Back in 1948 the psychologist Bertram Forer asked his students to take a personality test.  He then gave each one a description of their personality type and asked them to score it from 1 to 5 for accuracy.  Why not try to score the following evaluation against your own personality?

You have a need for other people to like and admire you, and yet you tend to be critical of yourself. While you have some personality weaknesses you are generally able to compensate for them. You have considerable unused capacity that you have not turned to your advantage. Disciplined and self-controlled on the outside, you tend to be worrisome and insecure on the inside. At times you have serious doubts as to whether you have made the right decision or done the right thing. You prefer a certain amount of change and variety and become dissatisfied when hemmed in by restrictions and limitations. You also pride yourself as an independent thinker; and do not accept others’ statements without satisfactory proof. But you have found it unwise to be too frank in revealing yourself to others. At times you are extroverted, affable, and sociable, while at other times you are introverted, wary, and reserved. Some of your aspirations tend to be rather unrealistic.

The chances are that you’ve marked it somewhere between 4 and 5.  Forer’s students did and they all received exactly the same evaluation, made up from different newspaper horoscopes – exactly the same one that you have just read.  What Forer had just measured through his experiment was how far people will regard general comments as uniquely applicable to them, and how they will forget that these comments apply to many other people too.

Even today, with the test having been repeated hundreds of times, the average score for the description is still 4.2 out of 5.  The degree to which people accept it can be raised simply by marking it “for you”, and any negative traits are more readily accepted if delivered by an authority figure.  What though lies behind this willingness to accept vague “Barnum statements” such as “You are sometimes insecure, especially with people you don’t know very well” as being highly individual?

One major cause of the Forer effect seems to be confirmation bias.  This is our natural tendency to accept statements which support our self-image and our view of the world.   It is what makes us interpret ambiguous evidence as being supportive, and the effect is even greater where it affects strongly held beliefs or emotions.

As well as filtering out “unhelpful” information our brains also look for connections and patterns in the information it accepts.  Whilst this is crucial in many basic mental processes it can cause people to see relationships and cause and effect where they don’t exist.

A further major cause of the Forer effect appears to be people’s discomfort with uncertainty.  Where people need reassurance, either about themselves or their future, their increased appetite for advice can also prompt them to accept statements that they might otherwise treat with more rigour.  This is why so many people report divination, such as astrology or dream interpretation, or clairvoyance as being so accurate.  Partly it’s a desire for reassurance and partly the natural tendency to discount or forget aspects that don’t support that reassurance.

So next time someone claims that an online test or a psychic will reveal your personality, arrange to see their results as well as your own without knowing which is which.  If they’re accurate they should be as distinctive and precise, and have as little common ground, as possible.   In short, if you can’t tell whose is whose then it’s a very strong sign there’s not a lot of weight to it.