In part because the scientific portrait of the brain remains so patchy, it has long been embellished with numerous myths and misconceptions. For example, there’s no truth to the idea that the brain is half android and half artist, with a left hemisphere dedicated to logic and analytical thinking and a right hemisphere for intuition and creativity. You don’t have a primitive reptilian brain tucked inside your more sophisticated mammalian tissues. You can’t increase brainpower by eating nuts, blueberries, fish and other so-called brain foods. Entire books have been written to counter such falsehoods.

Misinformation about the brain is not isolated to the general public; it is surprisingly prevalent in academia too. By the time Herculano-Houzel was old enough to pursue graduate studies in science, she had long been inoculated with a strong dose of skepticism. When she was growing up in Brazil, her parents emphasized that “it was a good thing to not take somebody’s word, no matter how respected they were,” she recalls, “and rather ask: ‘Why? How do you know that?’ ” It was not until she earned a Ph.D. in neuroscience in Europe and returned to Rio de Janeiro in 1999, however, that she confronted neuromythology head on.

Instead of pursuing postdoctoral studies — which she thought would be too intellectually restricting — she persuaded the city’s recently opened Museum of Life to offer her a job giving presentations on the brain to the public. One of her first projects was a survey regarding general beliefs about the brain: E.g., did consciousness depend on the brain? Did drugs physically alter the brain? She was shocked to learn that 60 percent of college-educated people in Rio de Janeiro believed that humans used only 10 percent of their brains — a longstanding fallacy. In truth, the brain is highly active across its entirety just about all the time, even when we are spacing out or sleeping. She couldn’t let it go. Where did such a prevalent falsehood come from? How did it spread?

She started looking for clues in research papers and popular science writing. In the foreword to the first edition of Dale Carnegie’s “How to Win Friends and Influence People,” the American psychologist William James is misquoted as declaring that “the average man develops only 10 percent of his latent mental ability.” In the ’30s and ’40s, another pioneering psychologist, Karl Lashley, discovered that he could scoop out large portions of a rat’s brain without seriously impairing its ability to solve a maze. Herculano-Houzel also recalled that early editions of the textbook “Principles of Neural Science,” along with countless studies, claimed that the human brain contained at least 10 times as many glial cells as neurons. Glia are now known to be every bit as important as neurons, facilitating electrical and chemical communication, clearing cellular detritus, protecting and healing injured brain cells and guiding the development of new neural circuits. But until the mid- to late 20th century, scientists mostly regarded glia as passive scaffolding for neurons. Perhaps the widely cited fact that glia outnumbered neurons by at least 10 to one helped cement the notion that only 10 percent of the brain really mattered. But where were the studies establishing the oft-repeated glia-to-neuron ratio?

After an exhaustive search, Herculano-Houzel concluded that there was no scientific basis for the claim. She and her collaborator Christopher von Bartheld, a professor at the University of Nevada School of Medicine, published a paper last year summing up their detective work. In the 1950s and ’60s, a few scientists proposed that glia were about 10 times as common as neurons, based on studies of small brain regions, ones that happened to have particularly high glia-to-neuron ratios. In a decades-long game of telephone, other researchers repeated these estimates, extrapolating them to the entire brain. Science journalists parroted the numbers. Soon this misconception spread to textbooks and educational websites run by the government and respected scientific organizations. Even the latest edition of “Principles of Neural Science” states that the brain as a whole contains “two to 10 times more glia than neurons.” The truth is that not a single study has ever demonstrated this. “I realized we didn’t know the first thing about what the human brain is made of, much less what other brains were made of, and how we compared,” Herculano-Houzel says.

So she decided to find out herself. For decades, the standard method for counting brain cells was stereology: slicing up the brain, tallying cells in thin sheets of tissue splayed on microscope slides and multiplying those numbers by the volume of the relevant region to get an estimate. Stereology is a laborious technique that works well for small, relatively uniform areas of the brain. But many species have brains that are simply too big, convoluted and multitudinous to yield to stereology. Using stereology to take a census of the human brain would require a daunting amount of time, resources and unerring precision.