011 min
Where the Barnum effect shows up
An illustrative case. A product manager at a wellbeing app launches a feature that gives each new user a "personality insight" after ten onboarding questions. The insight reads: "You value close relationships but need time alone. You can be hard on yourself, yet you push through challenges. At times you doubt your choices." In a pilot, most users rate the insight as accurate. The product manager concludes that the scoring model understands users.
Now change one input. Suppose each pilot user receives an insight that was written for a different user, and nobody is told. If the accuracy ratings stay about the same, the ratings never measured how well the text fitted the person. They measured how general the text was. The first pilot could not tell those two apart, because it never compared the insight with anything.
That is the Barnum effect at work in a product decision. It shows up wherever a system gives a general statement to one person and then asks whether it fits:
- Personality quizzes and year-in-review summaries.
- Automatically generated performance feedback that is not tied to specific work.
- Persona documents that describe "a busy professional who values simplicity".
- Chat assistants that give a confident, general answer to a personal question.
- Horoscopes, fortune telling and some personality tests.
022 min
Where the Barnum effect comes from
Two names matter here, Ross Stagner and Bertram Forer, and one more, Paul Meehl, who gave the effect its common name.
In 1947, psychologist Ross Stagner asked a number of personnel managers to take a personality test. He did not give them feedback based on their answers. He gave each of them the same generalised feedback, based on horoscopes, graphological analyses and similar sources. Then he asked how accurate the assessment was. More than half described the assessment as accurate, and almost none described it as wrong.
In 1948, psychologist Bertram Forer ran the better-known version. He gave his 39 psychology students a test called the Diagnostic Interest Blank and told them each would receive a short personality sketch based on their results. One week later each student received a sketch and rated how well it applied to them. In reality, every student received the same sketch. Forer built it from a newsstand astrology book.
The sketch contained lines such as "You have a great need for other people to like and admire you" and "At times you are extroverted, affable, sociable, while at other times you are introverted, wary, reserved." The students rated its accuracy on a scale from 0 (very poor) to 5 (excellent). On average, they rated it 4.30. Only after the ratings were turned in did Forer reveal that all students had received an identical sketch.
Forer published the result in 1949 as "The Fallacy of Personal Validation". He attributed the effect to gullibility. Paul Meehl coined the term "Barnum effect" in 1956 in an essay called "Wanted - A Good Cookbook", because he related the vague personality descriptions used in some psychological tests to those given by the showman P. T. Barnum. This is why the same effect has two names: the Barnum effect and the Forer effect.
032 min
Why the Barnum effect happens
The effect has three main conditions. Each one gives the reader more room to find a match.
1. The statements are vague. The effect is consistently found when the assessment statements are vague. A statement of this kind is called a Barnum statement: a general description that could apply to almost anyone. People read their own meaning into it, so it becomes personal to them. Statements with the phrase "at times" work well. "At times you feel very sure of yourself, while at other times you are not as confident" is true of nearly everyone, because every person can recall one such time.
2. The description is mostly positive. People rate a description as more accurate when it lists mainly positive traits. Accepting a flattering description costs nothing and feels good.
3. The reader believes it is personal and trusts the source. Replication studies of Forer's work point to two factors: the ratio of positive to negative traits in the description, and whether the person trusts the honesty of whoever gives the feedback. People also accept negative assessments more readily when they see the source as a high-status professional.
In every step the reader does the matching. The description supplies a statement that fits many lives, and the reader picks the life it fits. This process is called subjective validation: the statement seems true because the reader makes it true for themselves. It also works together with confirmation bias, the habit of noticing information that fits what you already expect. A reader who expects the description to fit will count the lines that match and pass over the lines that do not.
041 min
What the Barnum effect costs
The first cost is that a practitioner can seem to know a person without knowing anything about them. Such techniques are used by fortune tellers, astrologers, and other practitioners to convince customers that they, the practitioners, have a paranormal gift. In 1977, Ray Hyman wrote about how hucksters use the Forer effect to take advantage of victims. He listed factors that help them succeed, such as confidence, a prop like a crystal ball or tarot cards, attention to a client's clothing and speech, and flattery. This is the basis of cold reading.
The second cost is a wrong conclusion in product and research work. A general result gets read as proof that a method works. A test that gives the same output to everyone and then asks whether it describes them cannot tell a good model from a bad one. The same mistake happens in teams: a persona or customer summary that is vague enough to feel right to everyone gives no direction for a decision.
The third cost is that vague claims collect support. When a believer rates a vague claim as accurate, the believer counts that rating as evidence. This is part of why the effect gives a partial explanation for the wide acceptance of some paranormal beliefs and practices, such as astrology and some types of personality tests.
051 min
How to guard against the Barnum effect
Awareness alone does not remove the effect, because it comes from how people match statements to themselves. These steps change the test instead. They are practical guidance based on the mechanism above, not findings from a study.
Run a swap test.
Show each person two unlabelled descriptions, one made for them and one made for someone else, and ask which is theirs. If people cannot tell, the description is too general.
Look for statements that could be false.
"At times you doubt yourself" cannot be wrong. "You changed jobs more than twice in the last five years" can be wrong, so a correct answer carries information.
Check the balance.
If every trait is positive, expect a high rating for that reason alone.
Ask for specifics.
Ask the user to point to the part that fits and say what it predicts about their behaviour.
Do not treat an accuracy rating as validation.
Michael Birnbaum, a professor of psychology at California State University, Fullerton, summarises the lesson of the Barnum demonstration as: self-validation is no validation.
The same applies to your own writing. If you write personas, summaries or feedback, make each statement specific enough that it could turn out to be wrong.
061 min
The Barnum effect compared with nearby biases
The Barnum effect is often mixed up with three other ideas. The axis that separates them is what triggers the judgment.
| Concept | What it is | What triggers it |
|---|---|---|
| Not in the library yetBarnum effect | ||
| PsychologyConfirmation bias | ||
| PsychologyHalo effect | ||
| PsychologyNarrative fallacy |
Confirmation bias helps the Barnum effect along, but it does not need vagueness. The Barnum effect does. A flattering description can also raise trust in its source, which is where the halo effect overlaps.
071 min
Common misunderstandings about the Barnum effect
"It only fools gullible people." Forer attributed the effect to gullibility. Later replication studies point instead to conditions: how vague and positive the text is, whether it seems personal, and whether the source is trusted. Wikipedia's summary also says evidence suggests people with authoritarian or neurotic personalities, or a greater than usual need for approval, are more likely to show it. That is a tendency, not a split between fooled and unfooled people.
"A personal result that feels accurate must be accurate." A correct description and a general one can feel the same to the reader. A rating of fit alone cannot separate them. That is why the guard steps above compare, test or ask for specifics.
"Specific text is always safe." The effect depends on vagueness, but a specific text can still be wrong. The swap test checks fit, not truth.
?5 questions
Questions people ask
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▶3 videos
Watch
Understanding Barnum Effect
The Barnum Effect - Why Do People Believe In Horoscopes?
The Infographics Show
The Forer Effect or Barnum Effect (Intro Psych Tutorial #135)
PsychExamReview
§3 sources
Sources
Forer, B. R. (1949). "The fallacy of personal validation: a classroom demonstration of gullibility." Journal of Abnormal and Social Psychology, 44(1), 118-123.
Meehl, P. E. (1956). "Wanted - a good cookbook." American Psychologist, 11(6), 263-272.
Barnum effect — Wikipedia.
