The principle can be described as: A spectator is told to write a word or question on a slip of paper or card. On this piece of paper are instructions about what information he is to write; e.g., phone number, address, birth date, a friend’s name, a color, a city he has visited. The spectator doesn't question or mention this, and the rest of the audience isn’t aware of these written instructions.
This principle was first explained by Burling Hull in How To Answer Questions for Crystal Gazing and Mind Reading Acts, 1921 (p. 36 in the 1927 third edition), under the title of “The Telephone Number Feature”. There the idea was used less expansively than it would later be, with a line for a phone number included on a minority of the question slips passed out to the audience.
In 1926, Paul Kara expanded the idea to include on the question cards lines for a street address and birth date, as well as a phone number. He called the idea “Karaism Effect #9: Mental Census” in his manuscript Karaism. This is reprinted in The Collected Mental Secrets of C. A. George Newmann, edited by Leo Behnke, 1990, p. 98.
A verbal approach using billets was employed by Dr. Edward Ervin and recorded in 1937 by Dariel Fitzkee in The Strange Inventions of Dr. Ervin, p. 29. Ervin whispered a suggestion that the spectator write more information than the audience was led to believe. Fitzkee writes that Ervin “invented [this technique] many years ago”.
Nineteen years after Kara published Karaism, James G. Thompson, Jr. used the idea in "Strongest Thought", marketed in 1945.
A more advanced version was devised in the early 1970s by John Pomeroy. In Pomeroy’s version, each slip secretly requests a different bit of information than the others, along with one piece of information openly asked from all the participants. This leaves not only the audience puzzled but the persons who filled out the slips, since their slip did not ask for the information being revealed to others. John Pomeroy’s method is explained in his book Mentology, 1973, p. 16. Further versions of the concept have been developed since.