Elsevier

Social Science & Medicine

Volume 62, Issue 7, April 2006, Pages 1768-1784
Social Science & Medicine

Income inequality and population health: A review and explanation of the evidence

https://doi.org/10.1016/j.socscimed.2005.08.036Get rights and content

Abstract

Whether or not the scale of a society's income inequality is a determinant of population health is still regarded as a controversial issue. We decided to review the evidence and see if we could find a consistent interpretation of both the positive and negative findings.

We identified 168 analyses in 155 papers reporting research findings on the association between income distribution and population health, and classified them according to how far their findings supported the hypothesis that greater income differences are associated with lower standards of population health. Analyses in which all adjusted associations between greater income equality and higher standards of population health were statistically significant and positive were classified as “wholly supportive”; if none were significant and positive they were classified as “unsupportive”; and if some but not all were significant and supportive they were classified as “partially supportive”. Of those classified as either wholly supportive or unsupportive, a large majority (70 per cent) suggest that health is less good in societies where income differences are bigger.

There were substantial differences in the proportion of supportive findings according to whether inequality was measured in large or small areas. We suggest that the studies of income inequality are more supportive in large areas because in that context income inequality serves as a measure of the scale of social stratification, or how hierarchical a society is.

We suggest three explanations for the unsupportive findings reported by a minority of studies. First, many studies measured inequality in areas too small to reflect the scale of social class differences in a society; second, a number of studies controlled for factors which, rather than being genuine confounders, are likely either to mediate between class and health or to be other reflections of the scale of social stratification; and third, the international relationship was temporarily lost (in all but the youngest age groups) during the decade from the mid-1980s when income differences were widening particularly rapidly in a number of countries. We finish by discussing possible objections to our interpretation of the findings.

Introduction

Whether or not the extent of income inequality in a society is a determinant of population health remains a controversial issue despite a large body of research. Although the findings of a substantial majority of studies suggest that more egalitarian societies do have better health and longevity (Lynch, Smith, & Harper, 2004a; Subramanian & Kawachi, 2004), a minority conclude otherwise and several authorities remain skeptical as to whether inequality has any implications for population health (Deaton, 2003; Lynch et al., 2004a). To gain a clearer understanding of the evidence and the nature of the disagreement, we decided to review all the research reports published in peer reviewed journals, and then to see if we could arrive at an interpretation of them which made sense of both the supportive and unsupportive findings.

Section snippets

The review

We compiled a list of 155 published peer reviewed reports of research on the relation between income distribution and measures of population health. This is much the most comprehensive list of studies yet compiled: as well as containing all the eligible studies listed in three previous reviews of parts of the literature (Hsieh & Pugh, 1993; Lynch et al., 2004a; Subramanian & Kawachi, 2004), we also found 37 additional papers either by using electronic searches or through informal contacts.

Findings

Table 1 provides a summary of the 168 analyses according to classification and the type of area over which inequality was measured. Table 2 lists all the analyses included according to their classification. A tally of numbers showed 87 wholly supportive analyses, 44 partially supportive, and 37 unsupportive. Almost three-quarters of all analyses were classified as either wholly or partially supportive. Of all analyses classified as wholly supportive or unsupportive, 70 per cent were wholly

The size of area

Table 1 shows the per centage of analyses classified as either wholly supportive or unsupportive according to whether they were international analyses using data for whole countries, whether their data were for large subnational areas such as states, regions and metropolitan areas, or whether they were for smaller units such as counties, census tracts or parishes. The proportion of analyses classified as wholly supportive falls from 83 per cent (of all wholly supportive or unsupportive) in the

Discussion

Taking account of the size of the area and the use of control variables reveals a high degree of consistency in the research findings. Thus, if we confine our attention to the 128 analyses which use data for areas the size of metropolitan areas or larger, only 23 fail to find some support for the hypothesis. If we were to reclassify analyses on the basis of results before the use of potentially problematic control variables (including individual income in multilevel models), then only eight (6

Mechanism

Low social status and the quality of the social environment are both known to affect health (Berkman & Kawachi, 2000; Marmot & Wilkinson, 1999). Not only are more unequal societies likely to have a bigger problem of low social status, but there is now substantial evidence to suggest that inequality is socially corrosive, leading to more violence, lower levels of trust, and lower social capital (Wilkinson, 2005). Psychosocial factors, many of which are associated with low social status, are

Conclusions

Our interpretation of 168 analyses of the relationship between income inequality and health is that income distribution is related to health where it serves as a measure of the scale of social class differences in a society. In small areas, where income inequality is unlikely to reflect the degree of social stratification in the wider society, it is—as Table 1 shows—less likely to be related to health. The overwhelmingly positive evidence from studies of larger areas suggests that this

References (178)

  • H. Gravelle et al.

    Income, income inequality and health: What can we learn from aggregate data?

    Social Science & Medicine

    (2002)
  • S. Hales et al.

