This is a really interesting (and not a little disquieting) thought. In the case of the Sistene Chapel ceiling - the scale and artistry of which really is difficult to comprehend if you haven't seen it firsthand (truly, pictures do not in any way do it justice) - it was, at least, a commissioned piece of work, which I believe Michelangelo was initially reluctant to accept.
He rejected it because he was a sculptor and not a painter. Eventually he gave in and had to teach himself how to paint.
A lot of artists were mentally ill, and took up painting as a form of therapy.
In the USA they used to put the stupid with the smart in special schools and asylums for people with a mental illness. Once they changed that there are more disabled people now in society with a mental illness.
