The paradox of cognitive change.

Properties of cognitive change scores were compared in adults over age 70, for whom longitudinal changes are often negative, and in adults in two age groups under age 70, for whom the changes are often close to zero. Longitudinal assessments of three measures of memory and three measures of speed across an average interval of 2.4 years were obtained from a sample of 1,282 healthy adults between 18 and 92 years of age. Although substantial longitudinal declines were primarily apparent in adults 70 years of age and older, adults under and over age 70 were similar with respect to the variability and reliability of the cognitive changes, and in the magnitude of the correlations of the changes with each other and with variables that have been identified as risk factors for late-life cognitive decline and dementia. It is suggested that longitudinal changes in cognition can be considered to represent a paradox in that the mean values of the changes are more negative at older ages, but the change scores have similar measurement properties, and appear to be just as systematic, among adults under and over 70 years of age.