Defining the Barnum Effect and the Experimental Evidence
The Barnum effect, also known as the Forer effect, is the cognitive bias by which a person presented with a vague and general personality description comes to believe that it is an accurate portrait of them specifically. The phenomenon is widely known through a classic experiment carried out in 1948 by the psychologist Bertram Forer. After administering a personality test to his students, Forer handed every one of them the identical personality sketch, assembled from newspaper astrology columns, and presented it as "your individual result." On average the students rated the accuracy of that description as it applied to them at 4.26 out of 5.00. The effect is also called the P.T. Barnum effect, after the circus showman P.T. Barnum; no record confirms that he himself said the line usually attributed to him. The Barnum effect is one of the psychological mechanisms that allow unscientific personality classifications such as fortune-telling, blood-type personality typing, and the MBTI to be so widely believed.
The Conditions and Psychological Mechanisms That Produce the Effect
Several conditions are commonly cited as making the Barnum effect stronger. (1) The description contains positive content, since people readily accept flattering statements about themselves. (2) The recipient believes the description was "prepared individually for you," which lends it both authority and a sense of personal specificity. (3) The source of the description is perceived as credible, as in the case of an expert or a fortune-teller. (4) The description is sufficiently vague and open to multiple readings. As for the underlying mechanisms, several are thought to be involved: confirmation bias, whereby one selectively recalls the aspects of oneself that fit the description; subjective validation, whereby an ambiguous statement is interpreted so as to match one's own experience; and self-serving bias, whereby positive descriptions are accepted more readily than negative ones. Beyond that, the human appetite for self-understanding is strong, and critical thinking is easily blunted in any situation where we are being "told something" about ourselves.
Distinguishing Scientific Compatibility Assessment from the Barnum Effect
Understanding the Barnum effect is important in telling a scientific compatibility assessment apart from unscientific fortune-telling and personality typing. To avoid falling into the effect, a scientific assessment has to satisfy the following conditions. (1) It provides descriptions that are specific and testable, avoiding statements such as "you are sometimes sociable and sometimes withdrawn" that apply to absolutely everyone. (2) It makes individual differences explicit, describing a person's own tendencies rather than retreating into adjectives that fit anyone. (3) It says plainly that it rests on scientifically validated instruments. (4) It states its limitations openly, giving an honest account of what the assessment cannot predict. A compatibility assessment built on the Big Five starts from a different place than fortune-telling, because it rests on results measured with standardised instruments. Even so, however statistical the measurement itself is, the interpretive text that explains it can still be Barnum-ised. When that text is written, it is important to avoid phrasing that would fit anyone and to stay with what is specific to the person.