"Scientists" don't say it. One or two people may have said it in the past, and a lot of people have quoted this fact without knowing where it came from. It is true that we do not yet know what all the brain does. It is true also that in a few cases some people have managed to live fairly normal lives despite a large part of their brain being missing. It is true also that if one part of the brain is damaged or destroyed, the functions of the damage part can sometimes be taken over by other parts. The answer , therefore, is that we do not yet know how the brain does what it does, so we cannot answer how much of it is used. However, recent technological advances mean that brain research is advancing very fast at the moment. From an eviolutionary standpoint, the human brain is a remarkably expensive organ to maintain. It represents only 10% of our mass, yet consumes 25% of the energy. The relatively huge head makes childbirth much more dangerous for human women than for other animals. That weight on top of the neck requires a lot of muscle to keep upright and m ove around, and is relatively easily damaged. So if we could get on without 90% of it, it seems likely that we would evolve to do so.