Cognitive dispersion in established adulthood: etiology and implications for cognitive aging

Abstract Background and Objectives Cognitive dispersion, reflecting intraindividual variability across tasks, may signal dementia risk in older adults, but its origins and relevance before midlife are unclear. We examined dispersion as a potential marker of cognitive reserve in established adulthood and distinguished it from general ability using heritability analyses. Research Design and Methods 1,245 primarily non-Hispanic White twins and siblings were assessed at 33.3 years (SD = 5.0; 53% female) using two cognitive batteries. Bayesian mixed effects location scale models estimated mean performance and dispersion across cognitive tasks, with tasks standardized against current sample-based norms (age 33) and adolescent norms (age 16). The mean of task scores standardized to current norms reflects current ability, while the mean of the same scores standardized to adolescent norms reflects growth in ability. The respective dispersion estimates represent task-specific deviations from general ability and task-specific gains. Results Across four models fitted by battery and standardization, greater cognitive dispersion was associated with higher general ability (r = 0.23–0.56) and developmental gains (r = 0.62–0.93). Verbal tasks showed lower dispersion relative to performance tasks (β = 0.83–0.87). Dispersion was strongly heritable overall (43%–84%), and moderately heritable independent of general ability (36%–37%) and growth (2%–16%). Dispersion did not predict education or occupational outcomes beyond general ability and growth. Discussion and Implications Before midlife, cognitive dispersion is distinct from general ability and moderately heritable. Dispersion is associated with higher cognitive ability and growth, but not directly with educational or occupational attainment, suggesting dispersion may relate to cognitive reserve through other pathways.