In preparation for the American Crossword Puzzle Tournament, I've been reading a lot of crossword books, including Cruciverbalism by Stanley Newman, which I finished yesterday. In a section on the need for "a robust, wide-ranging, malleable, and ever-growing vocabulary," Newman writes:

Our conversational vocabulary of 3,000 words represents less than one-sixth of the total words with which we're familiar; most of us actually know and recognize about 20,000 words. That national average may drop substantially when William F. Buckley finally shuffles off this mortal coil...but there's no reason why we should rely on the estimable Mr. Buckley to prop up our stats.

Well, great. So what's the average now? (For more thoughts on Mr. Buckley, see here.)