Fluid Intelligence in a Norwegian Sample of Middle-Aged and Older Adults

Based on results from earlier studies, fluid intelligence (Gf) is expected to decline with age and symptoms of depression, while crystallized intelligence (Gc) is expected to remain stable. Gf is expected to show a relation to cortical thickness in parieto-frontal areas. Method: N = 125 (83 females) healthy middle-aged and older adults completed WASI Matrix Reasoning, Cued Discrimination Task, California Verbal Learning Test-II and Color Word Interference Test, giving the means to derive Gf. WASI Vocabulary was used to derive Gc. Beck's Depression Inventory provided self-reported symptoms of depression. MRI measures of cortical thickness of n = 91 were attained. Results: Aging was associated with a decline on Gf (r = -.436, p = .001), but not Gc. Symptoms of depression were significantly correlated with Gf (r = -.514, p = .001) and Gc (r = -.365, p = .017) in males, but not in females. There were correlations between cortical thickness and Gf, but these disappeared when correcting for age. Discussion: The Gf factor relies heavily on measures of cognitive speed, known to decline with age. The effect of symptoms of depression might challenge the current theoretical approach to intelligence, and give implications for clinical practice. Methodological factors may explain the non-significant correlations between cortical thickness and Gf when age was accounted for