Preplanned Studies: Early-Life Circumstances and Cross-Country Disparities in Cognition Among Older Populations — China, the US, and the EU, 2008–2018

With accelerated aging across the globe (1), it is critical to improving health in older populations (2), including aspects relating to cognitive ability (3). Growing scientific evidence found associations between childhood circumstances and cognition in old age (4-6). However, it has yet to be discovered to what extent childhood circumstances may explain cross-country disparities in cognition in older populations, especially between countries in different stages of development. This study applied the Oaxaca-Blinder decomposition to a validated measure of cognition in harmonized global aging surveys to explore the contributions of childhood circumstances to cross-country differences in cognition. Our results demonstrated a large share of cross-country differences in cognition among older adults being explained by childhood circumstances. Specifically, childhood disadvantages contributed 65.4% to the China-US difference in cognition, while childhood disadvantages contributed 38.2% to the China-EU difference in cognition. In both cases, family socioeconomic status explained the largest share of cross-country differences in cognition. The large gap in the cognition of older adults across these countries and their risk factors in childhood stress the urgency of mitigating childhood disadvantages to achieve more equitable, healthy aging.

This study used three Health and Retirement Study (HRS)-family surveys: the China Health and Retirement Longitudinal Study (CHARLS); the U.S. HRS; and the Survey of Health, Aging, and Retirement in Europe (SHARE). In the past three decades, the U.S. National Institutes of Health and HRS-family survey teams in each country have collected and harmonized data for aging populations to offer an opportunity for cross-national comparisons (7-8). Our analysis was restricted to individuals aged 60 years and older, with no proxy respondents. We matched CHARLS Life History Survey 2014 (9,846 respondents) with CHARLS core survey 2018 (11,096 respondents); HRS Life History Mail Survey 2015/2017 (8,579 respondents) with HRS core survey 2016 (7,744 respondents); and SHARELIFE 2008/2017 (30,706 respondents) with SHARE core survey 2017 (27,173 respondents). For the three HRS-family surveys, the earliest and latest years in which cognition data were collected for this analysis were 2008 and 2018, respectively. To make the cognitive assessments most comparable across China, the U.S., and the EU, we measured cognitive ability using the validated Mini-Mental State Examination (MMSE). Our cognitive score included the typical dimensions and items across the three HRS-family surveys, ranging from 0 to 29. The larger the value, the better the cognitive ability. Considering salient differences in the distribution of cognitive scores between China and the U.S. and between China and the EU, standardized cognitive scores were measured. Z-scores were determined to facilitate cross-country comparisons, using the combined mean and standard deviation of cognition scores in all three HRS-family surveys. We used the Oaxaca-Blinder decomposition to estimate the extent to which various childhood circumstances were explained in the cognitive abilities of older adults across countries (9). Originally proposed in 1973, the Oaxaca-Blinder decomposition offered a regression-based approach to attributing cross-country differences in cognition of older adults to differences in childhood circumstances (composition effect) versus differences in the effects of these circumstances (association effect) (10). A package iop in STATA (version16.0, Stata Corp, College Station, U.S.) was used to perform the analyses (9).

We measured cognitive abilities among older populations in China, the U.S., and the EU, all of which exhibited rapid aging processes, though at different stages of development. While the cognitive scores of older adults in the U.S. and EU are relatively similar in level and distribution, Figure 1 displays the large differences with their Chinese counterparts who tended to have lower cognitive scores. As shown in Table 1, Chinese older adults in the sample also tended to receive lower education (on average 4.5 years, versus 12.0 years in the U.S. and 11.2 years in the EU). Education has been recognized as a profound, protective factor of cognitive ability (11).

Figure 1.  Distributions of cognitive score for older adults — China, the United States, and the European Union, 2008–2018.

Notes: The kernel density plots represent the distributions of assessed cognitive ability for older adults in China, the US, and the EU. This figure is plotted using raw cognitive assessment data without accounting for differences in education across the countries. The MMSE score was calculated based on common items in three dimensions in the CHARLS, HRS and SHARE surveys. Specifically, the cognitive ability score ranges from 0 to 29, and the larger the value, the better the cognitive ability. The three dimensions included functions of orientation to time and attention, episodic memory, and computational ability. There were 4 points for orientation function, i.e., correct answers to the year, month, day, and day of the week; 20 points for episodic memory, i.e., 10 points for immediate memory and 10 points for delayed memory; and 5 points for computational ability, i.e., subtracting 7 from 100 for five consecutive times, and the number of correct answers is the computational ability score.

Abbreviation: MMSE=Mini-mental State Examination; CHARLS=China Health and Retirement Longitudinal Study; HRS=Health and Retirement Study; SHARE=the Survey of Health, Aging, and Retirement in Europe.

In addition to the gaps in educational attainment, a key predictor of cognitive disparities for older populations, other economic and social development aspects may also widen the cross-country cognitive disparities. To better understand these economic and social conditions at an individual level, we considered the role of various childhood circumstances of our study subjects before completing their education. Table 1 shows the summary statistics, a significantly higher proportion of Chinese older adults experienced war or famine compared to those in the U.S. and the EU. Five levels of schooling were created across China, the U.S., and the EU to indicate parental educational attainment to harmonize parental educational data for comparison. There were significant cross-country differences in parental education. Specifically, for older Chinese adults above age 60, about 60% of their fathers and 90% of their mothers received no formal education. By contrast, only 3% of fathers and 3% of mothers of older U.S. adults received no formal education. Similar differences existed when comparing other childhood circumstances, including parental and individual health. Chinese and European older adults experienced more abuse in childhood than their U.S. counterparts. Childhood circumstance variables in Table 1 were defined according to the literature (12).

Using the three HRS-family surveys and the Oaxaca-Blinder decomposition based on the standardized cognitive scores (Z-scores), Table 2 further illustrates the differences in cognition of the elderly and the contribution of childhood circumstances: 1) Chinese older persons had significantly lower assessed cognition (8.9 out of a total score of 29) than their U.S. and EU counterparts, respectively, at 16.5 and 17.5 (Table 1); 2) comparing China to the U.S. and the EU, overall childhood circumstances respectively explained 65.4% [95% confidence interval (CI): 59.4%, 71.4%] and 38.2% (95% CI: 35.1%, 41.2%) of the China-US and China-EU disparities in cognitive ability; 3) various childhood circumstances significantly contributed to cross-country differences in the cognitive ability of older adults. Of the childhood circumstances considered, family socioeconomic status, measured by parental educational attainment, contributed the largest — 57.6% (95% CI: 51.8%, 63.4%) to the China-US difference and 34.9% (95% CI: 31.9%, 38.0%) to the China-EU difference in cognitive ability. Of the other factors, individual health, parental health, and war or famine experiences respectively explained 4.4% (95% CI: 3.2%, 5.6%), 3.5% (95% CI: 2.6%, 4.4%), and 1.3% (95% CI: 0.7%, 1.9%) of the China-US difference, while war or famine experiences, individual health, and parental health explained 1.6% (95% CI: 1.3%, 2.0%), 1.0% (95% CI: 0.8%, 1.3%), and 0.6% (95% CI: 0.2%, 1.0%) of the China-EU difference.