel information consistent with our views is more credible — regardless of the actual evidence.
Separate confidence of tone from quality of evidence
Charisma, confidence, and fluent speech (human or AI) are not evidence. Theranos is the most expensive lesson of this principle in history.
Use the SIFT method instead of intuition
Stop, check the source, look for independent confirmation, trace it back to the original — we describe this in detail in our guide.
Frequently asked questions
It’s a documented psychological phenomenon (Hasher, Goldstein, Toppino, 1977) in which repeating a claim increases the subjective sense of its truthfulness — regardless of whether the claim is actually true. It has been confirmed in more than 80 scientific studies.
A lie is a conscious statement of something untrue. Truthiness (Stephen Colbert’s term, Merriam-Webster’s 2006 Word of the Year) is a situation in which someone judges something to be true based on a feeling or a desire for it to be so — often without being aware that they’re ignoring the available evidence.
No. Research shows that the illusory truth effect works regardless of the participant’s prior knowledge — even experts in a given field succumb to it in areas they know well.
Replace intuitive judgment with a specific procedure — e.g., the SIFT method (Stop, Investigate, Find, Trace). The habit alone of asking “how do I know this?” instead of “do I feel this is true?” significantly reduces susceptibility to this mechanism.
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Sources: Hasher, Goldstein & Toppino (1977), Journal of Verbal Learning and Verbal Behavior; reviews of research on the illusory truth effect (ScienceDirect, 2023; Collabra: Psychology, 2020); Merriam-Webster (“Truthiness” — Words of the Year 2006); Wikipedia (the “Truthiness” entry); Focus Law LA, Forbes, IMD Business School, Harvard Law School Forum on Corporate Governance (analyses of the Theranos case); Nixon Peabody LLP (a summary of the Elizabeth Holmes trial).