educational attainment and occupational status, post-tax family income was associated with a 3.6-fold mortality risk among working-age adults in the Panel Study of Income Dynamics, comparing the top (>$70,000 in 1984 dollars) to the bottom (<$15,000) categories of income (Duncan et al., 2002). The association between income and mortality also has been described as a “gradient” (Adler et al., 1994). That is, the excess risks of poor health are not confined simply to individuals below the official poverty threshold of income. Rather, an individual’s chances of having good health (e.g., avoiding premature mortality) improve with each incremental rise in income (although the relationship is also steepest at lower levels of income and tends to flatten out beyond incomes that are about twice the median level).

Also, as with education, the causal direction of an association between income and health does not entirely run from income → health. That is, the relationship between the two variables is acknowledged to be dynamic and reciprocal. Ill health is a potent cause of job loss and reduction in income. Indeed, income as an indicator of SES is more susceptible to reverse causation than education, which tends to be completed in early adult life prior to the onset of major causes of morbidity and functional limitations.

Nonetheless, tests of the income/health relationship in different datasets suggest that lower income is likely to be a cause of worse health status. For example, children do not normally contribute to household incomes, yet their health is strongly associated with levels of household income in both the Panel Study of Income Dynamics and the National Health Interview Surveys (Case et al., 2002). Furthermore, the adverse health effects of lower income accumulate over children’s lives, so that the relationship between income and children’s health becomes more pronounced as children grow older (Case et al., 2002).

An alternative possibility is that the relationship between income and health is explained by a third variable—such as inherited ability—that is associated with both socioeconomic mobility and the adoption of health maintenance behaviors. However, even inherited ability is unlikely to entirely account for the income/health association. If inherited ability is the sole explanation for the income/health relationship, we would not expect to find any association between family income and health among children who are adopted soon after birth by nonbiological parents (assuming that adoptive parents do not get to choose the children they will adopt based on their background, including their socioeconomic circumstances). Yet, in the National Health Interview Survey, the impact of family income on child health has been found to be similar among children who were adopted by nonbiological parents compared to children who were reared by their biological parents (Case et al., 2002). Other types of tests of the income/health association—such as the use of instrumental variable estimation (Ettner,

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