Intelligence

Findings from age-comparative (cross-sectional) studies of intellectual performance are used to compare adults of different ages at a single point in time. Because of substantial generational Figure 1 Longitudinal age changes on six latent intellectual ability dimensions. SOURCE: Schaie, K. W.."The Course of Adult Intellectual Development." American Psychologist 49 (1994): 304-313. Reprinted with permission of the American Psychological Association. differences, these studies show far greater age differences than the within-individual changes observed in longitudinal data. Ages of peak performance are found to be earlier (for later-born cohorts) in cross-sectional studies. Modest age differences are found by the early fifties for some, and by the sixties for most, dimensions of intelligence. On the WAIS, age differences are moderate for the verbal part of the test, but substantial for the performance scales. Because of the slowing in the rate of positive cohort differences (a later-born cohort performs at a higher level than an earlier-born cohort at the same ages), age difference profiles have begun to converge somewhat more with the age-change data from longitudinal studies. Both peak performance and onset of decline seem to be shifting to later ages for most variables.

Figure 2 presents age differences over the age range from twenty-five to eighty-one for samples tested in 1991, found in the Seattle Longitudinal Study, which can be directly compared to the longitudinal data presented in Figure 1. Particularly noteworthy is the fact that cross-sectional age differences are greater than those observed in longitudinal studies—except for numerical skills, which show greater decline when measured longitudinally.

More recent studies of the WAIS with normal individuals (the WAIS has been widely applied to samples with neuropathology or mental illness, so it is important to state that the work referred to is on normal individuals) use approaches that involve latent variable models (see McArdle & Prescott, 1992; Rott, 1993), while other analyses have been conducted at the item Figure 2 Cross-sectional age differences on six latent intellectual ability dimensions. SOURCE: Schaie, K. W.."The Course of Adult Intellectual Development." American Psychologist 49 (1994): 304-313. Reprinted with permission of the American Psychological Association. level. A study by Sands, Terry, and Meredith (1989) investigated two cohorts spanning the age range from 18 to 61. Improvement in performance was found between the ages of 18 and 40 and between 18 and 54. Between ages 40 and 61, improvement was found for the information, comprehension and vocabulary subtests, while there was a mixed change (gain on the easy items and decline on the difficult items) on picture completion and a decline on digit symbol and block design (with decline only for the most difficult items of the latter test). The discrepancies between the longitudinal and cross-sectional findings on the WAIS, as well as on the primary mental abilities, can be attributed largely to cohort differences in attained peak level and rate of change arising as a consequence of the different life experiences of successive generations.

Since women, on average, live longer than men, one might ask whether there are differential patterns by sex. Most studies find that there are average-level differences between men and women at all ages, with women doing better on verbal skills and memory, while men excel on numerical and spatial skills. The developmental course of intellectual abilities, however, tends to have parallel slopes for men and women.