My radar went off on this one, because I've heard it attributed to both Mencken and P.T. Barnum. But it turns out Mencken <i>did</i> say it - or rather, he said something that resembled it:<p><i>No one in this world, so far as I know — and I have researched the records for years, and employed agents to help me — has ever lost money by underestimating the intelligence of the great masses of the plain people. Nor has anyone ever lost public office thereby.</i> (Chicago Tribune, Sept. 19, 1926)<p>This is another case of a quote that mutated over the years into a pithier, more rhythmic form. An interesting divergence is that Mencken never explicitly said the <i>American</i> public.