What would happen if we used 100% of our brain?

You already do, across a normal day. The idea that switching on the 'other 90%' would give you a far higher IQ, perfect memory or special powers comes from fiction — films such as Lucy (2014) and Limitless (2011) — not from neuroscience.

Where did the 10% myth come from?

No scientist ever measured it. The best-documented trail runs through self-improvement writing: the psychologist William James wrote that people use only part of their mental energy, and in his 1936 foreword to Dale Carnegie's How to Win Friends and Influence People, Lowell Thomas turned that into a figure, writing that James 'used to say that the average man develops only ten percent of his latent mental ability'. A 1929 almanac had already printed a similar claim.

Did Albert Einstein say we only use 10% of our brain?

There is no documented source for that. The quote is one of many misattributions attached to the myth to make it sound authoritative.

Do people with a higher IQ use more of their brain?

Not in the way the myth suggests. In an early brain-scan study, people who scored higher on a hard reasoning test used less glucose in the cortex while solving it, and later research describes more able people as often using their brains more efficiently on easy-to-moderate tasks. Intelligence is about how well the brain works, not how much of it is switched on.

Do We Only Use 10% of Our Brain? Why It's a Myth

Q: Do we only use 10% of our brain?

No. Brain scans show activity in every region over the course of a day, damage to almost any part of the brain causes some loss of function, and the brain uses about 20% of the body's energy while making up about 2% of its weight. There is no unused 90% waiting to be switched on.

Q: How much of our brain do we actually use?

All of it, but not all at the same moment. Different regions do different jobs, and their activity rises and falls with what you are doing. Because each nerve impulse costs energy, only a small share of cortical neurons can be strongly active at any instant — one estimate puts it below 1%. That is how an efficient brain is built, not a sign that the rest is idle.

No — the idea that we only use 10% of our brain is a myth. Over the course of a day you use virtually all of it: every region has a job, damage to almost any part causes problems, and the brain burns about 20% of the body’s energy while making up about 2% of its weight. What is true is that only a small share of neurons fire strongly at the same instant — because every signal costs energy. That is how an efficient brain works, not a hidden reserve that could make you smarter if you switched it on.

How much of our brain do we actually use?

Three independent lines of evidence point the same way:

Brain scans. PET and functional MRI show activity throughout the brain. Which regions are busiest changes with the task — seeing, speaking, moving, remembering — but no region stays silent across a normal day, including during sleep and rest.
Energy cost. The human brain is about 2% of body weight but takes about 20% of the body’s metabolic energy, and per gram it is roughly ten times as expensive as muscle. Evolution does not keep an organ that costly with 90% of it unused.
Brain injury. Strokes, tumours and injuries in almost any location produce some loss — of movement, speech, vision, memory or personality. If 90% of the brain were spare, most damage would go unnoticed.

The grain of truth: not all neurons fire at once

The myth survives partly because there is a real, narrow fact underneath it. In 2003 the neuroscientist Peter Lennie estimated the energy cost of a single nerve impulse in the human cortex and compared it with the cortex’s known energy budget. His conclusion: spikes are so expensive that probably fewer than 1% of cortical neurons can be strongly active at the same time.

That is a statement about timing, not about unused parts. The brain represents information with very few active neurons at any moment and moves its energy between regions as the task demands. Across a day, different neurons take their turn. A brain in which every neuron fired at once would not be a genius — it would not be able to carry information at all.

Where the 10% figure came from

No experiment ever found that we use 10% of the brain. The number came from self-improvement writing, not from a laboratory:

William James. In his essay The Energies of Men, the Harvard psychologist argued that people have a “wider potential range” than their “habitually narrow actual use” — a point about effort and energy. He gave no percentage and said nothing about unused brain tissue.
1929. The World Almanac printed the claim that scientists and psychologists say we use only about ten percent of our brain power.
1936. In his foreword to Dale Carnegie’s How to Win Friends and Influence People, the broadcaster Lowell Thomas wrote that James “used to say that the average man develops only ten percent of his latent mental ability”. A best-selling book carried the number to millions of readers.
Einstein. The line is often credited to Albert Einstein. There is no documented source for it.

From there the figure moved into advertising, motivational courses and films. Limitless (2011) and Lucy (2014) are built on the premise that unlocking the rest of the brain produces superhuman intelligence. They are entertaining fiction.

How many people still believe it?

Surveys keep finding the belief, even among people who teach:

In a 2012 survey of 242 teachers interested in brain research, 48% in the UK and 46% in the Netherlands agreed that we only use 10% of our brain.
In a 2017 online survey in the United States, 36% of 3,045 members of the public and 33% of 598 educators endorsed it, against 14% of people with substantial neuroscience training.

Training reduces the belief but does not remove it — the same pattern seen with the myths about learning styles and left-brained and right-brained people.

Would using “more” of your brain raise your IQ?

The research on intelligence points the other way. In a 1988 PET study led by Richard Haier, people who scored higher on a demanding reasoning test (Raven’s Advanced Progressive Matrices) used less glucose in the cortex while solving it. The authors suggested that some differences in ability may reflect the efficiency of neural circuits.

A 2009 review by Aljoscha Neubauer and Andreas Fink found that this “neural efficiency” pattern shows up mainly on easy to moderately difficult tasks and mainly in frontal areas, so it is not a universal rule. Either way, intelligence is about how well the brain is organised and how flexibly it uses its resources — not about a dormant share that could be switched on.

If you want to know what can and cannot change, our guides to neuroplasticity and whether you can increase your IQ go through the evidence. The Mozart effect is another promise of an easy upgrade that did not survive testing.

Why the myth is so persistent

The 10% idea is flattering and hopeful: it suggests that everyone has untapped genius and that the right course, pill or trick can release it. It also sounds scientific, comes with a precise number and is attached to famous names. Those are exactly the features to check before believing any claim about the brain: who measured it, how, and where it was published.

Trust and scope notes

This page is general information about brain science, not medical advice. The IQ Revealed test is an online reasoning test of 40 original items for adults aged 18 and over; it is not a clinical assessment and it does not measure brain activity. Our guide to how IQ tests work explains what an online test can and cannot tell you.

IQ Revealed is not affiliated with, endorsed by or connected to Harvard University, the publishers of the World Almanac or of “How to Win Friends and Influence People”, the makers and distributors of the films “Lucy” and “Limitless”, the publisher of Raven’s Progressive Matrices, or the researchers and institutions named on this page. Names are used only to identify the works and research being described.

Do We Only Use 10% of Our Brain? The Myth, Explained