Are people with Intellectual disabilities getting more or less intelligent II: US data

There is now good evidence that the intellectual ability of the population as a whole is increasing from one generation to the next (Flynn 1984, 1987, 2006), a phenomenon now known as the Flynn effect after James Flynn, who conducted much of the work on the effect. This increase in intellectual ability has clearly occurred at the low range of intellectual ability, at least in western industrialised countries (Flynn, 1985, 2006, 2009), over at least the last 50 years.