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Common Myths #1: We only use 10% of our brains

10%?

"Humans only use 10% of their brains." Who said it first, and did they really say that?

The claimwe use only 10% of the brain, the other 90% lies "dormant"
Who spread itLowell Thomas, preface to Dale Carnegie's book (1936)
The factsevery brain region has a job; the brain uses about 20% of the body's energy
Main sourcesWilliam James (1907), Beyerstein (1999), Raichle and Gusnard (PNAS 2002)
One-minute recap

Essence

James never said 10%In 1907 he wrote that we use "only a small part" of our resources. No number at all.
The number came laterA 1936 preface pinned "10%" on James. An ad turned "ability" into "brain-power".
2% of weight, 20% of energyEvolution is unlikely to keep such an expensive organ and leave 90% of it idle.
Sparse, not spareOnly a small share of neurons fire hard at any moment. They take turns; the whole brain is used.
Lesson: go to the sourceA sentence passed through three people has already changed meaning. Find the original before you quote.

Sources: W. James, "The Energies of Men" (1907); Lowell Thomas's preface to Dale Carnegie's book (1936), per Beyerstein (Scientific American); Raichle and Gusnard, PNAS 2002 (2% and 20%); Lennie, Current Biology 2003 (sparse neural activity).

Part 1

The question

A claim nobody ever proved, rejected by scientists themselves, has survived for nearly 100 years. Why? Because it comes with a famous name, a round number and a pleasant promise: you still have 90% left to use.

1907The original ideaJames: we use "only a small part" of our mental and physical resources.
→
1936A number is addedPreface to Carnegie's book: "the average man develops only ten per cent of his latent mental ability".
→
TodayThe subject changes"Ability" becomes "brain". A point about motivation becomes an "anatomical fact".

Sources: James (1907), Philosophical Review 16(1); Lowell Thomas's preface (1936) per Beyerstein, Scientific American, and The Psychologist (BPS).

Part 2

Where the myth came from

The sentence changed shape with every retelling. Each milestone below comes with its source.

Timeline of the sentence. 1907: James writes we use a small part of our resources, with no number. 1936: Lowell Thomas's preface adds 10 per cent of latent ability and credits James. 1944: a Pelman ad says science tells you that you use one tenth of your brain-power. From then on: we only use 10 per cent of our brains. 2002: about 59 per cent of college-educated people in Rio de Janeiro believe it. 2014: the film Lucy builds its plot around unlocking 100 per cent. 1907 "a small part ofour resources"James · no number 1936 "10% of latentmental ability"Lowell Thomas preface 1944 "1/10 brain-power","Science says"Pelman ad "10% of the brain" ability becomes brain 2002 ~59% of graduatesin Rio believe itHerculano-Houzel survey 2014 film Lucy:"unlock 100%" each step changes only a little: add a number, add "science says", swap "ability" for "brain"
Nobody invented the whole sentence at once. Each retelling added one detail that made it easier to remember and more "scientific" sounding.Sources: James (1907); Lowell Thomas's preface (1936) and Beyerstein's analysis (1999, Mind Myths; Scientific American); the 1944 Pelman ad per C. Wanjek, Bad Medicine (2002), via The Psychologist (secondary source); Herculano-Houzel survey, The Neuroscientist (2002), via The Psychologist (secondary source); the film Lucy (2014).
1890s · "Reserve energy"William James and Boris Sidis argued that people have reserves of mental energy, and tested the idea by educating the child prodigy William Sidis early. (Secondary source.)
1907 · "The Energies of Men"James: "We are making use of only a small part of our possible mental and physical resources." He meant motivation and habit, not a percentage of brain tissue.
1936 · Preface "A Short-Cut to Distinction"Journalist Lowell Thomas, in How to Win Friends and Influence People: "Professor William James of Harvard used to say that the average man develops only ten per cent of his latent mental ability." No source given.
1944 · A self-improvement course adThe Pelman Institute in London: "as Science says, you are only using one-tenth of your real brain-power!" (Secondary source.)
1970s onward · Cell countsTextbooks repeat "100 billion neurons and ten times as many glial cells". That is easy to misread as "neurons are only a tenth of the brain". The figure was never measured.
Later · Credited to Einstein"Einstein said we only use 10% of our brains" spread widely. Beyerstein searched the Albert Einstein Archives and found nothing.
2014 · The film LucyIts whole plot rests on a woman gradually "unlocking" 100% of her brain.