    National infant mortality rates in relation to gross national product and distribution of income

    The Lancet

    (1999)
  • C. Henderson et al.

    The effects of US state income inequality and alcohol policies on symptoms of depression and alcohol dependence

    Social Science & Medicine

    (2004)
  • D.R. Holtgrave et al.

    Social determinants of tuberculosis case rates in the United States

    American Journal of Preventive Medicine

    (2004)
  • F. Hou et al.

    Neighborhood inequality, neighborhood affluence and population health

    Social Science & Medicine

    (2005)
  • K. Judge et al.

    Income inequality and population health

    Social Science & Medicine

    (1998)
  • H.S. Kahn et al.

    Are geographic regions with high income inequality associated with risk of abdominal weight gain?

    Social Science & Medicine

    (1998)
  • I. Kawachi et al.

    The relationship of income inequality to mortality: Does the choice of indicator matter?

    Social Science & Medicine

    (1997)
  • B.P. Kennedy et al.

    Social capital, income inequality, and firearm violent crime

    Social Science & Medicine

    (1998)
  • A. Laporte

    A note on the use of a single inequality index in testing the effect of income distribution on mortality

    Social Science & Medicine

    (2002)
  • A. Laporte et al.

    Income inequality and mortality: Time series evidence from Canada

    Health Policy

    (2003)
  • C. Larrea et al.

    Does economic inequality affect child malnutrition? The case of Ecuador

    Social Science & Medicine

    (2005)
  • J. Legrand

    Inequalities in health—some international comparisons

    European Economic Review

    (1987)
  • R. Lopez

    Income inequality and self-rated health in US metropolitan areas: A multi-level analysis

    Social Science & Medicine

    (2004)
  • V. Lorant et al.

    Deprivation and mortality: The implications of spatial autocorrelation for health resources allocation

    Social Science & Medicine

    (2001)
  • J. Lynch et al.

    Income inequality, the psychosocial environment, and health: Comparisons of wealthy nations

    The Lancet

    (2001)
  • J.A. Macinko et al.

    Wage inequality, the health system, and infant mortality in wealthy industrialized countries, 1970–1996

    Social Science & Medicine

    (2004)
  • S.E. Mayer et al.

    Some mechanisms linking economic inequality and infant mortality

    Social Science & Medicine

    (2005)
  • W.R. Avison et al.

    Population diversity and cross-national homicide

    Criminology

    (1986)
  • W.C. Bailey

    Poverty, inequality and city homicide rates

    Criminology

    (1984)
  • M.H. Baldani et al.

    Association of the DMFT index with socioeconomic and dental services indicators in the state of Parana, Brazil

    Cad Saude Publica

    (2004)
  • J.W. Balkwell

    Ethnic inequality and the rate of homicide

    Social Forces

    (1990)
  • L. Baron et al.

    Cultural and economic sources of homicide in the United States

    The Sociological Quarterly

    (1988)
  • J. Beckfield

    Does income inequality harm health?

    Journal of Health and Social Behavior

    (2004)
  • T. Blakely et al.

    No association of income inequality with adult mortality within New Zealand

    Journal of Epidemiology and Community Health

    (2003)
  • T.A. Blakely et al.

    What is the lag time between income inequality and health status?

    Journal of Epidemiology and Community Health

    (2000)
  • T.A. Blakely et al.

    Socioeconomic inequality in voting participation and self-rated health

    American Journal of Public Health

    (2001)
  • J.R. Blau et al.

    The cost of inequality: Metropolitan structure and violent crime

    American Sociological Review

    (1982)
  • P. Bourdieu

    Distinction: A social critique of the judgement of taste

    (1984)
  • P.H. Brodish et al.

    Income inequality and all-cause mortality in the 100 counties of North Carolina

    Southern Medical Journal

    (2000)
  • E. Brunner et al.

    Social organization, stress & health

  • D. Canadine

    Class in Britain

    (1998)
  • S.J. Charlesworth et al.

    Living inferiority

    British Medical Bulletin

    (2004)
  • T. Chiang

    Economic transition and changing relation between income inequality and mortality in Taiwan

    British Medical Journal

    (1999)
  • R.S. Cooper et al.

    Relationship between premature mortality and socioeconomic factors in black and white populations of US metropolitan areas

    Public Health Reports

    (2001)
  • M. Daly et al.

    Income inequality and homicide rates in Canada and the United States

    Canadian Journal of Public Health

    (2001)
  • M.C. Daly et al.

    Macro-to-micro links in the relation between income inequality and mortality

    Milbank Quarterly

    (1998)
  • G. Davey Smith et al.

    Commentary: Understanding it all—health, meta-theories, and mortality trends

    British Medical Journal

    (1996)
  • R. De Vogli et al.

    Income inequality and life expectancy: Evidence from Italy

    Journal of Epidemiology and Community Health

    (2005)
  • Cited by (1259)

    View all citing articles on Scopus
    View full text