Using a large representative sample of the older Chinese population, we found that unfavourable early-life factors were linked to lower late-life IC value over thirty years later, both directly and through the cumulative effects of socioeconomic inequities over a person’s life course. Unfavourable early-life factors were more likely to decrease an individual’s cognitive and sensory capacities than locomotor functioning, psychological capacity or homeostasis. As a consequence, significant inequalities in IC were observed across the population with a higher current socioeconomic position associated with better outcomes. These inequalities varied substantially across the various IC subfactors, with more inequalities found in cognitive and sensory capacities. Although a participant’s current socioeconomic position accounted for most IC inequalities, we found that early-life factors directly explained a substantial share of IC inequalities.
To the best of our knowledge, this is the first comprehensive life-course analysis of the socioeconomic determinants of inequalities in intrinsic capacity, a more holistic and strengths-based construct of human ageing than approaches considering specific conditions 41. Although a moderate proportion of our sample was excluded from our analysis, the sensitivity analyses indicated that our results were reliable and robust.
Our findings were consistent with previous research linking early-life factors to specific late-life health outcomes as we found a robust relationship between early-life factors and late-life IC values that remained statistically significant after controlling for a variety of demographic, lifestyle and current socioeconomic factors. For example, unfavourable socioeconomic position, sustained starvation, low parental education, poor health status and the absence of essential healthcare services in childhood have been found to increase the likelihood of being frail in old age 27,30,42. Meanwhile, our findings significantly associated better childhood friendships, healthy behaviour and receiving mentorship and support from others in childhood with a higher prognostic IC value in later life. However, our analysis of the broad IC construct also allowed us to examine the variation of these associations across subdomains and the factors across the life course that may influence these.
We found that early-life factors were more likely to impair cognitive and sensory capacities compared with locomotor functioning, vitality and psychological capacity. This is of interest since, in western Europe, cognitive capacity appears to be improving among older people when compared to previous generations of people of the same age 43, although the mechanism for this is unclear. Cognitive capacity in older age may be influenced by cognitive development in early life 44, which may be associated with child brain structural alterations of the cerebral cortex 45. Although exposure to stressors is protective in that they promote short-term adaptation, it can lead to a long-term dysregulation of allostasis that promote maladaptive wear and tear on the body and brain under chronically stressful conditions 46. Emerging evidence in neuroscience suggests that childhood adversity alters trajectories of brain development and negatively damages brain structure, function and connectivity 47. For example, childhood adversity reduces the volume of the hippocampus, the volume of anterior cingulate and ventromedial and dorsomedial cortices 48,49; affects the development of key fibre tracts 50; appears to alter the development of sensory systems that process and convey stressful experiences 51. Many of the early life characteristics associated with cognitive inequalities in our analysis (for example, literacy of the father or starvation in childhood) provide obvious routes to support this. The main early life characteristics associated with poor sensory capacity concerned starvation and illness which physiologically may have impacted the development of hearing and vision.
The smaller negative effect of early-life disadvantages identified for locomotor functioning, vitality and psychological capacities is somewhat surprising. Psychological capacity was partly dominated by early life determinants although less influence was identified, suggesting the influence of childhood development on the ability to manage stressors. Our measure of psychological capacity was dominated by a depression score 52, which itself is heavily influenced by current life events and genetic predisposition 53. While diminished locomotor capacity was also associated with adverse early life or life course exposures, the impact was less than for cognitive capacity. Locomotor capacity is influenced by many factors with conflicting associations with socioeconomic position 54. Moreover, the dysregulation derived from stressors in childhood could be masked by the growth of organs since locomotion covers skeletal muscle physiology, cardiovascular system energy homeostasis, osteoarticular and neurophysiology 55. Finally, vitality could be less likely to be affected because of a person’s underlying resilience to challenges, vigour and stamina, and thus maintain homeostasis in the face of daily stressors 56 compared with cognitive and sensory capacities, which are two major channels connecting individuals to their environments.
Our findings also lent support to the biological weathering theory that ambient stress of disadvantaged exposures fosters biological wear and tear throughout the life course 57. Specifically, prolonged exposure to socioeconomic disadvantage fosters an adverse physiological response that damages cells and induces oxidative stress 58 and provides highly salient cues of threat that trigger negative emotionality and a cascade of biological responses that, over time, cause wear and tear on physiological systems, increasing the risk for biological ageing 59.
