This sounds crazy - what gives??
90% is memory storage.
It’s a popular enough meme, but it’s also absolutely untrue.
Here’s a website that goes through some of it:
I’m going green! In these tough economic times, I’ve decided to use less energy and reduce my brain usage to 5%.
Interesting. How many miles per cheeseburger are you getting now?
Yep. Presumably, the meme got started by a Theosophist, perhaps saying, “well, most humans use 10% of their brain, but I’m up at 15% and see how awesome I am, gimmie some money and I can take you to 12% overnight” And stupid people (and who isn’t a little stupid, at least once in a while,) decided it was worth a shot. I was just a child when I first heard it, and grown ups said it, and I believed it, because … grown ups said it.
Yeah, all you can really do is try to guess what grain of truth (if any) it might have been based on, but without tracking down the original source, that’s just a guessing game, and it’s quite possible that, like most BS, it doesn’t have an original grain of truth at all.
The only thing I can think of is that perhaps something in the ballpark of 10% is only active at once? That or early neuroscientists looked at half the brain, couldn’t find a 1-1 mapping between it and the body and went “wtf is this?” and declared it inactive. (These are total shots in the dark, I doubt they’re correct)
FWIW, that’s what I always thought, and wiki thinks so, too.
You can get a lot more than 10% if you use NZT.
One of the most commonly used factoids in my Neuroscience department is that there are 10 times more Glial cells then Neurons in the human brain. A bit a math would show that this puts Neurons in ~10% range. Could be a possible start of the myth…?
(However despite that this factoid is commonly told, this paper seems put the Glial:Neuron ratio to 1:1 or lower.)
The other 90% is overhead for the operating system! 
This seems likely to me too. Its the same as saying that a book only uses 10% of its pages because there is more white space than ink covered space. Having a brain that had 100% of neurons firing would be as useful as reading an entirely black page.
Or it could be referring to the fact that people can lose large portions of their brain and still function. Maybe somewhere, sometime, there was someone who lost 90% of his brain and still survived, thus proving that all that was “unused”. The thing is, there are a lot of possible explanations, and no way of telling which (if any) of them was the actual origin.
wow, I am sooo there… Check this out:
Anyway, I have always suspected that the personal coaches industry nowadays is based on some dubious premises. But at least, that industry could hypothetically evolve into something that works, if they put their mind to it. But here we go - why try to improve things that don’t work but could have, when you can make plenty of money selling snake oil of the purest sort. Made of real snakes or your money back!
In some cases 10% is a very generous usage.
You know what we call it when someone uses 100% of their brain all at once?
A seizure.
Mythbusters did an episode on this as well. It was pretty interesting. To give the short answer to the OPs question, it was BUSTED…
-XT
Did they blow up someone’s brain at the end?