Here is the ‘per­son­alised’ per­son­al­ity pro­file as used in a 1948 exper­i­ment by Bertram Forer:

You have a great need for other peo­ple to like and admire you. You have a ten­dency to be crit­i­cal of your­self. You have a great deal of unused capac­ity which you have not turned to your advan­tage. While you have some per­son­al­ity weak­nesses, you are gen­er­ally able to com­pen­sate for them. Your sex­ual adjust­ment has pre­sented prob­lems for you. Dis­ci­plined and self-controlled out­side, you tend to be wor­ri­some and inse­cure 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 pride your­self as an inde­pen­dent thinker and do not accept oth­ers’ state­ments with­out sat­is­fac­tory proof. You have found it unwise to be too frank in reveal­ing your­self to oth­ers. At times you are extro­verted, affa­ble, socia­ble, while at other times you are intro­verted, wary, reserved. Some of your aspi­ra­tions tend to be pretty unre­al­is­tic. Secu­rity is one of your major goals in life.

On a scale of 0 (very poor) to 5 (excel­lent), par­tic­i­pants in the study rated the accu­racy of the above state­ment as 4.26 (mean). Only after these rat­ings were pro­vided did Forer reveal to the par­tic­i­pants that all of them had been pro­vided with the exact same statement.

It was after this exper­i­ment that Forer famously described the per­sonal val­i­da­tion fal­lacy (or: the Forer effect)

In Tricks of the Mind (an excel­lent Christ­mas present for those inter­ested in such things, by the way), Der­ren Brown dis­cusses an updated ver­sion of this exper­i­ment that he con­ducted for his TV show of the same name. The fif­teen par­tic­i­pants in this exper­i­ment (from the U.K., U.S. and Spain) pro­vided per­sonal items to Brown (a traced out­line of their hand, the time and date of their birth, and a small, every-day ‘per­sonal object’), and in return were pro­vided with per­son­al­ity pro­files such as that above and were asked to mark its accu­racy out of 100.

Three par­tic­i­pants scored it poorly, between 40 and 50, while the remain­ing twelve rated the pro­file as highly accurate–one rat­ing it as 99% accu­rate, while another was so drawn in to the pro­file that she believed the TV crew had secretly read her diary. Two more felt so revealed by the state­ment that they refused to dis­cuss their pro­file on film.

Even though all par­tic­i­pants in Brown’s exper­i­ment expected to receive a series of “vague and ambigu­ous state­ments” that could apply widely, they all still fell foul of the per­sonal val­i­da­tion fallacy.

No mat­ter how much we know, we seem unable to account for our biases and beliefs.