Sources: James (1907), Philosophical Review 16(1) and Science 25(635), p. 323; Lowell Thomas's preface (1936) per Beyerstein, Scientific American, and Chudler, Neuroscience for Kids; Pelman 1944 per Wanjek (2002) via The Psychologist; Sidis per myth round-ups (secondary source); Azevedo et al. (2009) on the unmeasured "100 billion, ten times as many" figure; Beyerstein (1999) on Einstein.

Part 1 of the series

Where the myth came from

James 19071936 preface1944 adEinstein?
Part 2

What the evidence says

2% and 20%86 billion neuronssparse, not sparefact check
Part 3

Work lesson

5-minute source checkred flagsMark's score
Part 3

What the evidence says

The brain is the most "expensive" organ

energy budgetRaichle and Gusnard, PNAS 2002
The brain is about 2 per cent of body weight but uses about 20 per cent of the body's oxygen, and so its energy. body weight brain ~2% energy (oxygen) used brain ~20% about ten times its "share", and the rate barely changes whether we think hard or not
Evolution does not keep costly things that do nothing. An organ that is about 2% of the body yet burns about 20% of its energy is unlikely to leave 90% of itself idle.Source: Raichle and Gusnard, "Appraising the brain's energy budget", PNAS 99(16):10237–10239, 2002 (about 2% of weight, about 20% of oxygen and calories, a "remarkably constant" metabolic rate).

Counting the cells again: no "90% empty space"

neurons and gliaAzevedo et al., 2009
The old textbook figure: 100 billion neurons and 1 trillion glial cells, a ratio of 1 to 10. The actual 2009 count: about 86 billion neurons and about 85 billion other cells, close to 1 to 1. old figure, never measured 100 bn neurons 1 trillion glia actual count (2009) ~86 bn neurons ~85 bn other cells close to 1:1, and glia have jobs too: feeding, cleaning and protecting neurons
"Neurons are only a tenth of the brain" is wrong too. Suzana Herculano-Houzel's group counted an average of 86.1 billion neurons and 84.6 billion non-neuronal cells. They note that the "100 billion, ten times as many glia" figure had never been measured.Sources: Azevedo et al., J Comp Neurol 513(5):532–541, 2009; von Bartheld, Bahney and Herculano-Houzel, J Comp Neurol 524, 2016 (review: a ratio of about 1:1).

Sparse, not spare

sparse codingLennie, Current Biology 2003
Three moments, only a few cells lit at each, a different group each time. Over a whole day, every cell has lit up. moment 1+moment 2+moment 3=whole dayonly a few cells light up at any moment, a different group each time: add them up and every cell has work
This is the myth's grain of truth. Every nerve impulse costs energy, so only a small share of neurons fire hard at any moment; Lennie estimates the ceiling may be under 1%. But that is how the brain saves energy, with groups taking turns, not 90% left idle.Source: Lennie, "The cost of cortical computation", Current Biology 13:493–497, 2003 (an estimated ceiling, not a direct measurement).

No "spare" regions

no silent reserveBeyerstein, 1999
Four kinds of evidence against the myth: brain imaging finds no permanently dormant region; damage almost anywhere leaves a deficit; evolution does not keep a costly organ to waste it; electrode recordings of single neurons find no dead areas. 1 · brain imagingPET, fMRI:no region stays"asleep" 2 · brain damagedamage almostanywhere costssome function 3 · evolutiona costly organis not keptto be wasted 4 · electrodessingle neurons:no "dead"areas found if 90% of the brain were unused, damage to that 90% would do no harm; in reality it does
Four independent lines of evidence, one conclusion. Psychologist Barry Beyerstein gathered these arguments while tracing the myth.Sources: Beyerstein, "Whence cometh the myth that we only use ten percent of our brains?", in Della Sala (ed.), Mind Myths, Wiley, 1999; his summary in Scientific American (secondary source for the chapter's content).

