One of the most enduring myths about the brain is that we use only 10 percent of it. Presumably the other 90 percent just sits idly by, or perhaps it serves for spare parts—the originators of this myth never said why it was there or what it was for. By inference the myth suggests that people can harness and activate this unused potential in order to boost IQ or even develop “psychic” skills or other extraordinary abilities.
There are many false ideas about the brain circulating out there. For instance, most of what is said in popular culture about differences between the right and left brain hemispheres is inaccurate, but the idea that we use only a fraction of our brainpower is a doozy. It is patent nonsense. And yet this particular myth endures. Two-thirds of the public believe the 10 percent myth, and, according to a recent study in Frontiers in Psychology, even 47 percent of secondary school teachers take it as true! If schoolteachers don’t have their facts right then what hope is there of setting the public record straight? What accounts for the ten–percent myth’s persistence and durability despite ample proof of its falseness?
No one knows for sure, but I think this particular idea got started in the early days of the last century when neurology was just developing as a science. For a long time it was known that a motor area controls the opposite side of the body, and that a sensory area sitting on the strip of cerebral cortex just behind it mirrors the motor area. The primary brain areas for hearing and vision were also well known. But then there were the many parts of the brain that collectively we now know as association areas given that they perform high–level calculations important for perception, thought, and behavior. In other words they are the basis for a person’s “smarts.” 
Take the largest of the association areas, the two frontal lobes that account for a full third of brain tissue inside the skull. Because damage to this large expanse of brain produces no obvious motor or sensory symptoms, medical scientists from decades ago concluded that it served no obvious purpose. Together with the other regions whose functions weren’t apparent they became known as the “silent areas.” (This is an example of circular logic: because we can’t figure out what they are doing, they must not be doing anything.)
On the face of it the conclusion is absurd—how could such a significant percentage of brain tissue do nothing? But then even scientists are not immune to idiotic thinking. The history of science is peppered with bloopers. For example, as late as the 1950s serious and well-respected individuals claimed that the brain had nothing to do with cognition or overt behavior. That is, they claimed it dealt with physical functions, movement, and reflexes—and nothing more. Another idea we look back on in amazement deals with the corpus callosum, the enormous bundle of nerve fibers that carries cross-traffic between the right and left hemispheres. Simply on an anatomical basis the idea that it was inert should have been dismissed, because the number of nerve fibers in the callosum is far greater than all the fibers from the incoming senses combined. Yet because nothing appears outwardly wrong in people who had had these massive connections severed (in an early type of split-brain operation that sought to control seizures), neurosurgeons and others working in the 1950 and 1960s mistakenly concluded that the corpus callosum was without function. One eminent scientist suggested that it acted as a buttress and served a purely mechanical purpose: keeping the two hemispheres from collapsing into each other.
To be fair, encounters with split-brain patients are strongly counterintuitive. Logic says that you have done something drastic by cutting their brains in half. Yet judging by conversation, ordinary interactions, and even the standard neurological exam, they don’t seem different at all. The answer to this riddle is that patient’s weren’t tested the right way. Examiners weren’t looking for the right symptoms. When you restrict test input and patient response to one hemisphere at a time, then patients in fact exhibit profound symptoms.
How could our scientific ancestors have made such seemingly simple errors? Looking back we can see that their mistake was giving primacy to motor and sensory ability at the expenses of everything else. Measured this way those functions account for only a small percentage of brain tissue, and thus perhaps the 10 percent myth was born and has persisted ever since.
Astute readers may ask what prompted such narrow-minded thinking in the first place. Around the time of Freud in the last century a turn in thinking occurred that was strongly anti-biological and resisted associating the brain with higher thought. Unbelievable, but true. But that is a topic for a different blog post.