The Debunker: Do We Use Only 10 Percent of Our Brains?

by Ken Jennings

Did you know that the second week of March is Brain Awareness Week around the globe? You didn’t? You weren’t aware of your brain? Conscious of your consciousness? Well, get with the program. March is perhaps the brainiest month of the year—it’s also when we celebrate the 1879 birthday of famous smarty-pants Albert Einstein, and the 1946 beginning of Mensa intelligence testing. But it turns out people will believe just about anything they hear about what’s going up between their ears. We’ve asked Ken Jennings to fact-check some particularly lame-brained misconceptions about gray matter.

The Debunker: Do We Use Only 10 Percent of Our Brains?

Harvard psychologist William James used to claim that people “use only a small part of our mental and physical resources.” This is hard to argue with: of course, humans are born with an abundance of time and talent and possibility and sadly, most of us spend a lot of it on dumb stuff like Facebook or fantasy football. But in 1936, Professor James’s soundbite suddenly went viral. Journalist Lowell Thomas misquoted James to say that “the average person develops only 10 percent of his latent mental ability”—and then added the now very scientific-sounding claim to his introduction to Dale Carnegie’s How to Win Friends and Influence People. The book, with Thomas’s information attached, became the biggest bestseller of its time.


As a result, this is now the received wisdom that has launched a thousand self-help books: we all have amazing, untapped potential in our noggins. Only laziness and bad technique are keeping us from Professor X-style superpowers! But today we have the MRI scans to test James’s platitude, and the science doesn’t back him up. As you go through your day—even if you’re just ordering a sandwich and not calculating the entropy of a black hole—almost all of your brain is active almost all of the time. You could be sound asleep and the neurons would still be firing on all cylinders. Even the most severe injuries, the kind that can knock out whole areas of the brain, don’t get you down anywhere near to Lowell Thomas’s 10 percent figure.

We also know that brain cells atrophy fairly quickly when they’re not used, and the fact that evolution gave you a giant brain and that 90 percent of it isn’t melting away right now are signs that (luckily!) those neurons are still in use. So there’s no need to feel guilty, like you could be accomplishing ten times more with your life if only you had read The Secret or something. There are certainly things we can all improve on mentally—for example, I have no idea where my car keys are at the moment—but that has nothing to do with brain anatomy.

Quick Quiz: Your brain may not be 90% useless, but it is composed of 73% what, by weight?

Ken Jennings is the author of Because I Said So!, Brainiac, Ken Jennings's Trivia Almanac, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at or on Twitter as @KenJennings.