Why not indeed. The idea that we only use a small portion of the brain, usually quantified by a very specific number, is completely false. I don’t even have a guess as to where it originated, but it has since spread and infected the public consciousness. We do, in fact, use all of our brain.
Of course, this implies that we can’t just “switch on” the rest of the dormant brain and magically become smarter and more handsome, like Bradley Cooper’s character in Limitless. If it sounds too good to be true, it probably is.
However, that doesn’t mean that the way in which our brain operates is at all times completely optimal for our goals. Increasing or decreasing activity in certain parts of the brain, or certain neurotransmitter pathways, could plausibly make at least some of us happier or more productive. Which, of course, is nothing new, since we have been using psychoactive drugs for such purposes since the dawn of medicine. As we learn more about the brain, we will come closer to the level of understanding required to really mess with it in ways that can, possibly, make us smarter or happier without risking dangerous side effects. But we aren’t really there yet. Most current drugs, or non-drug methods of altering the brain come with a long sheet of possible adverse reactions.
Obviously it would be easier if there really were large swaths of the brain going unused all the time, just sort of hitchhiking on the evolutionary trail, a sort of parasitic neural network gobbling up nutrients and energy—the brain is the part of our body that uses the most energy as compared to its volume—that we could activate to become superhuman. But that really isn’t the case.
And if you think about it, that really makes no sense at all on two levels. First, why would we have a huge organ that consumes huge amounts of precious (at least in prehistoric times) energy if we only used a small portion of it? If we could do with the brain of a baboon, we would never have retained, or evolved such a big brain in the first place. And secondly: consider the extremely implausible-even-for-a-hypothetical scenario that we all were actually carrying around a huge brain but only using a small portion of it. That would constitute normal experience. What would happen if we suddenly activated the rest? In the movies, the obvious answer is that we’d be superhuman. But maybe we’d actually become emotional wrecks, or maybe we’d become intellectually impaired because the mind could not integrate all the new activity into a coherent picture.
Luckily for us, no such dilemma faces us. The 10% or whatever number is making the rounds is completely fabricated.
However, while it’s not the case that ordinary healthy people go around not using a large chunk of their brain, it is possible to survive and even thrive with minimal loss of cognitive function with only half your brain. A procedure known as hemispherectomy involves removing or severing one hemisphere of the brain. This surgery is only performed in extreme cases of epilepsy where the source of seizures has been found to be localized to one hemisphere, due to the obvious risks of cutting out or off one half of someone’s brain. Remarkably, the brain, especially if the surgery is performed at a young age, is able to adapt and allow basically all of the functions of the other hemisphere to be taken over by the one remaining.