I've heard claims that the average person uses only 10% of their "brain power," whatever that means. The implication, though, is that there the majority of our brain's potential is untapped. However, I've often heard this refrain from people who are selling "training" or "secrets" to unlock the unused potential of our brain.
It seems unlikely that there would be evolutionary advantages to having a brain whose capabilities lie largely untapped.
I'm curious - what percentage of our brain do we use, or can it even be quantified in such measures?