Income and health
If income is causally related to health (and that depends on whether one is convinced by the empirical evidence—see later section), the proposed mechanisms generally involve either access to material resources (e.g., the ability to purchase higher quality diets, or better housing), or access to symbolic resources (status and rank within one's community), or both. Broadly speaking, three different hypotheses can be spelled out regarding the relationship between income and health.1
First, the absolute income hypothesis posits that
where hi is an individual's level of well-being (e.g., years of life), and yi refers to that individual's own level of income. The relationship between individual income and individual health is shown as a diminishing function of increasing levels of income because above the level where basic needs are met, added income has fewer health benefits. By contrast, the relative income hypothesis posits that
where the term (yi−yr) denotes the relative gap between an individual's income, yi, and the income of some reference group, yr. The reference population could be the income of co-workers, neighbors, or the national population. In this instance, there is no asymptote; the greater the gap, the poorer one's health.
The absolute income hypothesis is primarily identified with the so-called “materialist” theory2 that attributes income effects on health as resulting from access to tangible resources such as food, clothing, and shelter; while the relative income hypothesis is primarily identified with “psycho-social” theory, which posits that the effects of income on health are mediated through symbolic resources such as status, prestige, and control. However, matching these hypotheses to one or the other theory is problematic and likely to be counterproductive. Both the absolute and relative income hypotheses are consistent with either neo-material or psychosocial explanations. Indeed, it is doubtful that an empirical study could ever succeed in teasing out neo-material from psychosocial processes because of colinearity and measurement error. It is difficult, if not impossible, to conceive of an experiment in which neo-material resources could be manipulated without also affecting psychosocial responses, and vice versa. Under the absolute income hypothesis, a given increase in income could improve health because it improves access to material resources but it simultaneously improves a person's sense of financial security. Conversely, under the relative income hypothesis, raising an individual's income relative to their reference group could improve health because it elevates their prestige but it also expands access to a broader range of goods and services that others cannot afford.3
The principal merit of distinguishing between the absolute and relative income hypotheses lies in the ability to make separate predictions about health effects even if the explanations are not obvious. This can be seen in the following thought experiment: If your current income is US$ 10,000 and everybody else's income in your community is US$ 20,000, what would happen to your health status if your neighbors’ incomes were doubled but you were left with the same income (assuming equal purchasing power in the new scenario)? The absolute income hypothesis would predict that your health would remain unaffected. The relative income hypothesis would predict adverse health effects because the gap between your income and your reference group has been doubled. However, the reasons for this could reflect either psychosocial or material pathways. Your neighbors can now afford to purchase cell phones, an internet connection, obtain loans for a car or home, and so on. This could have an adverse impact on your health as a result of psychosocial effects of envy and frustration. At the same time, lack of access to goods and services that you cannot afford—but everybody else now can—could deleteriously affect your ability to participate and function within your community (a neo-material explanation). In practice, teasing out the absolute income effect from the relative income effect is tricky because of colinearity. Nevertheless, emerging empirical evidence suggests that absolute and relative income independently predict mortality, disability, and high-risk coping behaviors.
A few studies have now analyzed the related concept of relative deprivation (RD) using the Yitzhaki construct4 based on the difference between own income yi and mean income of those individuals j with higher incomes within the reference group of size N (weighted by the proportion of the reference group with income greater than i's)
This relative deprivation construct focuses on the gap between ones own income and incomes of those richer than oneself, but ignores information on the magnitude of the income gap compared to those poorer than oneself. Eibner and Evans5 find that relative deprivation compared to one's state-demographic reference group is associated with higher adult male mortality after controlling for own income. It is also associated with other adverse health outcomes such as seeking care for mental health problems, increased cigarette smoking as well as higher BMI, consistent with heightened stress which is one of the pathways by which deprivation could affect health outcomes.5,6 However, these results are sensitive to the measure of relative income used, and the design does not rule out omitted variables bias and reverse causality concerns discussed later. Further work in this vein would be valuable; for example, Kondo et al.7 find that a similar relative deprivation measure is associated with higher disability incidence in a prospective cohort study of older Japanese adults.
