Income inequality and mortality: importance to health of individual income, psychosocial environment, or material conditions

British Medical Journal, April 29, 2000 by John W Lynch, George Davey Smith, George A Kaplan, James S. House

Summary points

Income inequality has generally been associated with differences in health

A psychosocial interpretation of health inequalities, in terms of perceptions of relative disadvantage and the psychological consequences of inequality, raises several conceptual and empirical problems

Income inequality is accompanied by many differences in conditions of life at the individual and population levels, which may adversely influence health

Interpretation of links between income inequality and health must begin with the structural causes of inequalities, and not just focus on perceptions of that inequality

Reducing health inequalities and improving public health in the 21st century requires strategic investment in neo-material conditions via more equitable distribution of public and private resources

Studies on the health effects of income inequality have generated great interest. The evidence on this association between countries is mixed,[1-4] but income inequality and health have been linked within the United States,[5-11] Britain,[12] and Brazil.[13] Questions remain over how to interpret these findings and the mechanisms involved. We discuss three interpretations of the association between income inequality and health: the individual income interpretation, the psychosocial environment interpretation, and the neo-material interpretation.

Methods

We reviewed the literature through traditional and electronic means and supplemented this with correlational analyses of gross domestic product and life expectancy and of income inequality and mortality trends based on data from the World Bank,[14] the World Health Organization,[15] and two British sources.[16 17]

The individual income interpretation

According to the individual income interpretation, aggregate level associations between income inequality and health reflect only the individual level association between income and health. The curvilinear relation between income and health at the individual level[18 19] is a sufficient condition to produce health differences between populations with the same average income but different distributions of income.[3 20] This interpretation assumes that determinants of population health are completely specified as attributes of independent individuals and that health effects at the population level are merely sums of individual effects.[21 22] In contrast, research on income inequality recognises that there may also be important contextual determinants of health. To understand these potential multilevel effects, analyses are needed that use measures of income distribution and individual income to examine health differences across individuals and aggregated units.

In examinations of health differences among individuals, contextual health effects of income distribution have remained after adjustment for individual income in most studies[8-11]--but not all.[23] Not surprisingly, these studies found that individual income was more strongly related to individual differences in health than to income distribution. Only one study has examined the role of individual income and income distribution on health differences among aggregated units: Wolfson and colleagues used a simulation technique to explore the contribution of individual income to aggregate health differences.[24] They showed that the individual mechanism explained only a modest proportion of the observed aggregate variation in mortality at the level of US states.

Though empirical tests of this hypothesis indicate that the association between income and health at the individual level is important in understanding differences in health between individuals, they also indicate that individual income may be less important in understanding variation in health across aggregated units. Policies on wages, investments, and taxes help determine the extent of unequal income distribution across the population, and this distribution then influences individual incomes. The statistical adjustment for individual income reveals an important pathway linking aggregate income inequality and individual health--but it may also encourage underestimation of the overall population effects of unequal income distribution.

The psychosocial environment interpretation

The psychosocial environment interpretation proposes that psychosocial factors are paramount in understanding the health effects of income inequality. Wilkinson has argued that income inequality affects health through perceptions of place in the social hierarchy based on relative position according to income.[25] Such perceptions produce negative emotions such as shame and distrust that are translated "inside" the body into poorer health via psycho-neuro-endocrine mechanisms and stress induced behaviours such as smoking. Simultaneously, perceptions of relative position and the negative emotions they foster are translated "outside" the individual into antisocial behaviour, reduced civic participation, and less social capital and cohesion within the community. In this way, perceptions of social rank--indexed by relative income--have negative biological consequences for individuals and negative social consequences for how individuals interact. Perceptions of relative income thus link individual and social pathology.