Plenty is doing.
"If, as Flynn (1984, 1987) demonstrated, test performance has been improving steadily over the 27 years between the standardization of the WAIS and the WAIS-Revised, then the empirical results [of my literature survey] indicate that this improvement is largest in the average range and is much less or non-existent as you go up or down the IQ scale. Contrary to the general trend, performance has actually decreased in the retarded range, at least for the WAIS / WAIS-R comparison."
Disclaimer: this quote appears here only to spark discussion. It is not endorsed one way or the other. Make up your own mind. Or just refresh the page for another viewpoint. From a collection assembled by the late Chris Brand.