It has been reported that the age-specific prevalence and incidence of dementia and cognitive impairment in the United States have either remained stable or even declined during the 1980s–1990s (1,2). A recent Dutch study also showed age-adjusted dementia incidence rates to be consistently, although nonsignificantly, lower in the subcohort assessed in 2000 than that assessed a decade earlier (3). In behavioral sciences, one of the persistent predictors of disease prevalence, incidence, and mortality is years of educational attainment. Although there is still a debate regarding potential mechanisms underlying the association between education and overall health (eg, income, accessibility to health care, lifestyle and environment, mother’s nutrition during prenatal period, nutrition during infancy, etc.), we expect that cognitive functions can be very much affected by educational attainment. Besides the above potential effects of education on overall health, test-taking skill (which influences performance on neuropsychological tests) can be also associated with educational attainment. Furthermore, where educational opportunities are uniform, higher educasstional level may reflect higher intelligence and cognitive reserve (4), and more highly educated individuals may undertake occupations, which are more cognitively stimulating and require “continuing education” throughout life. The cognitive stimulation may itself further stimulate synaptic density and dendritic branching (4). Yet, there is a paucity of studies, which examine the extent to which educational attainment explains observed cohort effects in cognitive aging, specifically age-associated cognitive decline measured longitudinally over time. Facing a rapid rate of population aging, there is growing interest in projecting future trends in dementia prevalence and incidence. These projections would be benefit from information on whether educational attainment explains cohort effects on cognitive trajectories. In a population-based cohort of older adults, the present study assessed whether (i) cohort effects could be observed in age-associated trajectories of cognitive functions and (ii) the observed cohort effects could be explained by changes in educational attainment among cohorts. Practice or learning effects, which refer to the improvement in cognitive test scores over repeated administrations of cognitive tests, could possibly mask or distort the age-associated cognitive trajectories (5,6). Therefore, we also examined cohort differences in practice effects in the above assessments. Trajectories of neuropsychological tests tapping three cognitive domains (psychomotor speed, executive function, and language) were compared among cohorts born between 1902 and 1911, 1912 and 1921, 1922 and 1931, and 1932 and 1943. Data Data come from two large epidemiological studies of dementia: the Monongahela Valley Independent Elders Study (aka, MoVIES) and the Monongahela-Youghiogheny Healthy Aging Team study (aka, MYHAT). The two studies were conducted in geographically contiguous areas of southwestern Pennsylvania between 1987 and 2012. Both studies recruited age-stratified random samples of individuals aged 65 and older from the Voter Registration lists for targeted communities. Brief descriptions of each study are given below. MoVIES. This project recruited and assessed 1,681 individuals during the years 1987–1989 from a group of largely rural communities and followed them biennially until 2001 to investigate incidence, risk factors for cognitive impairment, and dementia. Details of sampling, recruitment, assessments, and follow-up have been reported previously (7,8). At baseline, the study response rate was approximately 60%. MYHAT. This project recruited and assessed 1,982 individuals during the years 2005–2007 from a group of small-town communities and followed them annually to investigate outcomes and predictors of outcomes in mild cognitive impairment. Details of sampling, recruitment, assessments, and follow-up have been reported previously (9,10). The study is currently in the sixth wave of data collection. At baseline, the study response rate was approximately 63% (11). Pooling data from the two studies, we categorized participants into the following four 10-year birth cohorts: those born between 1902 and 1911, between 1912 and 1921, between 1922 and 1931, and between 1932 and 1943. We excluded 46 participants born before 1901 from longitudinal analyses due to small sample size. Educational attainment. We used three education categories: less than high school education, completed high school education but less than college education, and completed college or more education. Neuropsychological tests. The following four neuropsychological tests were administered in the identical manner across two studies and were, therefore, examined in this study: Trail Making Test A (attention/psychomotor speed) (12); Trail Making Test B (executive function) (12); verbal fluency for initial letters P and S, aka letter fluency (executive function); and verbal fluency for the category of animals, aka category fluency (language) (13). The Trail Making Tests are conventionally scored in the time (in seconds) to complete the test, but in our population-based cohort, this measure showed skewed distributions and ceiling effects. Hence, we calculated the number of correct connections per second (connections/s) to use in the current study. Although both studies assessed memory in detail, they used different memory tests as MoVIES addressed dementia and MYHAT was focused on mild cognitive impairment; we, therefore, did not include memory measures in the present study. For fair comparisons of magnitudes of coefficients across four cognitive tests, all tests were standardized using mean and standard deviation of each test score at baseline of the combined data set. Statistical Analysis Differences in educational attainment by birth cohorts were compared using Pearson chi-square statistics. We used mixed-effects models with the outcomes being standardized scores on each cognitive test to examine age-associated cognitive trajectories. We fit two models (Models 1 and 2) for each outcome. In both models, we controlled for sex, practice effects, which identify second and third assessments (two dummy variables, each indicating second or third assessment) and its interaction with a variable indicating MYHAT cohort. The interactions were included because annual assessment in MYHAT (as opposed to biannual assessment in MoVIES) could lead to larger practice effects. In Model 1, we included age at each assessment, cohort, and Cohort × Age interactions (for assessing cohort effect on age-associated cognitive trajectories) and Cohort × Practice effects (for assessing cohort differences in practice effects observed at second and third assessments). In Model 2, we added education, Age × Education, and Age × Education × Cohort interactions. In exploratory analysis, we also examined nonlinear effects (age2, age3), but these were not significant and not included in the final model; practice effects were adequate to capture nonlinearity. Age was centered at age 80 for efficient convergence. Intercept and age were treated as random effects. Model fitness was examined through visual inspection of residuals and formal statistical tests. We used SAS version 9.2 (SAS Institute, Inc.) and R version 2.11 (R Foundation) for statistical analyses.