Correlated and coupled cognitive change in older adults with and without preclinical dementia.

Common factor aging theories state that correlations among cognitive age effects signify a single underlying causal process. The logic underlying this proposition was evaluated by examining correlated cognitive change in a sample of 391 initially nondemented older adults who were tested annually for up to 16 years. Between-person correlations among rates of change (range = .56-.61) were partly attributable to model misspecification and the aggregation of heterogeneous groups of individuals. Correlated within-person cognitive change was much stronger in the cases (.45-.51) than in the noncases (.07-.18). These results demonstrate that correlated change may either signify causal commonality or the cumulative effects of multiple age-related conditions that can affect multiple cognitive systems.