The Barnum effect is our tendency to apply personal meaning to statements that could apply to many people.
For statements such as the results of a personality test, the effect is stronger if:
You may recognise these tricks from a horoscope or your star sign, magicians, fortune-tellers, fortune cookies, con artists and, perhaps, marketing. There's a fascinating magician's technique you may have seen called cold reading that uses the Barnum effect with other tricks ("I see someone with heart pain in your family.") to make it seem like they know a lot of personal details about someone at a first meeting.
Originally from Bertram Forer, who called it "the fallacy of personal validation," it was later known as the Forer effect until Paul Meehl suggested it should be named the Barnum effect after P. T. Barnum (now of The Greatest Showman fame) for the way he could easily fool an audience with these techniques and to emphasize that we should all know and be on the lookout for being fooled. For that reason, I called it the same here.
Bertram Forer ran an experiment with his students (pdf), giving everyone the same results for their individual personality assessment. Generally, the students all thought the same description applied very well to themselves. He used these statements taken "largely from a newsstand astrology book":
Not a bad description of me...
I learned about it from a discussion of the Meyers-Briggs test (I'm an ENTP btw).
Here's Paul Meehl advocating for the Barnum effect:
"Many psychometric reports bear a disconcerting resemblance to what my colleague Donald G. Paterson calls “personality description after the manner of P. T. Barnum” ... I suggest—and I am quite serious—that we adopt the phrase Barnum effect to stigmatize those pseudo-successful clinical procedures in which personality descriptions from tests are made to fit the patient largely or wholly by virtue of their triviality; and in which any nontrivial, but perhaps erroneous, inferences are hidden in a context of assertions or denials which carry high confidence simply because of the population base rates, regardless of the test’s validity. I think this fallacy is at least as important and frequent as others for which we have familiar labels (halo effect, leniency error, contamination, etc.). One of the best ways to increase the general sensitivity to such fallacies is to give them a name. We ought to make our clinical students as acutely aware of the Barnum effect as they are of the dangers of countertransference or the standard error of r."
Meehl, P. E. (1956). Wanted—a good cook-book. American Psychologist, 11(6), 263–272. https://doi.org/10.1037/h0044164