/Is Your Brain Goofing Off?

Is Your Brain Goofing Off?

OK, concentrate: How much of your brain are you using right now—or at any moment in time?

If you said 10%, you’ve repeated a popular, but inaccurate, myth.

The origin of the tale is murky but might be rooted in an outdated theory about the structure of the brain that was repeated in a bestselling self-help book more than 80 years ago.

Today, the trope is still trotted out in cartoons, books and movies.

In the 2014 thriller “Lucy,” a scientist repeats the 10% claim while speculating about the promise of accessing a larger portion of the mind. The 2011 film “Limitless,” about a struggling writer, pegs the fraction at a slightly more encouraging 20%. And the 1991 comedy “Defending Your Life,” about a deceased man’s efforts to prove his worth in the afterlife, lowers it to a demoralizing 3% to 5%.

Sorry, Hollywood. Science doesn’t buy it.

Head Case

The myth that humans use only 10% of their brains persists even though scans reveal how the different areas function.

Frontal lobe

Thinking, problem solving, emotions, decision making

Parietal lobe

Perception, spelling, arithmetic

Temporal lobe

Memory, understanding, language

Occipital lobe

Vision, color identification

Brain stem

Regulates body temperature, heart rate, breathing

“It’s absurd,” said Daphna Shohamy, a neuroscientist at Columbia University’s Zuckerman Institute. “It’s as absurd as if I told you that you use only 10% of your body. Biology doesn’t work that way. Evolution doesn’t work that way.”

According to Dr. Shohamy and other neuroscientists, the entire brain is always at work, although perhaps at different levels of intensity.

So why does the idea persist?

“I think we find it plausible because we believe we have untapped potential,” Dr. Shohamy said.

And in some cases, the brain does appear capable of superhuman feats.

In a rare procedure known as a hemispherectomy, half the brain is removed, yet the individual remains able to function.

In 2007, French neurologists documented the case of a man leading a normal life despite having a tiny brain tucked into a skull largely filled with fluid. He was married, the father of two and worked as a civil servant.

In those extreme circumstances, the brain adapts, but it doesn’t tap into unused reserves.

“Our brain has a tremendous capacity to change and rewire itself, but it’s not that it’s sitting there and waiting for something to happen for it to do,” said Eric H. Chudler, a neuroscientist at the University of Washington who has written about the 10% myth in the book “Brain Bytes.”

The idea that a healthy brain functions at only 10% of its capacity might have grown out of an early misconception about brain cells, an influential psychologist’s reference to the theory, and a writer’s account that attached an irresistible number to it.

In the past, researchers believed neurons—the nerve cells responsible for transmitting information between different areas of the brain and the rest of the nervous system—were outnumbered by glia—the cells named after the Greek word for “glue” that support neurons.

“Old textbooks say that there are 10 glial cells for every nerve cell, making nerve cells 10% of the brain,” Dr. Chudler said.

Today, scientists believe the two kinds of cells are roughly equal in number, and researchers have documented the brain’s activity using scans, disproving that a significant portion of the organ is dormant.

“In a 5-year-old child, maybe 40% of energy is used to maintain the brain, to keep it going,” said Jon Kaas, a psychology professor at Vanderbilt University who studies brain architecture. “In an adult, maybe 20% of all energy is used to maintain the brain. It’s extremely costly.”

Particularly for an organ that weighs just 3 pounds.

Even when someone is resting or sleeping, the brain continues to work.

“Activity goes way down when a person is unconscious, but still those systems are working,” Dr. Kaas said. “Neurons are always active.”

The obsolete information contained in old textbooks might have penetrated pop culture after the general idea was disseminated by William James, known as the father of American psychology.

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At the turn of the last century, James wrote that people used only a small portion of their mental and physical resources. He never pinned the amount at 10%, Dr. Chudler said, but the figure was attributed to him by Lowell Thomas in his introduction to Dale Carnegie’s 1936 bestseller “How to Win Friends and Influence People.”

Thomas, at the time a radio broadcaster, wrote: “Professor William James of Harvard used to say that the average person develops only 10% of his latent mental ability,” while crediting Carnegie for teaching people how to unleash their full potential.

Today, nearly 33 million copies of the book have sold world-wide in print, ebook and audio formats, according to Simon & Schuster. Not all contain Thomas’s introduction, and whether it was responsible for cementing the myth isn’t clear.

But whatever the origin, scientists continue to dispel it.

When Dr. Shohamy teaches Frontiers of Science, a requirement for all Columbia freshmen, she asks whether it’s true that people use only 10% of their brains.

“Usually, it’s 50-50 or 30-70 who believe it,” she said. “But if they graduate from college, I want them to know we use our whole brain, all the time.”

Because, as scientists have proven, that’s a no-brainer.

Write to Jo Craven McGinty at Jo.McGinty@wsj.com

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