Is it possible that crazy people are the normal ones?

Sanity is a relative term, where one state of mind is accepted and others shunned. Naturally, the state of mind deemed ‘normal’ is that of the most powerful people. For example, historically those who were homosexual were considered insane, and left handedness was ‘corrected’. Although these are no longer ‘treated’ in most of the world, is it possible that other so called disorders that we treat are in fact more normal than ourselves? Maybe depressed people have a better understanding of the nature of life. Maybe ADD affected people are reacting to changes in the environment that we do not perceive, and feel as if it is a disorder because of social pressures.

As always, I do not necessarily believe this is such, but even hypothetically how would we know it they were the normal ones?