Intelligence

Cohort differences in psychometric abilities have been most intensively examined in the Seattle Longitudinal Study. Cumulative cohort differences Figure 3 Cumulative cohort differences on six latent intellectual ability dimensions for cohorts born from 1897 to 1966. SOURCE: Schaie, K. W.."The Course of Adult Intellectual Development." American Psychologist 49 (1994): 304-313. Reprinted with permission of the American Psychological Association. for cohorts born from 1897 to 1966 are shown in Figure 3 for the abilities discussed above. There is a linear positive pattern for inductive reasoning and verbal memory, a positive pattern for spatial orientation, but curvilinear or negative patterns for numeric ability and perceptual speed. Factors thought to influence these cohort differences include changes in average educational exposure and changes in educational practices, as well as the control of early childhood infectious diseases and the adoption of healthier lifestyles by more recent cohorts. Similar differences have also been found using biologically related parent-offspring dyads compared at approximately similar ages.

The effect of these cohort differences is to increase age differences in intelligence between young and old for those skills where there have been substantial gains across successive generations (e.g., inductive reasoning), but to decrease age differences in instances were younger generations perform more poorly (e.g., number skills). Hence, it should be kept in mind that some older persons seem to perform poorly when compared with their younger peers not because they have suffered mental decline, but because they are experiencing the consequences of obsolescence.