Kaplan GA, Pamuk E, Lynch JW, Cohen RD, Balfour JL. Inequality in income and mortality in the United States: analysis of mortality and potential pathways. British Medical Journal 1996; 312: 999-1003

This paper, and another published simultaneously with another independent effort (Kennedy & Kawachi below), provide important substantiations of the income inequality health relationship because they are carried out on consistent data, collected in one country, the US, and statistically corrected for potential confounders. The age adjusted mortality by state is found to be highly correlated with the proportion of household income received by the least well off 50% of households. This was also found for death rates in specific age ranges, being strongest in the productive 25-64 year olds. The association remained strong after adjusting for the different median incomes among states. Similar strong associations were found between income inequality and homicide, violent crime, low birthweight, smoking, sedentary behavior, disability, expenditure on police protection, and surprisingly expenditure on medical care. That is, states with greater income inequality had higher rates or expenditure on those factors. Greater inequality was also correlated with more unemployment, incarceration, people receiving income assistance and food stamps, as well as less spending on education, with corresponding poorer education outcomes. Income inequality seems to be growing in importance as a factor affecting health of populations, for the correlation with mortality was less strong in 1980, than in 1990. As well, more unequal states had less good health improvements during that decade. Some might argue that absolute income affects mortality, but there is no association for median income by state once the modest initial correlation is statistically corrected for the different income distributions by state. Others would suggest race explains the difference, but the same association is seen for whites as well as for African Americans. The authors suggest in considering the ecologic fallacy, namely that the data do not reflect individual experiences, that their variables (age adjusted mortality rates, income distribution levels) can only be measured at the population level. They suggest that from a prevention point of view, to improve health it may be more important to deal with the structural features of inequality, than with the psychosocial consequences. 

Correction (see abstract and next paper below)

On page 1253, BMJ 18 May 1996, are important corrections, resulting from production errors which change the signs of some of the correlation coefficients.

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Abstract

OBJECTIVE--To examine the relation between health outcomes and the equality with which income is distributed in the United States. 

DESIGN--The degree of income inequality, defined as the percentage of total household income received by the less well off 50% of households, and changes in income inequality were calculated for the 50 states in 1980 and 1990. These measures were then examined in relation to all cause mortality adjusted for age for each state, age specific deaths, changes in mortalities, and other health outcomes and potential pathways for 1980, 1990, and 1989-91. 

MAIN OUTCOME MEASURE--Age adjusted mortality from all causes. 

RESULTS--There was a significant correlation (r = -0.62 [corrected], P < 0.001) between the percentage of total household income received by the less well off 50% in each state and all cause mortality, unaffected by adjustment for state median incomes. Income inequality was also significantly associated with age specific mortalities and rates of low birth weight, homicide, violent crime, work disability, expenditures on medical care and police protection, smoking, and sedentary activity. Rates of unemployment, imprisonment, recipients of income assistance and food stamps, lack of medical insurance, and educational outcomes were also worse as income inequality increased. Income inequality was also associated with mortality trends, and there was a suggestion of an impact of inequality trends on mortality trends. 

CONCLUSION--Variations between states in the inequality of the distribution of income are significantly associated with variations between states in a large number of health outcomes and social indicators and with mortality trends. These differences parallel relative investments in human and social capital. Economic policies that influence income and wealth inequality may have an important impact on the health of countries.

Keywords

• cause specific mortality
• community
• confounding
• ecological studies
• education
• homicide
• income
• income distribution
• income inequality
• medical care
• mortality
• population health
• race
• regional
• relative deprivation
• smoking
• social capital
• socioeconomic status
• unemployment
• violent crime
• wealth inequality