THE EFFECT OF TIME AND COHORT DIFFERENCES ON THE INTERPRETATION OF AGE CHANGES IN COGNITIVE BEHAVIOR.

The Primary Mental Abilities Test and the Test of Behavioral Rigidity were administered to 1060 Ss obtained by stratified-random sampling of the membership of a prepaid medical plan. Independent samples of 26 men and 26 women in each five-year interval in the age range from 21 to 76 years were tested in 1966 and 1963. Composite longitudinal and cross-sectional padients were constructed and examined for the effect of time and cohort differences. Time difference is defined as that component of change between two successive measurements within the same generation which is due t o environmental experience. Cohort difference is defined as the difference in scores obtained by successive generations when measured at the same age level. Positive cohort and time differences were found for most unspeeded tasks while negative cohort and time differences occurred for the speeded tasks. It was concluded that cross-sectional studies tend to overestimate decrement on unspeeded tasks due to the effect of increasingly favorable life experience or genetic improvement in the species. As a result, within generation decrements appear to be relatively small. Decrement on speeded tasks is under- estimated by the cross-sectional method for the opposite reasons; i.e., un- favorable life experience or decrease in ability over successive generations. The results of repeated measurement and independent random sampling methods for the short-term study of developmental change were compared and the adequacy of the latter approach seems substantiated.