Finally, yet a third formulation of the relationship between income and health is the relative rank hypothesis, which posits that an individual's level of health is determined by the relative position within a hierarchy that a given income confers on that individual. Related to the relative income hypothesis, evidence for the relevance of hierarchical rank on health derives from studies in nonhuman primates, for example, macaques and baboons, in the wild and in captivity. In both settings, higher-ranked animals have better health than do those who are lower on the dominance hierarchy. It is not simply that dominant individuals enjoy greater access to food and mates, however; adverse health effects of lower rank occur even when there are abundant resources. Rather, lower-ranked animals suffer a different set of slings and arrows of their subordinate status which appear to expose them to more stress.8 Higher physiological cost of subordinate rank turns out to vary depending on the pattern of social organization in different species, the stability of the environment, and the temperament of the individual animal. For example, low-ranking individuals tend to be more stressed in stable hierarchies, whereas high-ranking individuals experience greater stress in unstable arrangements.
Complexities such as those described earlier raise questions about the relevance of evidence on dominance hierarchies in nonhuman primate species for human society. In addition, humans differ from other primates in having multiple bases of social ordering. There is not a single hierarchical order affecting a given individual; he or she may occupy different positions depending on the domain and reference group. Even within the domain of socioeconomic status, the various components (income, education, and occupation) are only moderately correlated with one another. Despite this, research using the MacArthur scale of subjective social status (SSS), on samples from a wide range of populations, has shown that individuals appear to have an overall sense of their relative position in the socioeconomic hierarchy and that this perception shows significant associations with health outcomes. The SSS scale asks individuals to place themselves on one of the rungs of a l0-rung ladder where the top of the ladder is occupied by individuals with the most money and education and the most prestigious jobs and the bottom by those with the least money and education and the worst jobs or no job at all. The higher people place themselves on the ladder, the better their health. Scores on the ladder have been linked to self-reported global health and disease cross-sectionally,9,10 as well as to change in health over time.11 Ladder scores also relate to biological indicators of stress arousal including elevated heart rate and blood pressure, greater abdominal fat deposition, and morning rise in cortisol, and reduced gray matter volume in the anterior cingulate portion of the brain which modulates stress response.12–14 At an ecological level it has been linked to mortality rates.15
Many of the associations between ladder scores and health-related outcomes remain significant when adjusted for objective indicators of SES. Subjective status may be linked to health above and beyond objective status because it provides a more sensitive and complete measure of social status than do the traditional indicators. Alternatively (or in addition), it may be that the experience of lower status is itself distressing and the physiological responses associated with feelings of relatively low status may themselves be harmful. There is no research to date which allows us to test these competing possibilities. Intriguing studies in human populations have suggested possibilities of status effects. For example, Redelmeier and Singh16 reported that, among those nominated for Academy Awards in acting, those who won the Oscar subsequently lived longer than those who did not. However, there is controversy over potential methodological flaws (see, e.g., Sylvestre et al.17 and difficulties in distinguishing status effects from other risk protections that accrue to people with higher status).
Education and health
As in the case with income and health, there is consistent evidence linking more education with better health. The causal mechanisms underlying the link between schooling and health may operate through both material and psychosocial mechanisms. Education equips individuals with general as well as specific knowledge and skills that are useful for prevention of disease. At the same time, higher educational attainment confers greater prestige and status within the community as well as serving as a credential for employment. Earning a degree increases one's chances for obtaining a job that pays well, has prestige, and exposes workers to fewer safety hazards. Data showing that the association of education and health is not perfectly linear (i.e., not every year of additional education contributes the same amount to better health) but is discontinuous at the times of degrees (e.g., 12 years, l6 years) suggests that a “sheepskin effect” may be responsible for some of the health benefits of education.