IAWYC, and so does Wikipedia:

One of the main effects of illusory superiority in IQ is the Downing effect. This describes the tendency of people with a below average IQ to overestimate their IQ, and of people with an above average IQ to underestimate their IQ.

(I personally am a very good example of this, because although I think I'm not terribly bright, I am in fact a genius.)