Are people with intellectual disabilities getting more or less intelligent?

It has been well documented that over the last 60 years there has been a gradual increase in the intellectual ability of the population as a whole which has included people with low intellectual ability. The present study examined if this trend was still continuing in the UK by comparing the scaled scores given for individual raw scores on three common subtests (Symbol Search, Coding, and Digit Span) of the UK versions of the WISC-III and WISC-IV. It was found that over the 12.5 years between the two assessments being standardised there was an overall increase in intellectual ability. However, this increase was greater for those with high intellectual ability. For those with low intellectual ability there was a decline in their intellectual ability.