Cognitive dedifferentiation in later life: Longitudinal evidence from the Lothian Birth Cohort 1936

In the cognitive ageing literature, the dedifferentiation hypothesis refers to cognitive skills becoming more interrelated in older adulthood. Here, we find evidence for cognitive dedifferentiation in the Lothian Birth Cohort 1936 (LBC1936). The LBC1936 is a narrow-age longitudinal cohort, across five waves of data collected approximately every three years between the ages of 70 and 82 years. Here, we use the data from N = 418 participants (49% male) who provided cognitive data at each of the five waves. We modelled a latent factor of general cognitive functioning (g) at each wave in a structural equation modelling framework. The percentage of variance that g accounts for across 13 cognitive tests increases by wave; wave 1 to wave 5: 25%, 27%, 29%, 31%, 36%. Here, g is a structural equation model-derived latent trait on which all included cognitive tests have standardized loadings > 0.3, p < .001. The mean standardised loadings of individual tests on g increased from wave 1 to wave 5 as follows: 0.49, 0.51, 0.53, 0.55, and 0.59. We report strong negative associations between the wave-based increase in variance explained and the mean g score estimates (r = -0.991, p = .001). Thus, the group-level rate of dedifferentiation closely tracks the group-level rate of cognitive decline. Future longitudinal research will be crucial in clarifying the incremental validity, determinants, mechanisms and implications of cognitive differentiation and dedifferentiation across the lifespan.