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I have often heard it said humans “use only 10 percent of our brains.” (Why people make a point of saying this to me I’m not sure.) But for all the times I’ve run across this statement, no one has ever cited a source nor explained precisely what it means. Does it mean only 10 percent of the neurons ever fire at all, leaving the other 90 percent to atrophy? This would explain quite a bit about politics and college athletics, but it doesn’t seem appropriate for most functioning adults. As someone with an above-average number of active brain cells, perhaps you can unravel these mysteries. –Eugene Dillenburg, Chicago

The 10 percent statistic has been attributed to pioneering psychologist and philosopher William James (1842-1910). I haven’t been able to confirm that he gave a specific percentage, but he did say “we are making use of only a small part of our possible mental and physical resources” (The Energies of Men, 1908). Anthropologist Margaret Mead supposedly said we use 6 percent, and similar numbers have been mentioned by various lesser known parties.

Whatever the source, such figures have no scientific basis except in the most limited sense. Serious brain researchers say that while we perhaps don’t use our brains as efficiently as we might, there’s no evidence we have vast unused abilities.

Admittedly no one has ever tested all the tens of billions of neurons in a given brain. You’ve certainly got a few spares; otherwise no one would recover from a stroke. But attempts to map out the cerebral cortex, the center of the higher mental functions, have not found large areas that don’t do anything. The general view is that the brain is too small (just three pounds), uses too many resources (20 percent of body oxygen utilization though it accounts for just 2 percent of weight), and has too much to do for 90 percent of it to be completely comatose.

Obviously not all of the brain is in use at once. At any given time only about 5 percent of the neurons are active, the only sense in which the old saw is even close to true. (Good thing, too, or you’d have the equivalent of a grand mal seizure, a mental electrical storm in which all the neurons fire continually.) The parts of the brain are highly specialized, and some areas are more active than others depending on the task at hand. But all the parts do something, and it seems safe to say that over time you use pretty much all of your brain, just as most people use all their muscles to at least some degree.

In fact, muscles are a useful analogy. While we probably don’t have much extra capacity in the sense of unused neurons, it’s possible we have untapped potential. Studies with rats suggest that just as muscles grow stronger with exercise, so does the brain. Rats raised in stimulating environments had thicker cerebral cortexes, larger neurons, more connections between neurons, more glial (support) cells, and so on. In other words, good books, snappy conversation, and a regular dose of the Straight Dope may make you smarter. But don’t get your hopes up. Skeptics say what the rat studies prove is not that an enriched environment will make you smarter but only that a deprived one will make you dumber.

But wait, you say. What about memory? Obviously we accumulate memories; obviously the brain is also finite and has some limit to its capacity. What percentage of memory capacity do we use? We don’t know enough even to hazard a guess. Old people find it harder to learn, but that’s probably more due to deterioration and mental rigidity (which may or may not have some neurological basis) than to a lack of capacity.

Some popular beliefs about brains do have a basis in fact. Though the question is still disputed, it’s possible that after age 30 you do lose 100,000 brain cells a day (or at least some large number). Studies suggest that between early adulthood and age 90 the cortex loses between 10 and 30 percent of its neurons. The remaining neurons develop more cross connections with other cells, presumably to help pick up the load. Booze probably snuffs a few brain cells, too–at any rate it kills nerve cells in rats. I can believe it. I can still remember a great deal, of course. But Lord, the things I’ve forgot.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.