Had a random thought today:
People often will refuse to sing, even a casual setting with plenty of people around also singing (picture for instance the cliche of the campfire). They'll be self-depricating, saying "Oh, I can't sing"... "Can't carry a tune in a bucket"... "Don't want to peel the paint off the wall", whatever. Well, I'd have to say that most people can't speak like James Earl Jones, John Houseman, or Maya Angelou either -- but it doesn't stop them from flapping their gums all day.
Based on what I've seen of American Idol, however, some people probably should be kept from singing outside of said campfire scenarios.