kate asked:

What is relevant data that supports the inferences about “Do we use 10 per cent of our brain?”

Answer by Massimo Pigliucci

The claim is simply false, amounting to no more than an urban legend. In reality, it is pretty clear that we use the entirety of our brain, though not necessarily all at the same time. There are several empirical lines of evidence that refute the 10% idea. To begin with, it would be very strange if natural selection had created an organ that consumes a whopping 20% of our metabolism (while accounting for only 2% of our body mass), and 90% of it went unused. Such waste should have massive consequences on an individual’s viability and reproductive competitiveness, and would have therefore been quickly eliminated during the early stages of human evolution.

If you don’t find a priori arguments too convincing, then consider that when brain cells become inactive they degenerate, and even superficial scans or living brains (or autopsies of dead ones) clearly show that not to be the case. Speaking of brain scans, nowadays we can do fMRI and similar investigations while subjects are actually using their brains on a variety of tasks, and such techniques clearly show large portions of the human brain to be active at any given time. Moreover, scientists have studied a number of people with extensive brain injuries, and invariably these people suffer from reduced cognitive functionality. If 90% of their brains were normally untapped one would expect to see no impairment following even extensive brain damage. Barry Gordon, a neurologist at the Johns Hopkins School of Medicine in Baltimore, interviewed by Scientific American, said that the very idea that we use only 10% of our brains is “so wrong it is almost laughable.”

Where, then, does this idea come from, and why does it persist? Different scenarios have been proposed for the origin of the 10% myth, and they may all be correct, since it is possible that the idea has popped up independently a number of times. One of the best documented stories traces it back to philosopher and pioneer psychologist William James, who carried out research on the unrealized potential of human IQ back in the 1890s. James concluded – on the basis of his study of a child prodigy – that ordinarily we may achieve a fraction of our mental potential, which may be a defensible claim as far as it goes. However, in 1936 the American writer Lowell Thomas referred to this idea in a foreword he wrote for How to Win Friends and Influence People, by Dale Carnegie, falsely stating that “Professor William James of Harvard used to say that the average man develops only ten per cent of his latent mental ability.”

Another plausible root for the origin – or perhaps persistence – of the myth is the well known fact (since the early part of the 20th century) that only about 10% of brain cells are actually neurons, the remainder being made of glial cells, which play crucial supportive and protective roles for the neurons themselves.

Ezequiel Morsella in Psychology Today describes yet another possible origin story for the 10% myth, this one tracing it to more recent research conducted by American neurosurgeon Wilder Penfield. Penfield developed techniques to treat severe forms of epilepsy and was interested in exploring the possible damage that a surgical mistake may cause in the patient. He therefore embarked in research that involved the localized stimulation of areas of the brain with electrodes, while the subject was awake and could report the effects. Penfield and his colleagues discovered that stimulating a small number of areas of the brain (accounting for about 10% of the total) was sufficient to generate detectable effects. Needless to say, this is not at all the same as saying that we only use that percentage of our grey matter.

The 10% myth is popular both among New Age believers and paranormalists, as well as in the entertainment industry. The suggestion has been made that paranormal phenomena, like telepathy, clairvoyance and telekinesis (for which there is no convincing evidence) become possible when one somehow “taps” into the normally (allegedly) non-functioning 90% of one’s brain (see this article by Ben Radford in Skeptical Inquirer). New Age believers explain the (again, alleged) existence of psychic powers in the same fashion.

A number of movies and television shows have been loosely based on the premise that the 10% myth is true, including the pilot episode of Heroes and the movies Defending Your Life (1991), The Lawnmower Man (1992), Limitless (2011), and Lucy (2014). The 10% use is not the only widely accepted but incorrect notion about the human brain, this article by Laura Helmuth in Smithsonian magazine lists another nine.