Intelligence

A number of longitudinal studies have been conducted in the United States and in Europe covering substantial age ranges. An important new addition is the initiation of longitudinal studies in the very old, providing hope for better information on age changes in the nineties and beyond.

Longitudinal studies of intelligence usually show a peak of intellectual performance in young adulthood or early middle age, with a virtual plateau until early old age and accelerating average decline thereafter. However, it should be noted that different intellectual skills reach peak performance at different ages and decline at different rates. Figure 1 provides an example from the Seattle Longitudinal Study, a large scale study of community-dwelling adults. This figure shows age changes from twenty-five to eighty-eight years of age for six primary mental abilities. Note that only perceptual speed follows a linear pattern of decline from young adulthood to old age, and that verbal ability does not reach a peak until midlife and still remains above early adult performance in advanced old age. Similar patterns have been obtained in meta-analyses of the WAIS (cf. McArdle, 1994).

It should also be noted that there are wide individual differences in change in intellectual competence over time. For example, when change in large groups of individuals was monitored over a seven-year interval, it was found that the ages 60 to 67 and 67 to 74 were marked by stability of performance, while even from ages 74 to 84 as many as 40 percent of study participants remained stable. However, it was also found that by their mid-sixties almost all persons had significantly declined on at least one ability, while remaining stable on others.