Changes in Fluid and Crystallized Intelligence Across the 20- to 90-Year Age Range on the K-Bit

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Abstract
Changes in ability across the adult life span were investigated for the adult portion of the K-BIT standardization sample (500 individuals aged 20 to 90 years) to permit comparisons with the results of similar investigations conducted with the WAIS and WAIS-R. These changes were explored for separate measures of crystallized intelligence (Vocabulary) and fluid intelligence (Matrices). Two two-way ANCOVAs (age and gender) were conducted, in which raw scores on Vocabulary and Matrices served as dependent variables and educational attainment as the covariate. Results agreed with previous findings on the Wechsler scales: Crystallized intelligence basically was maintained across the age range, while fluid intelligence peaked early and declined steadily despite the control for education. Age x gender interactions were nonsignificant. This study showed that fluid intelligence seems to decline with increasing age, even when the test is untimated and does not rely on visual-motor coordination.