Accumulating knowledge on the social determinants of health and infectious disease.

The importance of social factors—particularly absolute poverty and its consequences—as determinants of infectious disease has been recognized since the 19th century. Although awareness of the social determinants of noninfectious disease is far more recent, a sizable body of knowledge has accumulated in the past two decades identifying diverse social factors—including, but not limited to, socioeconomic factors—and plausible mechanisms that may explain how those factors influence health and both infectious and noninfectious disease. I will briefly mention only a few of the areas of rapidly expanding knowledge—the socioeconomic gradient in health and potential material and psychosocial pathways that may explain it, including the role of stress and the life-course perspective—and implications for public health research and policy. It may not be surprising that the poor or those with little schooling have worse health than the non-poor or those with at least some college education. Mirroring results of earlier European research, however, recent U.S. studies reveal a stepwise incremental gradient of health improving—albeit not necessarily linearly—with each increment of income or education, for diverse health outcomes among children and adults.1–3 The prevailing pattern has no cutoff at the poverty or low-income line, although health is often much worse below those thresholds. For example, for most health outcomes, those with some college education (but without a degree) are less healthy than college graduates; and those with incomes more than four times the federal poverty level have better health than those with incomes three to four times the federal poverty level. The socioeconomic gradient is seen for most health outcomes among both non-Latino black and white people studied separately. These patterns cannot pinpoint the underlying causes, but, when considered in light of the body of relevant literature now available, suggest a dose-response relationship between health and income, education, and/or factors closely associated with them. An effort to explain the gradient has been a strong force driving much research. Despite ongoing controversies about causality, current knowledge supports the notion that both education and income could influence health in myriad ways, through complex, biologically plausible pathways that do not always involve medical care.4,5 Although medical care and behaviors—both of which are associated with income and education—are important, the gradient persists after accounting for them.6,7 Some correlates of absolute poverty, such as unhealthy housing and poor nutrition, do not explain the differences among those who are not poor. Evidence suggests a role for a number of psychosocial factors that are consistent with the gradient pattern (i.e., factors that vary by income, education, and other markers of socioeconomic resources and position). These psychosocial factors include effort-reward imbalance or high external demands combined with low control at work,6,8 one's perceived relative position in a social hierarchy,9 and stress.10 One of the most significant recent scientific advances with the potential to explain the socioeconomic gradient is an increased understanding of the physiology of stress and how, over time, stress can lead to wear and tear on multiple organs and systems (for example, through neuroendocrine and immune/inflammatory processes).10 Stressful events and circumstances follow a socioeconomic gradient, decreasing as income increases.11,12 It is plausible that greater economic resources would provide individuals with more resources to cope with both everyday and special challenges, and that limited economic resources in themselves can create stress (e.g., due to residential crowding, food insecurity, worry about making ends meet in general, and family conflict related to these stressors). Schooling is generally a crucial path to greater economic resources and also may reflect greater knowledge and skills that can be applied to problem solving and, hence, to stress reduction. Higher income and education both reflect and can provide access to social networks that have more resources and make fewer demands on one's own resources, thus potentially reducing stress. While definitive answers are lacking in all these areas, sufficient evidence exists to keep them under serious consideration. The life-course perspective refers to an emerging awareness of how a multitude of health determinants—social and otherwise—may interact with each other over the entire course of an individual's life, and potentially across generations, to shape health and illness. For example, considerable evidence has linked chronic disease in adulthood to social and economic experiences in early life.13,14 A mother's experiences in her own childhood also may be key determinants of her newborn's birth outcomes, which then predict the newborn's health outcomes in adulthood.15,16 What are the implications for public health? First, these perspectives can inform our understanding not only of socioeconomic, but also of racial/ethnic disparities in health. Socioeconomic status/position and race/ethnicity are distinct but intertwined. The legacy of centuries of legal racial discrimination has left deep traces that persist even when conscious intent to discriminate is no longer evident. For example, racial/ethnic residential segregation tracks black and Latino children into inadequate schools and exposes them to unhealthy neighborhood conditions, including the hopelessness generated by seeing poverty, joblessness, and incarceration as inescapable. This legacy explains the marked and persistent racial/ethnic differences in income, wealth, education, and occupation, which often are not adequately considered in health research. While some racial/ethnic disparities in health have greatly diminished or disappeared after adjustment for socioeconomic factors,17,18 other differences appear to persist after accounting for socioeconomic factors.19 These disparities could reflect unmeasured socioeconomic differences, for example, in accumulated wealth, neighborhood conditions, and/or past socioeconomic circumstances, which are rarely measured in health research, yet could have substantial health effects. Persistent racial/ethnic differences also could reflect the added stress, over and above that generated by socioeconomic disadvantage, of experiences of racial/ethnic discrimination, including not only overt incidents, but also more subtle experiences, such as maintaining chronic vigilance in anticipation of a threat.20 Investigations of the role of these and other social factors in racial/ethnic disparities in health are important areas of inquiry21 that have received relatively little research funding to date. We have made dismal progress in reducing racial/ethnic disparities in health; our next efforts need to be informed by a broader, deeper understanding of the likely determinants of those disparities. Much remains unknown about the precise pathways and mechanisms through which different social factors affect different health outcomes. We do know enough now, however, to say that achieving better health overall and reducing both socioeconomic and racial/ethnic disparities in health will require a more adequate understanding of the role of social factors.22 Therefore, we need the following: a more thorough consideration of social factors in surveillance and one-time studies, even when social factors are not the main focus of a given study; research to develop better measurement of social factors; and better conceptualization of both socioeconomic factors and race/ethnicity in health research. Researchers should not treat income, education, and race/ethnicity uncritically, but consider them thoughtfully as markers for different experiences and exposures, including different social advantages and disadvantages, and consider their potential roles as mediators and moderators in pathways and mechanisms resulting in health and disease. Furthermore, studies need to consider not just the proximate or immediate causes for diseases or risk factors, such as smoking and poor diet, but the causes of those causes—the underlying social factors that set pathways and, ultimately, biological mechanisms in motion. Public health practitioners and leaders should read the literature with a critical eye toward these issues and insist that their practice be guided by current scientific knowledge. The latter indicates that social factors generally play a crucial role in health and health disparities, and do so by affecting not only the exposures people have to health-promoting or -damaging experiences, but also their vulnerability to health damage if exposed. Such factors also affect an individual's access to timely treatment and the socioeconomic consequences of illness, which then can transmit poverty and ill health across lifetimes and generations. Public health leaders and practitioners at various levels should consider the possibilities for joint action with other sectors, such as education, housing, and child care; models exist for doing this.23,24 Finally, health leaders and practitioners can make substantial contributions to improving health by being vocal, visible advocates for policies to reduce social disadvantage. They can do this not by trying to become experts in the business of other sectors, but by understanding and speaking to existing knowledge of the likely deleterious effects of social disadvantage on health and health disparities.