The Barnum Effect in the Assessment of Personality Types

Dear Colleagues,

Several commonly used personality assessments categorize people into types.  The MBTI, for example, assesses which of 16 personality types best describes an individual, while the DiSC places test takers into one of four distinct categories. Even though studies by independent researchers have cast serious doubt on the scientific validity of personality measures that are based on types, the assessments remain popular.  Why is this?

The answer, quite simply, can be found in the scientific literature on the Barnum effect, a term which refers to a psychological phenomenon in which people view as valid general personality descriptions that supposedly pertain uniquely to them.  Interestingly, the name “Barnum effect” was coined as a tribute to P.T. Barnum, a famous circus owner who based his success on always having a little something for everyone.

The original study was conducted by Professor Bertram Forer, who administered the Diagnostic Interest Blank to 39 subjects.  When he gave the subjects their “individual” test results the following week, each subject actually received the same personality description consisting of 13 vague statements, many of which came from an astrology book.  For example:

You have a tendency to be critical of yourself.
While you have some personality weaknesses, you are generally able to compensate for them.
At times, you are extroverted, affable, sociable, while at other times you are introverted, wary, reserved.

Asked to rate how well the test results applied to them, the subjects, on average, rated the accuracy of the description as 4.3 on a scale of 0 (very poor) to 5 (excellent).

Many other studies have replicated the effect. Psychologist Ross Stagner, for example, asked a group of HR managers to take a personality test and then presented them with generalized feedback that had no relation to their actual responses but that instead included statements taken from horoscopes and handwriting analyses. The majority of the managers reported their “individual” personality description to be accurate, and almost none viewed the description as invalid.

Researchers have demonstrated the Barnum effect occurs when:

the statements describe personality traits frequently found in the general population
the analysis lists mostly positive traits
the wording allows the respondent to project his own interpretation onto the statements, and
the results are labelled as being specifically for the test taker.

The Barnum effect explains why the descriptions of personality types generated by assessments like the MBTI and the DiSC are viewed favorably by test takers.  Most of the statements in the MBTI and DiSC reports describe characteristics that are socially desirable and that have a high rate of occurrence in the general population.  The Supporter personality type in the DiSC, for example, is described as a patient listener who can work cooperatively with others, is willing to follow a trusted leader, and feels uncomfortable with aggressive people.  As summarized by Ronald Riggio, Ph.D., a Professor of Leadership and Organizational Psychology who has studied the MBTI: “When you read the basic descriptions, they’re all written in a positive way.  [Psychologists] call that the Barnum effect.  They all sound right, they’re all so positive and kind of generic, people say, ‘Oh, it totally applies to me.’”

Another group of studies has demonstrated that the Barnum effect influences perceptions not only of the personality descriptions but also of the assessment that produced those descriptions.  The more the subjects viewed the descriptions as accurate, the greater was the subjects’ belief in the validity of the test.  Thus, when the Barnum-like descriptions of the MBTI and DiSC personality types resonate with test takers, this produces a perception of the tests as valid.

In summary, the Barnum effect explains the continuing popularity of personality assessments that categorize people into types.  A positive perception of tests like the MBTI and the DiSC persists despite the firm conclusion of most independent researchers that these assessments are structurally flawed.  Unlike some practitioners, most scientists understand that personality traits do not divide into discrete types – for example, extroverts versus introverts – but rather are best measured along a continuum.  Although categorizing people into types is a simpler approach to describing personality, this approach is invalid because it fails to recognize the extraordinary individuality of human nature.

Maggi M. Reiss, President
IDS Publishing Corporation

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