Like most legends, the origin of this fiction is unclear, though there are some clues. According to Sam Wang, a neuroscientist at Princeton and the author of Welcome to Your Brain, the catalyst may have been the self-help industry. In the early 1900s, William James, one of the most influential thinkers in modern psychology, famously said that humans have unused mental potential. This completely reasonable assertion was later revived, in mangled form, by the writer Lowell Thomas in his foreword to the 1936 self-help bible How To Win Friends And Influence People. “Professor William James of Harvard used to say that the average person develops only 10 percent of his latent mental ability,” Thomas wrote. It appears that he, or perhaps someone else in his day, simply plucked the golden number out of the sky.

The 10 percent claim is demonstrably false on a number of levels. First, the entire brain is active all the time. The brain is an organ. Its living neurons, and the cells that support them, are always doing something. (Where’s the “you only use 10 percent of your spleen” myth?) Joe LeDoux, a professor of neuroscience and psychology at NYU, thinks that people today may be thrown off by the “blobs”—the dispersed markers of high brain activity—seen in functional magnetic resonance imaging (fMRI) of the human brain. These blobs are often what people are talking about when they refer to the brain “lighting up.”

Say you’re watching a movie in an fMRI scanner. Certain areas of your brain—the auditory and visual cortices, for instance—will be significantly more active than others; and that activity will show up as colored splotches when the fMRI images are later analyzed. These blobs of significant activity usually cover small portions of the brain image, often less than 10 percent, which could make it seem, to the casual observer, that the rest of the brain is idling. But, as LeDoux put it to me in an email, “the brain could be one hundred percent active during a task with only a small percentage of brain activity unique to the task.” This kind of imaging highlights big differences in regional brain activity, not everything the brain is doing.

In fact, the entire premise of only “using” a certain proportion of your brain is misguided. When your brain works on a problem—turning light that hits your retina into an image, or preparing to reach for a pint of beer, or solving an algebra problem—its effectiveness is as much a question of “where” and “when” as it is of “how much.” Certain regions of the brain are more specialized than others to deal with certain tasks, and most behavior depends on tight temporal coordination between those regions. Your visual system helps you locate that pint of beer, and your motor system gets your hand around it. The idea that swaths of the brain are stagnant pudding while one section does all the work is silly. The brain is a complex, constantly multi-tasking network of tissue.