Forer effect 

Psychological effect noted by psychologist Bertram R. Forer in 1948, under which any fairly large and diverse group of people may come to believe that a tenuously balanced personality reading (especially one which, for the purpose of the test, is copied and handed out for them to ponder individually) applies strongly to each of them as individuals. Later studies have shown that the effect tends to be strongest when the recipient is given to understand that the reading has supposedly been tailored to them as an individual, that the person making the propositions is an authority they feel they can trust, and/or that most or all of the personality traits listed are positive ones, in other words flattering. The Forer effect may be of import in understanding the popularity of such pseudosciences as fortune telling or astrology.

Personality reading used to illuminate the Forer effect (thanks to Wikipedia):

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.

Now ... how does this apply to YOU?

Forer effect by Fearman March 11, 2008