We have identified significant socioeconomic inequalities in late-life IC, a fine-grained proxy for the overall health of older adults, with a higher socioeconomic position linked to a higher overall IC value. These findings are consistent with previous studies that have provided comprehensive evidence across countries concerning socioeconomic determinants of various health domains 60,61, including self-rated health, quality of life 62,63, functional limitations in daily life 27, anthropometric measures 35 and even mortality 64. However, much of this research is inconclusive for both effect size and direction 65–67, and some findings present conflicting results, likely due to the confounding effects of environmental factors on health. Moreover, many of the health-related outcomes studied previously have either been subjective or limited to various components of what might be considered as an individual’s health status 27,30,65−68. The bulk of these analyses has also had difficulty teasing out the role of early life and life course socioeconomic influences.
Distressingly, we found that the negative effect of early-life disadvantages on cognitive and sensory capacities persisted and accumulated over thirty years as we estimated greater inequalities in the two subfactors compared with inequalities in locomotor, vitality and psychological capacities. In fact, durable exposure to concentrated disadvantages in adulthood may still damage the person’s cognition 69, and lower socioeconomic position in late life is related to an acceleration of ageing across a broad range of functional abilities and phenotypes, including cognition, locomotion, sensory and psychological functioning 70. These associations are underpinned by biological processes such as chronic allostatic load and sustained inflammation 71,72. Among life course determinants, the impact of education was also much greater for cognitive capacity than for all the other capacities. Combined, these late-life inequalities would reinforce the lasting impacts of early life educational opportunities. There is also a large body of evidence linking sensory impairments with later life socioeconomic disadvantage in a self-reinforcing relationship 73. In contrast, a sedentary occupation may have been associated with more advantaged participants in this cohort of the Chinese population 74, and occupations involving more manual activities or participation in strenuous activities in day-to-day life might have some protective influences on locomotor function 75, although this might not be true for the most strenuous of occupations.
Although a participant’s current socioeconomic position accounted for most IC inequalities, we found that early-life factors explained a substantial share of IC inequalities directly over thirty years later and via the cumulative effects of socioeconomic inequalities. Almost all of the determinants of these disparities can be shaped by public policies across an individual’s life course 3. Decomposing IC inequalities into life-course factors may help identify key components of proactive early-life interventions that aim to preserve IC in older age 5 because enshrining equitable early-life opportunities influences people’s capacity to make choices, contribute to society and receive support when needed 3. Importantly, reducing inequalities in family economic status and education and closing the gaps between urban and rural residents and urban hukou and non-urban hukou holders may contribute significantly to reducing overall IC inequalities. This evidence provides support for the Chinese government’s latest initiatives, including “common prosperity” and “rural revitalisation”, designed to reduce rural-urban disparities and wealth inequalities.
One of the strengths of this analysis is that it employs a rich set of early-life factors and current socioeconomic factors to examine an innovative measure for healthy ageing in the context of the world’s largest ageing population. Indeed, social determinants have been found to influence each other beyond their simultaneous effect on health, resulting from the complex interplay between physical environments and culturally mediated behaviours and psychological factors. However, there has been recognition that the effect of social determinants of health should be examined and understood from a complementary rather than exclusionary perspective 26. Furthermore, we adopted the novel approach of examining the mediating pathways of early-life factors and IC through the cumulative effects of socioeconomic inequalities.
We acknowledge several limitations. First, we used individual recall data for early-life factors, making recall bias inevitable. Second, IC trajectories are important for tracking health status over a lifetime; however, this study could not distinguish whether IC variations were caused by higher/lower peaks or faster/slower declining speeds of IC. Accordingly, we will further examine the longitudinal nature of IC when new CHARLS data becomes available. Third, potential cohort effects may limit the generalisation of our findings. Analysing age-period-cohort effects is beyond the scope of the study and will be further explored in an analysis using the longitudinal variations in IC. Fourth, genetics were not included in the study; instead, we included a comprehensive and rich set of variables to try our best to ‘mop-up’ individual heterogeneity. Finally, the current socioeconomic factors included in this study preclude the interpretation of our results as causal relationships.
To conclude, using a large representative sample of the older Chinese population, we found that unfavourable early-life factors were linked with lower IC in late-life directly and through the cumulative inequalities over time. Unfavourable early-life factors were more likely to damage an individual’s capacities connecting the environment than internal organism homeostasis. A substantial share of IC inequality was explained by early-life factors that could potentially be reshaped by early interventions. Further studies using the variations in higher/lower peaks or faster/slower declining speeds of IC are needed.