Plenty of people still believe it

how commonsurveys from 2002 and 2024
About 59 per cent of college-educated people in Rio de Janeiro in 2002 believed the myth. About 6 per cent of neuroscientists agreed too. In 2024, 46.7 per cent of 6,679 Italian teachers believed it. college-educated, Rio, 2002 ~59% Italian teachers, 2024 (6,679) 46.7% neuroscientists, 2002 ~6% 0%100%
Education is no automatic vaccine. Two surveys in two countries, 22 years apart, with different samples, so they cannot be compared directly.Sources: Herculano-Houzel, The Neuroscientist (2002), via The Psychologist, BPS (secondary source: about 59% and about 6%); Basta et al. (2024), survey of 6,679 Italian teachers, per the abstract (46.7%).
Part 4

Fact check: Solid, Caution, False

11 things people say about "10% of the brain", checked against the original sources. Tap each line to see why.

SOLIDThe brain is about 2% of body weight but uses about 20% of its energy

Raichle and Gusnard (PNAS 2002). This rate barely changes whether we are doing maths or resting.

SOLIDWilliam James never gave a 10% figure

"The Energies of Men" (1907) says only "a small part of our resources". Beyerstein found no percentage anywhere in James's writing.

SOLIDThe 10% figure was pinned on James in a 1936 preface

Lowell Thomas wrote it in the preface to Dale Carnegie's book, gave no source, and still spoke of "latent ability", not the "brain" (Beyerstein; The Psychologist).

SOLIDNo brain region lies "dormant" waiting to be woken

Imaging, lesion studies and electrode recordings all show that every region has work to do (Beyerstein 1999).

CAUTION"People don't reach their full potential"

This was James's actual point, and it is reasonable: practice, motivation and sleep all change results. But potential cannot be measured in percentages, and it does not sit in an "unused" part of the brain.

CAUTION"Only a small share of neurons are active at a time"

True under sparse coding: Lennie (2003) estimates the ceiling may be under 1% of neurons firing hard at once. But every group gets its turn. Think of a building that never has every light on at once, not one with 90% of its rooms abandoned.

CAUTION"A 1944 ad is where the sentence moved to the brain"

The Pelman Institute ad is cited by Christopher Wanjek in Bad Medicine (2002). Mark has only seen it through a secondary source, not the original newspaper. The sentence probably mutated in several places at once.

FALSE"Humans only use 10% of their brains"

No study has ever produced this figure. All the evidence in Part 3 points the other way.

FALSE"Einstein said so"

Beyerstein searched the Albert Einstein Archives and found no such quote. Pinning a line on a famous person is the fastest way to make it spread.

FALSE"Neurons are only 10% of brain cells, the rest are useless"

The 2009 count: about 86 billion neurons and 85 billion other cells, close to 1:1 (Azevedo et al.). Glia also feed, clean and protect neurons. Careful: don't correct this myth with another "1:10" figure.

FALSE"Unlock 100% of your brain and get superpowers" (Lucy)

There is no locked part to open. The way to get better is still learning, practice and enough sleep, not "activating" the rest of the brain.

Sources: Raichle and Gusnard (2002); James (1907); Beyerstein (1999; Scientific American); The Psychologist (BPS); Lennie (2003); Wanjek (2002) via The Psychologist (secondary); Azevedo et al. (2009).

Part 5

Verdict on the myth

Scores are Mark's assessment based on the sources in Part 8, not data from a study.

How wrong
5/5
How persistent
5/5
How harmful
2/5
Grain of truth
1/5
Why it is easy to believe
  • Attached to big names: James, Einstein
  • A round, memorable number
  • A pleasant promise: 90% still unused
  • A grain of truth: sparse neural firing
Where it does harm
  • Sells courses and "brain-activating" pills
  • Hides the real ways to improve: learn, practise, sleep
  • Teaches people to trust a line with no source

Almost entirely wrong about the brain, but a perfect lesson in how a sentence mutates when nobody goes back to the original.

