Should we be building more prisons or more psychiatric institutions?

We have over two million people incarcerated in prisons in America. Many of them have mental issues. Why are they in prisons when they should be in mental institutions where they can get real treatment and hopefully cured or at least have their disorders controlled?

It seems mental disease is still swept under the rug in this country and yet I would bet that most of us have a family member or a friend who has or has had some problems of this sort.

How can something so common be so ignored? Why do we keep throwing the sick into prisons? What do you think?