The Forer Effect

The Forer effect is the observation that individuals will give high accuracy ratings to descriptions of their personality that supposedly are tailored specifically for them, but are in fact vague and general enough to apply to a wide range of people. This effect can provide a partial explanation for the widespread acceptance of some beliefs and practices, such as astrology, fortune telling, and some types of personality tests.

In 1948, psychologist Bertram R. Forer gave a personality test to his students. Afterward, he told his students they were each receiving a unique personality analysis that was based on the test's results and to rate their analysis on a scale of 0 (very poor) to 5 (excellent) on how well it applied to themselves. In reality, each received the same analysis:

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.

On average, the rating was 4.26, but only after the ratings were turned in was it revealed that each student had received identical copies assembled by Forer from various horoscopes.

Sources
http://en.wikipedia.org/wiki/Forer_effect