Barnum / Forer Effect
The tendency to accept vague, general personality descriptions as uniquely accurate. Named after P.T. Barnum and demonstrated by Bertram Forer (1949).
In Forer's 1949 study, students were each given an identical generic personality description ("You have a great need for other people to like and admire you... you tend to be critical of yourself...") and asked to rate its accuracy. Average rating: 4.26/5. Every participant believed the description was personalised.
The Barnum Effect explains the perceived accuracy of horoscopes, cold-reading, and low-validity personality "tests" — descriptions written to apply to most people will be accepted as uniquely true.
Defence: ask whether the description distinguishes you from others. A high-quality personality result should make claims that would NOT apply to everyone — and that you could meaningfully disagree with.
Source: Forer, B. R. (1949). The fallacy of personal validation: a classroom demonstration of gullibility. Journal of Abnormal and Social Psychology.