The "we use only 10% of our brain capacity" is a myth with absolutely no scientifical evidence of any kind. It was originally invented (not deduced, not measured, but completely made up) by proponents of latent human supernatural psychic abilities.
Measurements of brain activity and usage give absolutely no support for this claim, much less for the 10 percent figure. PET-scans and MRI of the brain clearly show that the whole brain mass has full activity, and there simply isn't any scientifical evidence whatsoever that this isn't so. Even if one could argue that some part of the brain might not be used, the 10 percent figure just doesn't have any scientifical support. That number is completely made up.
In fact, if we used only 10 percent of our brain, we would be in a deep coma. When the brain of coma patients is scanned it shows most of the brain being inactive, with nothing going on, with just small parts having some activity (could well be that 10 percent). A human uses the entire brain just to be conscious and awake, and even in normal sleep the entire brain is in use, although in a different way. If parts of the brain would be completely unused and dead (ie. they show no activity in PET-scans) that's usually a symptom of a bad brain trauma and usually causes coma or other severe malfunctions.
What amazes me is how persistent this myth is even among people who don't believe in supernatural psychic phenomena nor that people have any potential for "superhuman" brain processing. Many of the most skeptic people I know still believe this myth, even to the point of actually defending it when they are confronted with evidence of the contrary.
Some of these people try to alleviate the claim by saying things like "ok, we use the entire brain, but we only use ten percent of our brain at any given time". Yet the ten percent figure is still completely made-up and based on no scientifical evidence. PET-scans and MRI show this claim to not have any truth in it. Yet these people persist in defending the myth.
Others alleviate it even more by conceding a bit, like "ok, the ten percent is just an approximation, it could be twenty percent or five percent, depends on what you are doing". This doesn't have too much scientifical evidence either.
What I don't understand is why. I can understand that new age fanatics want to believe the myth. I don't understand why rational, skeptic people do. They just don't want to let it go.
What's so hard in accepting this as just a myth? Is it some kind of disappointment that we use our whole brain, not just 10 percent of it? They certainly act like it was a big disappointment. I don't understand why, though.
Also even if you show them scientifical evidence against the 10 percent myth, including brain scans and such, they still doubt it and prefer believing the myth. Why they refuse to believe scientifical measurements and want to believe hearsay goes beyond my understanding. I just can't grasp the reason for this phenomenon.