Cohort differences in cognitive aging and terminal decline in the Seattle Longitudinal Study

Abstract

Life span researchers have long been interested in how and why fundamental aspects of human ontogeny differ between cohorts of people who have lived through different historical epochs. When examined at the same age, later born cohorts are often cognitively and physically fitter than earlier born cohorts. Less is known, however, about cohort differences in the rate of cognitive aging and if, at the very end of life, pervasive mortality-related processes overshadow and minimize cohort differences. We used data on 5 primary mental abilities from the Seattle Longitudinal Study (Schaie, 2005) to compare both age-related and mortality-related changes between earlier born cohorts (1886-1913) and later born cohorts (1914-1948). Our models covary for several individual and cohort differences in central indicators of life expectancy, education, health, and gender. Age-related growth models corroborate and extend earlier findings by documenting level differences at age 70 of up to 0.50 SD and less steep rates of cognitive aging on all abilities between 50 and 80 years of age favoring the later born cohort. In contrast, mortality-related models provide limited support for positive cohort differences. The later born cohort showed steeper mortality-related declines. We discuss possible reasons why often reported positive secular trends in age-related processes may not generalize to the vulnerable segment of the population that is close to death and suggest routes for further inquiry.

Figure 1

Frequency of observations in the Seattle Longitudinal Study in relation to chronological age (Panel A) and time-to-death (Panel B), separately for the two birth cohorts. For the age models, each cohort encompassed more than 700 participants who contributed more than 2,400 data points each over a 30-year observation period that was, on average, comparable across cohorts. For the time-to-death models, each cohort encompassed more than 280 participants who contributed more than 900 data points each over a 25-year observation period that was, on average, comparable across cohorts.

Figure 2

Illustrating cohort differences in cognitive aging from age 50 to age 80 on Thurstone’s five primary mental abilities of spatial orientation, inductive reasoning, word fluency, number, and verbal meaning, after residualizing for differences in education (years of schooling), health (circulatory disease and cancer), and gender. Except on number ability, later born cohorts (solid lines) outperformed earlier born cohorts (dashed lines) at age 70 by up to 0.50 SD and also showed shallower rates of cognitive decline on all abilities.

Figure 3

Illustrating cohort differences in terminal decline on Thurstone’s five primary mental abilities of spatial orientation, inductive reasoning, word fluency, number, and verbal meaning, after residualizing for differences in age at study entry, age at death, education (years of schooling), health (circulatory disease and cancer), and gender. Mortality-related models suggest no evidence for positive secular trends except on verbal meaning. In contrast, later born cohorts (solid lines) showed steeper mortality-related declines than earlier born cohorts (dashed lines) on four of the five abilities tested.