Cross-sectionally and longitudinally balanced effects of processing speed on intellectual abilities.

The processing speed theory of cognitive aging states that declines in intellectual abilities other than processing speed are mainly due to a slowing of processing speed. Cross-sectional studies have provided support for the processing speed theory as for age-related cognitive differences. Longitudinal studies, in turn, have provided weaker support for the processing speed theory as for cognitive age changes. The present study aims to reconcile this discrepancy of cross-sectional and longitudinal results by constructing a "fair" test of the processing speed theory, i.e., a test that balances the cross-sectional age range and the time period covered longitudinally. Data from 83 older adults came from the Bonn Longitudinal Study on Aging. Using hierarchical linear models, it is shown that, although cross-sectional age differences and longitudinal changes in the intellectual abilities were equal, processing speed only attenuated cross-sectional age effects in cognition but virtually did not reduce longitudinal change effects in cognition. This persisting difference in the explanatory power of processing speed regarding age-related differences and age-related changes is discussed with reference to other longitudinal studies and statistical issues regarding cross-sectional and longitudinal studies of aging.