How to Win Friends and Influence People
A likely origin for the "ten percent myth" is the reserve energy theories of Harvard psychologists William James and Boris Sidis who, in the 1890s, tested the theory in the accelerated raising of child prodigy William Sidis. Thereafter, James told lecture audiences that people only meet a fraction of their full mental potential, which is considered a plausible claim. The concept gained currency by circulating within the self-help movement of the 1920s; for example, the book "Mind Myths: Exploring Popular Assumptions About the Mind and Brain" includes a chapter on the ten percent myth that shows a self-help advertisement from the 1929 "World Almanac" with the line "There is NO LIMIT to what the human brain can accomplish. Scientists and psychologists tell us we use only about TEN PERCENT of our brain power." This became a particular "pet idea" of science fiction writer and editor John W. Campbell, who wrote in a 1932 short story that "no man in all history ever used even half of the thinking part of his brain". In 1936, American writer and broadcaster Lowell Thomas popularized the idea—in a foreword to Dale Carnegie's "How to Win Friends and Influence People"—by including the falsely precise percentage: "Professor William James of Harvard used to say that the average man develops only ten percent of his latent mental ability".