In order to discover how normal ageing of our brains affects our intellectual abilities we have to screen as many people of different ages as possible on as many different cognitive tests as they will agree to take. When we do this the most striking general conclusions are that variability in performance both between and within individuals sharply increases in successive age-decades. The increase in between individual variance implies that as a cohort ages the differences in ability between its most and least able members steadily widens. Many different factors must simultaneously contribute to this increased variability. There is good presumptive evidence that longevity, and consequently prolongation, of intellectual competence, are genetically inheritable; certainly women live longer, and probably retain intellectual competence proportionately later than do men. As people grow older they are increasingly likely to suffer from one or more chronic pathologies. There is also mounting evidence that illnesses such as diabetes or respiratory and cardiac insufficiency, which become increasingly common in old age, can accelerate cognitive decline. Widening gaps in intellectual ability between the most and least competent members of ageing populations may partly reflect different genetically programmed trajectories of ageing, but also, and to an even greater extent, increasingly wide variations in health between individuals. (Author/publisher)
Abstract