What matters more is what you do with the brain you have
Ever since it was discovered that the brain serves a purpose, humanity has been fixated on its size. Intuitively, it makes sense: the more of a resource you have, the more you should be able to do with that resource, right? And if the brain is what makes us intelligent, flexible, not only ready for anything but also able to think ahead about what might be needed and prepare for it, then the more brain, the more capable an animal species should be, and also an individual, right?
More or less. For fifteen years, I have been studying how different brains are made and what difference that makes. Among animal species, it seems quite clear to me that brain size is irrelevant in terms of cognitive ability, sleep requirements, and lifespan. Much more important is the number of neurons that make up the brain and its parts. It is the huge number of neurons in the cerebral cortex, for example, that distinguishes us from other animals, even elephants and whales—and incidentally also explains our long childhood and longevity.
If huge numbers of neurons cost time and energy, one would expect them to “serve” some purpose—that is, to offer some immediate advantage to the bearer. After all, this is how most biologists expect evolution to work: through the fixation over generations of small advantages of adaptive value. Such as, for example, more cortical neurons.
Five years ago, we discovered, to our surprise, that even mice raised in the laboratory, all from the same litter, are quite diverse in size and number of neurons—but the animals with the largest cortices are not necessarily those with the most cortical neurons. Kleber Neves and Gerson Guércio, both young doctoral students at the time, came into my office and asked, “But even so, animals with more neurons could have some cognitive advantage, even without a larger brain, couldn’t they? We want to test this.”
Ah, how delightful it is to have inquisitive young people in the lab. A few years and more than thirty carefully tested mice later, in collaboration with Prof. Rogério Panizzutti, from UFRJ, we have an answer: No. Not even a little bit. The exact number of neurons in an individual's brain does not predict how well they will perform on a test.
It's the kind of result that scientific journals don't find titillating—but I loved it. It sounds so egalitarian: what matters isn't the brain you're born with, but what you do with it.
Excerpt from Suzana Herculano-Houzel (2025) Neuroscience of Everyday Life, originally published in Folha de São Paulo in July 2020.