Evolutionarily, the idea that we use only a small portion of our brain is laughably fallacious. Brains are expensive to grow and maintain; it would make absolutely no sense for us to have such a large organ if it were not fully functional.
Here’s a whole bunch of sites that refer to the myth as debunked.
As to the origin, there’s this:
One possible origin is the reserve energy theories by Harvard psychologists William James and Boris Sidis in the 1890s who tested the theory in the accelerated raising of child prodigy William Sidis to effect an adulthood IQ of 250–300; thus, William James told audiences that people only meet a fraction of their full mental potential, which is a plausible claim.[5] In 1936, American writer Lowell Thomas summarized this idea (in a foreword to Dale Carnegie’s How to Win Friends and Influence People) by adding a falsely precise percentage: “Professor William James of Harvard used to say that the average man develops only ten percent of his latent mental ability.”[6] However, this book was not the first to use the 10% figure, which was already circulating within the self-help movement before then; for example, the book Mind Myths: Exploring Popular Assumptions About the Mind and Brain includes a chapter on the 10% myth which shows a self-help advertisement from the 1929 World Almanac containing the line “There is NO LIMIT to what the human brain can accomplish. Scientists and psychologists tell us we use only about TEN PER CENT of our brain power.”
In the 1970s, psychologist and educator Georgi Lozanov, proposed the teaching method of suggestopedia believing “that we might be using only five to ten percent of our mental capacity.”[8][9] The origin of the myth has also been attributed to Dr. Wilder Penfield, the U.S.-born neurosurgeon who was the first director of Montreal Neurological Institute of McGill University.
According to a related origin story, the 10% myth most likely arose from a misunderstanding (or misrepresentation) of neurological research in the late 19th century or early 20th century. For example, the functions of many brain regions (especially in the cerebral cortex) are complex enough that the effects of damage are subtle, leading early neurologists to wonder what these regions did.[11] The brain was also discovered to consist mostly of glial cells, which seemed to have very minor functions. Dr. James W. Kalat, author of the textbook Biological Psychology, points out that neuroscientists in the 1930s knew about the large number of “local” neurons in the brain. The misunderstanding of the function of local neurons may have led to the 10% myth.[12] The myth might have been propagated simply by a truncation of the idea that some use a small percentage of their brains at any given time.
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