“But We Only Use 10% of Our Brains!”

“It is estimated most human beings only use 10% of their brain’s capacity.”

That’s a line from Morgan Freeman’s character in the new Sci-Fi movie Lucy, coming out this summer. It’s a sentiment I’ve heard repeated by CEOs; it regularly pops up in commercials; and yes, it is a perpetual favorite of Hollywood (Heroes, Flight of the Navigator, The Lawnmower Man. Limitless).

But as a cognitive psychologist, there is no single piece of pop psychology that infuriates me more, and I cannot emphasize this enough:


The human brain weighs about 3 pounds; it consumes roughly 20% of the oxygen we breathe, 25% of the glucose we eat, and uses up about 15% of our cardiac output. In other words, lugging all that gray matter around is extremely biologically expensive. Evolution does not favor waste or excess (even those vestigial organs are proving to be not so vestigial), and it simply does not create an organ consuming that much of our resources for some prospective, undeveloped, potential purpose that is centuries or millennia away.

Think about it for a moment. If 90% of our brain is unused, then only 1 out of every 10 brain injuries should result in a disability. Unfortunately, that’s just not the case, as anyone who has ever watched a loved one suffer through a stroke or a traumatic injury knows. The effects are devastating. Yes, some miraculous recoveries happen, but we call them miraculous because they are so very, very rare. There is almost no area of the brain that can be damaged without resulting in impairment.

So how did this myth get started and why do people persistently choose to believe it?

How it got started is easy to understand. In the early days of neuroscience, imaging tools didn’t have the resolution that they have today, so we weren’t able to detect subtle activity throughout the brain. Additionally, think about what a person is typically doing when undergoing a CT or MRI scan. Are they running a marathon? Composing a symphony or writing a novel? Learning to knit or type or speak a foreign language? Of course not. They’re lying on an exam table, after being told not to move. In other words, brain activity is a bit on the quieter side. But thanks to newer technology, we now know that — barring damage — there is no part of our brain that isn’t active and isn’t active always. Even when we sleep, our brains show activity. Reduced, perhaps, but still there.

As for why this myth is perpetuated… I believe it all comes down to something so well exemplified by the movies perpetuating this myth: at some point in our lives, we all want to be better, smarter, faster, or to have more control over events around us. Maybe we even want to read minds or to move things just by thinking about it, or know instantly what’s happening in some faraway place. Wouldn’t it be nice to flip a switch and have that all be possible?

But there is no switch. Your brain is what it is. You’re already using it 100%. So make the most of it.

* Featured image: Brittany.lee2, CC BY-SA 4.0, via Wikimedia Commons