The figures show the distribution of vocabulary scores (General Social Survey (GSS) WORDSUM) of college graduates and those without college degrees for the periods 1974-1984 and 1998-2008. It looks like the average score for college graduates has dropped by a significant fraction of a standard deviation over 25 years. This effect is entirely predictable given the larger percentage of Americans attending college in recent times.
It should not be surprising that a shrinking percentage of college students can write well or do basic mathematics, let alone appreciate Proust or quantum mechanics.
Additional years of education have not increased verbal abilities in the general population. This observation supports, at least in part, the signaling and sorting model of higher education (the primary value of credentials is that they reflect more or less invariant qualities such as IQ and Conscientiousness), as opposed to the model that higher education builds human capital.
