A recent New York Times poll contained the startling result that, when asked what proportion of the U.S. population is black, 8% of white respondents and 17% of black respondents chose “more than 50%.”
Full poll here (.pdf file) The relevant question is #80.
Question: are there really this many people who think that the United States is more than half black? Or are there this many people who don’t know how much makes 50%?
According to this account of polls in 1990 and 1998, 24% of Jews and 58% of non-Jews think Jews make up more than 10% of the U.S. population. (It’s actually under 3%.) This one, I’d guess, really is a matter of people finding 10% hard to distinguish from 3%, and not some kind of general tendency to oversemitize one’s surroundings.