"Well," continued Dr. Hutton, "the Master described before both Houses of Parliament the real scientific objection to all existing legislation about lunacy. As he very truly said, the mistake was in supposing insanity to be merely an exception or an extreme. Insanity, like forgetfulness, is simply a quality which enters more or less into all human beings; and for practical purposes it is more necessary to know whose mind is really trustworthy than whose has some accidental taint. We have therefore reversed the existing method, and people now have to prove that they are sane."

--G. K. Chesterton, The Ball and the Cross.

This article fits nicely in my mind with Kate's sister's statement about eating disorders. I shall see if I can find a source for that figure.