From the blog,
I only wish they'd used my favorite sound bite, which went something like this: "Before I studied public opinion, I wondered why our policies were so bad. After I studied public opinion, I started wondering why our policies weren't worse. An important part of the answer is that the people who know less are less likely to vote."
Caplan's bit starts at ~2:30, but it's worth watching the entire clip, just for the enthusiasm with which people make wild guesses at the number of Senators per state.
Then, go read the book, which, for those in Oxford, is available in the Rothmere American Institute and the Social Sciences Library.
The title comes from Mencken's observation that 'democracy is the art and science of running the circus from the monkey cage.'
(Postscript: in linking to amazon, above, I encountered this review of the book, which rather makes Caplan's point for him. After all, "the university style of writing makes it difficult to understand what he is going on about, since you have to keep looking up a dictionary...")