Cognitive dedifferentiation with increasing age and proximity of death: Within-person evidence from the Seattle Longitudinal Study.

A central aim of life-span psychology is to understand ontogenetic changes in the structure of individuals' actions, thoughts, and behaviors. The dedifferentiation hypothesis of cognitive aging suggests that the structure of individuals' cognitive abilities becomes less differentiated in old age. Empirical tests have almost exclusively approached this hypothesis from a between-person difference perspective and produced a mixed set of findings. In the present study, we pursue a within-person test of the hypothesis using up to 8 repeated measures of cognitive abilities over up to 49 years, covering fluid (inductive reasoning), visualization (spatial orientation), and crystallized abilities (number, verbal meaning, and word fluency), obtained from 419 now-deceased individuals who participated in the Seattle Longitudinal Study (SLS) and have provided at least 4 observations for each cognitive test. Results revealed that with advancing age and proximity to death, within-person coupling increased (a) among the crystallized abilities, (b) between visualization and fluid abilities, (c) between visualization and crystallized abilities, and (d) between fluid abilities and crystallized abilities. We discuss the importance of within-person analyses for understanding changes in the structure of behavior and consider how our findings inform research on cognitive decline and dedifferentiation later in life.