Barry L. Beyerstein of the Brain Behavior Laboratory at Simon Fraser University in Vancouver explains.

Whenever I venture out of the Ivory Tower to deliver public lectures about the brain, by far the most likely question I can expect as the talk winds up is, "Do we really only use 10 percent of our brains?" The look of disappointment that usually follows when I say it isn't so strongly suggests that the 10-percent myth is one of those hopeful shibboleths that refuses to die simply because it would be so darn nice if it were true. I'm sure none of us would turn down a mighty hike in brainpower if it were attainable, and a seemingly never-ending stream of crackpot schemes and devices continues to be advanced by hucksters who trade on the myth. Always on the lookout for a "feel-good" story, the media have also played their part in keeping the myth alive. A study of self-improvement products by a panel of the prestigious National Research Council, Enhancing Human Performance, surveyed an assortment of the less far-fetched offerings of the "brain booster" genre and came to the conclusion that (alas!) there is no reliable substitute for practice and hard work when it comes to getting ahead in life. This unwelcome news has done little, however, to dissuade millions who are comforted by the prospect that the shortcut to their unfulfilled dreams lies in the fact that they just haven't quite found the secret to tap this vast, allegedly unused cerebral reservoir.

Why would a neuroscientist immediately doubt that 90 percent of the average brain lies perpetually fallow? First of all, it is obvious that the brain, like all our other organs, has been shaped by natural selection. Brain tissue is metabolically expensive both to grow and to run, and it strains credulity to think that evolution would have permitted squandering of resources on a scale necessary to build and maintain such a massively underutilized organ. Moreover, doubts are fueled by ample evidence from clinical neurology. Losing far less than 90 percent of the brain to accident or disease has catastrophic consequences. What is more, observing the effects of head injury reveals that there does not seem to be any area of the brain that can be destroyed by strokes, head trauma, or other manner, without leaving the patient with some kind of functional deficit. Likewise, electrical stimulation of points in the brain during neurosurgery has failed so far to uncover any dormant areas where no percept, emotion or movement is elicited by applying these tiny currents (this can be done with conscious patients under local anesthetic because the brain itself has no pain receptors).

The past hundred years has seen the advent of increasingly sophisticated technologies for listening in on the functional traffic of the brain. The goal of behavioral neuroscience has been to record electrical, chemical and magnetic changes in brain activity and to correlate them with specific mental and behavioral phenomena. With the aid of instruments such as EEGs, magnetoencephalographs, PET scanners and functional MRI machines, researchers have succeeded in localizing a vast number of psychological functions to specific centers and systems in the brain. With nonhuman animals, and occasionally with human patients undergoing neurological treatment, recording probes can even be inserted into the brain itself. Despite this detailed reconnaissance, no quiet areas awaiting new assignments have emerged.

All told, the foregoing suggests that there is no cerebral spare tire waiting to be mounted in service of one's grade point average, job advancement, or the pursuit of a cure for cancer or the Great American Novel. So, if the 10-percent myth is that implausible, how did it arise? My attempts to track down the origins of the 10-percent myth have not discovered any smoking guns, but some tantalizing clues have emerged (more are recounted in the references below). One stream leads back to the pioneering American psychologist, William James, in the late 19th and early 20th centuries. In addition to his voluminous scholarly work, James was a prodigious author of popular articles offering advice to the general public. In these exhortatory works James was fond of stating that the average person rarely achieves but a small portion of his or her potential. I was never able to find an exact percentage mentioned, and James always talked in terms of one's undeveloped potential, apparently never relating this to a specific amount of gray matter engaged. A generation of "positive thinking" gurus that followed were not so careful, however, and gradually "10 percent of our capacity" morphed into "10 percent of our brain." Undoubtedly, the biggest boost for the self-help entrepreneurs came when the famous adventurer and journalist Lowell Thomas attributed the 10-percent-of-the-brain claim to William James. Thomas did so in the preface he wrote, in 1936, to one of the best-selling self-help books of all time, Dale Carnegie's How to Win Friends and Influence People. The myth has never lost its steam since.