The graphs show the trend in scores on a ten question vocabulary test (WORDSUM) given by the General Social Survey (N = 24,196). This test is a good measure of IQ, and you can see how the share of the country in the highest IQ bracket has fallen. In 1990, it was 7.4%, and by 2008 it was almost half that--3.8%. The area of gain has been with the middle scores of 6 and 7.
Suspecting that the shrinking of the smartest group might be due to a larger number of immigrants--a group with a poorer English vocabulary--I removed them, but found the same trend. It looks like we're seeing Idiocracy in action.