Stability and change in cognitive ability over the life span: a comparison of populations with and without Down's...

Longitudinal studies show that in the general population IQ declines with age: early and rapidly in the case of performance IQ, later and more slowly in the case of verbal IQ. These populations have not apparently included people with intellectual disabilities (ID).

A literature search identified 11 studies, some cross-sectional and others longitudinal, which provided data on a variety of verbal and performance tests, over periods from 3 to 19 years, on older people with ID. Following statistical advice the results from the different tests were converted into the equivalent of, for verbal scores, British Picture Vocabulary Scale, and for performance scores, Leiter International Performance Scale, raw scores. Percentage change between earlier and later scores was then calculated.

With one exception the studies considered tend to show verbal ability declining relatively more, and performance ability declining relatively less, than has been shown to occur in the general population. Potential confounding factors, such as population attrition, cohort effects, etc., are thought not to have affected these results.

The pattern of change with age in verbal and performance ability appears different in people with ID from that seen in the general population. Some possible reasons for this difference are discussed.