It is sometimes said that “primitive peoples” (or welfare mothers, in a particularly obnoxious use of the trope) have a pathetically small vocabulary—a thousand words, perhaps. I’ve just found an excellent essay by linguist Geoffrey Nunberg (the language maven of Fresh Air, among other things) debunking this nonsense. By the way, although Nunberg doesn’t give a figure, the average adult vocabulary appears to be somewhere around 40,000-50,000 words (or, if you believe Steven Pinker, not one of my heroes, something closer to 60,000). (Thanks to Jonathan Mayhew for pointing me in this direction.)
Addendum. Check out the other essays at Nunberg’s homepage; there are eminently sensible ones on American attempts to pronounce “Iraq” and “Qatar” (and foreign names in general) and on the use of “Gallic” and other symptoms of our conflicted Francophobia, inter alia.