http://www.snopes.com/science/stats/10percent.asp

http://en.wikipedia.org/wiki/Human_brain

[Misconception]

"Humans use only 10% or less of their brain"

Even though many mysteries of brain function persist, every part of the brain has a known function.‏⁷‏⁸‏⁹

This misconception most likely arose from a misunderstanding (or misrepresentation in an advertisement) of neurological research in the late 1800s or early 1900s when researchers either discovered that only about 10% of the neurons in the brain are firing at any given time or announced that they had only mapped the functions of 10% of the brain up to that time (accounts differ on this point).

Another possible origin of the misconception is that only 10% of the cells in the brain are neurons; the rest are glial cells that, despite being involved in learning, do not function in the same way that neurons do.

Lower level of brain activation does not mean a lower performance of cognitive functions; this variable has confounded scientists, because some gifted individuals showed less activity than the average person. Haier proposed that indeed more gifted individuals might possess more efficient brain circuits.

Some New Age proponents propagate this belief by asserting that the "unused" ninety percent of the human brain is capable of exhibiting psychic powers and can be trained to perform psychokinesis and extra-sensory perception.