More: why Mark gives "harm" only 2/5

Believing you use only 10% of your brain rarely hurts anyone. The harm is elsewhere: it is bait for products promising to unlock the rest, and it gets people used to quoting a "fact" without knowing who said it.

Beyerstein called it one of those hopeful shibboleths that refuses to die. Precisely because it looks harmless, few people bother to check it.

Part 6

Work lesson: trace the source before you quote

Use it whenever you are about to put a number or a quote into a slide, report, post or email to your boss.

Four steps for a five-minute source check: find the original wording; check who said it, when and about what; compare the original number with the one being quoted; cite the source or drop it. 1 · find the originalwhere is the source?don't stop at a repost 2 · who, when, about what"ability" or "brain"?read the lines around it 3 · compare the numberdoes the source say 10%?or was it added later 4 · cite it or drop itname, year, pagesay when it's secondary "Everybody knows" is not a source.
Each step blocks one of the ways the "10%" line mutated. A number added (1936), "science says" added (1944), "ability" swapped for "brain", and credit given to Einstein.
Four red flagsA famous name with no source; "science says"; a round number; "everybody knows". Any one of them means: trace it.
Two steps backFrom what you read, go at least two steps back to where the number first appeared. By the second step the sentence has usually changed.
One source line per numberOn a slide: "Source: name, year". If you can't find it in five minutes, drop the number or label it "source unclear".
Don't fix one error with anotherWhen you debunk a myth, check the numbers you debunk it with. "Ten times as many glia" was never measured either.
We are making use of only a small part of our possible mental and physical resources.William James, "The Energies of Men" (1907). The real quote, with no number in it.
Part 7

Further reading

Barry Beyerstein, "Whence cometh the myth that we only use ten percent of our brains?", in Mind Myths (1999)The most thorough tracing of the myth, by a psychologist who specialised in debunking.
William James, "The Energies of Men" (1907)Read the original to see what he actually said: about motivation, "second wind" and habit.
Suzana Herculano-Houzel, The Human Advantage (2016)The story of recounting the neurons in the human brain, and why the old figure was never measured.
Common Myths #02: "Left-brained and right-brained people"From the Nobel-winning split-brain experiments to the quiz that tells you "which brain type you are".
Part 8

Sources

Primary documents first. Where only a secondary source was available, it is labelled right there.

  1. W. James, "The Energies of Men", Philosophical Review 16(1), January 1907; also in Science N.S. 25(635):321–332. Classics in the History of Psychology
  2. L. Thomas, preface "A Short-Cut to Distinction", in D. Carnegie, How to Win Friends and Influence People, 1936. Wording checked via Beyerstein and Chudler (secondary sources).
  3. B. L. Beyerstein, "Whence cometh the myth that we only use ten percent of our brains?", in S. Della Sala (ed.), Mind Myths, Wiley, 1999; and "Do we really use only 10 percent of our brains?", Scientific American. scientificamerican.com
  4. The Psychologist (BPS), "Great myths of the brain: We only use 10 per cent" (citing Wanjek 2002 on the 1944 Pelman ad, and Herculano-Houzel 2002). thepsychologist.bps.org.uk
  5. E. Chudler, "10% of the Brain Myth", Neuroscience for Kids, University of Washington. faculty.washington.edu
  6. M. E. Raichle and D. A. Gusnard, "Appraising the brain's energy budget", PNAS 99(16):10237–10239, 2002. PMC124895
  7. F. A. C. Azevedo et al., "Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain", J Comp Neurol 513(5):532–541, 2009. PubMed
  8. C. S. von Bartheld, J. Bahney and S. Herculano-Houzel, "The search for true numbers of neurons and glial cells in the human brain", J Comp Neurol 524, 2016. PMC5063692
  9. P. Lennie, "The cost of cortical computation", Current Biology 13:493–497, 2003.
  10. S. Herculano-Houzel, survey of public neuroscience literacy, The Neuroscientist, 2002 (figures via The Psychologist, secondary source).
  11. Basta et al., survey of brain myths among 6,679 Italian teachers, 2024 (per the abstract). iris.uniecampus.it