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A myth. Every region does a job. There is no idle 90%.
Where it came from
Nobody knows exactly where the number came from. Psychologist William James wrote in 1907 that people use only a small part of their mental resources, meaning untapped potential, not unused tissue. A 1936 preface to Dale Carnegie’s How to Win Friends and Influence People turned this into a tidy “ten percent”, and self-help has repeated it ever since.
What’s actually true
Brain imaging shows activity in virtually every region over the course of a day, even during sleep. Damage to almost any area, however small, has some effect on how we move, sense, speak or think.
The brain is also expensive: about 2% of body weight burning around 20% of our energy. Evolution would not keep paying for nine-tenths of an organ that does nothing.
Further reading
- Barry Beyerstein, “Whence cometh the myth that we only use 10% of our brains?” in Mind Myths (1999)
- Wikipedia: Ten percent of the brain myth