10%

In arguing that current theories of brain function cast suspicion on ESP,
psychokinesis, reincarnation, and so on, I am frequently challenged with
the most popular of all neuro-mythologies -- the notion that we ordinarily
use only 10 percent of our brains...

This "cerebral spare tire" concept continues to nourish the clientele of
"pop psychologists" and their many recycling self-improvement schemes. As
a metaphor for the fact that few of us fully exploit our talents, who could
deny it? As a refuge for occultists seeking a neural basis of the miraculous,
it leaves much to be desired.

-- Barry L. Beyerstein, "The Brain and Consciousness: Implications for
Psi Phenomena", The Skeptical Enquirer, Vol. XII, No. 2, pg. 171