Kennedy BP, Kawachi I, Glass R, Prothrow SD. Income distribution, socioeconomic status, and self rated health in the United States: multilevel analysis. British Medical Journal 1998;317(7163):917-21

One objection to the studies carried out in the US is that they are ecological, that is they use data about populations (see Ecologic Fallacy in Overview and Making Causal Inferences section on home page) and that they don't control for confounding (epidemiologic term for two effects not being separated in the analysis) of individual incomes.  This study overcomes those criticisms by looking at income distribution by states in the US, and correcting for these individual variables such as income, having health insurance, level of educational attainment, and using self-reported health as the outcome measure.  Studies have shown that self-reported health is a very good predictor  of mortality that stands independently of other risk factors.  This suggests that how people feel about their health averaged out over a population is a pretty good indicator of how healthy the population is.  Country wide surveys of individuals began asking the question about their health status in 1993, allowing this kind of broad-based study to be done.  The researchers showed that if you reported having fair or poor health (in contrast to very good or good health), that you were more likely to live in a state with greater income disparity and that this finding was maintained even if you statistically corrected for the level of household income, or were older, or were a smoker, or were obese, or had other adverse health risk factors.  Such statistical corrections are necessary because in some states such as Florida, for example, there are many retired people who would be expected to report poorer health.  The expected relationship with socioeconomic status was found, namely poorer people (whether measured by by income, or education, or race) reported poorer health.  The relationship with income inequality was sustained, that is people living in states with greater income inequality were more likely to report poorer health, and more importantly that the effects of income inequality on health were most pronounced among those with the lowest income but were also significant for those in the middle income group too.  This suggests that living where there is more income inequality doesn't just affect the health of the poor but those in the middle class.  This study validates the earlier findings, and strongly supports the premise of this web site.  It also answer many of the criticisms voiced in Section F below.

Abstract

OBJECTIVE: To determine the effect of inequalities in income within a state on self rated health status while controlling for individual characteristics such as socioeconomic status. 

DESIGN: Cross sectional multilevel study. Data were collected on income distribution in each of the 50 states in the United States. The Gini coefficient was used to measure statewide inequalities in income. Random probability samples of individuals in each state were collected by the 1993 and 1994 behavioural risk factor surveillance system, a random digit telephone survey. The survey collects information on an individual's income, education, self rated health and other health risk factors. 

SETTING: All 50 states. 

SUBJECTS: Civilian, non-institutionalised (that is, non-incarcerated and non-hospitalised) US residents aged 18 years or older. 

MAIN OUTCOME MEASURE: Self rated health status. 

RESULTS: When personal characteristics and household income were controlled for, individuals living in states with the greatest inequalities in income were 30% more likely to report their health as fair or poor than individuals living in states with the smallest inequalities in income. 

CONCLUSIONS: Inequality in the distribution of income was associated with an adverse impact on health independent of the effect of household income.
- Sociology of Health and Illness

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keywords

income distribution, socioeconomic status, self rated health, inequality