The existence of generational (cohort) differences in intellectual abilities has been conclusively demonstrated. Almost linear positive cohort shifts have been observed for inductive reasoning, with more spasmodic positive shifts for verbal ability and spatial orientation. A curvilinear cohort pattern has been found for number skills, which reach a peak for birth cohorts born in the 1920s and then follow a largely negative slope. A similar curvilinear cohort pattern has been observed for word fluency. As a consequence, cross-sectional studies of intellectual aging underestimate age changes before age 60 for abilities with negative cohort gradients and overestimate age changes for abilities with positive cohort gradients.
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