I have looked at all of the information I could find regarding this presumption and have come to the following personal conclusion. My reasoning may not satisfy those bent on empirical data, (for none exists) but it is the best my research can offer. I concur with the concept that we use only a fraction of our potential brain capacity and the following is my evidence.

First, have there been any definitive surgical studies on the human brain that test all physical aspects of the brain with the intention of mapping total brain usage? No! Nor could such tests exist, nor would they prove the point. It has been guestimated that there are approximately 100 billion brain cells.  Even if you attempted to take a random sampling of perhaps 100 people, test every portion of their brain against their cognitive usage, this data would be heavily biased towards culture, age and educational background. Thus the statistical norm of even this sampling of 100 would carry such heavy inherent biases that no definitive conclusion could be drawn.

What we can do is look at the work of those neuroscientists such as Dr. Karl Lashley and Dr. Wilder Penfield who have actually performed brain surgery on conscious subjects. Their work revolved around finding what parts of the brain may house memory. Much was learned by actually stimulating certain portions of the brain while asking the conscious subject questions. Though certain areas of the brain such as the hippocampus were discovered to play a large part in memory retention, more questions were left unanswered than information ascertained. Dr. Karl Pribram, having worked with Lashley, continued this work and came to the conclusion that the brain operates holographically, and that memory isn't stored in any one particular place but perhaps throughout the brain.

Science has come to some definitive conclusions on what certain portions of the brain are used for, i.e. the occipital lobes, temporal lobes, frontal lobes, etc. Yet, there are vast areas of the brain that are still a mystery to science, i.e. the pineal gland, the full potential of the pituitary gland, and portions of the midbrain limbic system to name a few. Thus to evaluate how much of our brain's capacity we are using when we are still unclear as to what vast areas are capable of is purely speculative.

One example of mapping brain usage compared to the norm was done in studying Einstein's brain. The one definitive difference they found in his brain compared to the norm was that he had an unusually high number of glial cells in his parietal lobe. Glial cells are the supporting architecture for neurons. High counts of glial cells could indicate that he was using this portion of brain cognitively and extensively. The parietal lobe is thought to facilitate abstract thought. We do know that whenever anything is learned there are new dendrite connections made between neurons. Greater usage of the brain through learning and stimulation creates greater dendrite connectivity. Einstein's brain indicated extensive dendrite connectivity.

Science has yet to have the opportunity to study under a microscope any brain whose entire neuronal and synaptic connection potentials were totally used. All potentially 100 billion. Yet this consideration itself is one reason to speculate that we are using only a small portion of our brains, since those brains that have undergone microscopic study show vast areas of the brain where there is little dendrite branching.

Another factor to weigh in is that of idiot savants, i.e. The Rainman. Rainman was the character played by Dustin Hoffman who was able to calculate dates in lightning speed, though otherwise appearing mentally retarded. These individuals have one unusual talent such as the ability to calculate incredible numerical equations instantaneously in their mind (a feat few humans possess) or incredible musical dexterity. The fact that there are humans who have demonstrated this ability show that the human brain is capable of such achievements.

Something else to consider is the incredible demonstrations of biological control exhibited by eastern Yogis and Tibetan monks over their autonomic nervous system. They can, for example, slow their heart rate to almost nil, or sit in freezing weather with no clothing and dry wet towels on their back because they are generating such intense heat within their bodies purely by mental concentration (this is called Tahumo). Science isn't clear what portions of their brain they are accessing to accomplish these feats, but they have been rigorously tested with the latest in technology and found to be able to exhibit what ordinary humans cannot.

The next area to consider is that of extra-sensory perception. There is a vast degree of mounting evidence that certain individuals have great capacity in this regard. Stanford University alone has many studies, as well as the Cognitive Research Institute. Yet what is not thoroughly understood is exactly what portions of the brain (though it is presumed the mid-brain limbic areas) are involved in perception beyond the five senses. Since most people don't exhibit great testable acuity in this area, it can be assumed that certain portions of the brain used to accomplish this phenomena are simply not functional in most people.

Genius in any area, be it artistic, musical, mathematical, scientific, linguistic, intellectual, etc. is more evidence that certain individuals are using portions of their brain that the majority are not. No thought can be processed without the use of the brain. Therefore, if demonstrable feats of extraordinary mental, artistic or psychic functioning exist in even a small group of people, it indicates that the human brain has capacities not tapped by the majority. The determination that less than 10% is the actual amount used may be an arbitrary number. Yet, it certainly appears plausible from those who have demonstrated exceptional abilities that we are not using anywhere near the total capacity of the brain in our ordinary daily thinking processes.

Those who chose to believe that they are using close to their full potential are welcome to do so. Yet I feel that the evidence thus far is overwhelming that we are only tapping a small portion of what the human brain can do. If each of us were operating with fully functional brains, meaning that we had all the capacities of any genius, we had total psychic functioning and complete control over our autonomic nervous system at will, we could be said to operating at full capacity. I find it heartening to realize that there is a great deal of potential that I have yet to realize, rather than to assume my present state of mind is nearly the best it gets. I delight in the notion that there is a great deal of room left for improvement, new experience and the flowering of genius. Accepting this I never expect to see the end of the horizon of mindful potentials.