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John W Lynch a Department of Epidemiology, School of Public
Health, University of Michigan, 109 Observatory Street, Ann Arbor,
MI 48109-2029, USA, b Survey
Research Center, Institute for Social Research, University of Michigan, c Department of Social
Medicine, University of Bristol, Bristol BS8 2PR
Correspondence to: J W Lynch jwlynch{at}sph.umich.edu
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.
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
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 studies8-11 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 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 Wilkinson's demonstration that absolute income was unrelated
(r=0.08) to health among developed countries has been
important in staking a claim for this psychosocial theory of health
inequalities.26 Figures 1 and 2 show the association
between gross domestic product per person and life expectancy for 155 countries and for the 33 countries where gross domestic product was
greater than $10 000
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
![]()
Methods
![]()
The individual income interpretation
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.
but it may also encourage
underestimation of the overall population effects of unequal income distribution.
![]()
The psychosocial environment interpretation
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.
the cut-off used by Wilkinson.26
Our results, however, include data for all the countries above
$10 000, not a selection of some countries in the Organisation for
Economic Cooperation and Development as used by Wilkinson. The
correlation between life expectancy and gross domestic product per
person in the complete sample is r=0.51 (P=0.003). Thus the
association between absolute income and life expectancy among wealthier
countries depends on which countries are
included.

View larger version (22K):
[in a new window]
Fig 1.
Gross domestic product per person in US dollars
(adjusted for purchasing power parity) and life expectancy in 155 countries, circa 1993

View larger version (22K):
[in a new window]
Fig 2.
Gross domestic product per person in all 33 countries with GDP/person greater than $10 000
For 15 developed countries with comparable income inequality data, Lynch and colleagues showed that indicators of social capital, such as trust and belonging to and volunteering for community organisations, were all much more strongly related to gross domestic product per person than to income inequality.27 Diener and colleagues showed that absolute income was a better predictor of subjective wellbeing than relative income, and concluded that "exposure in natural settings to others who are better off will not automatically influence one's moods in a negative way."28 In other analyses, social capital measured as trust and organisational membership mediated the cross sectional association between income inequality and mortality in US states.29 However, this association is difficult to interpret given that time series analyses of data from the same source show little decline in levels of trust, fairness, and helpfulness from the mid-1960s to 1994.30 The psychosocial hypothesis would lead to the expectation that these indicators of social capital should have deteriorated during this period of unprecedented increases in income inequality. In sum then, a broader consideration of relevant research raises questions about the evidence used to exclude absolute income and material conditions, and about the evidence in favour of a mainly psychosocial interpretation of health inequalities.
Areas of concern
We do not deny negative psychosocial consequences of income
inequality, but we argue that 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.
27 31-35
In this regard, the psychosocial
interpretation raises several areas of concern.
| |
The neo-material interpretation |
|---|
The neo-material interpretation says that health inequalities
result from the differential accumulation of exposures and experiences that have their sources in the material world. Under a neo-material interpretation, the effect of income inequality on health reflects a
combination of negative exposures and lack of resources held by
individuals, along with systematic underinvestment across a wide range
of human, physical, health, and social
infrastructure.
3 5 7 32
An unequal income distribution
is one result of historical, cultural, and political-economic
processes. These processes influence the private resources available to
individuals and shape the nature of public infrastructure
education,
health services, transportation, environmental controls, availability
of food, quality of housing, occupational health regulations
that form
the "neo-material" matrix of contemporary life. In the US, higher
income inequality is significantly associated with many aspects of
infrastructure
unemployment, health insurance, social welfare, work
disability, educational and medical expenditure, and even library books
per capita.5
Thus income inequality per se is but one manifestation of a cluster of
neo-material conditions that affect population health. This implies
that an aggregate relation between income inequality and health is not
necessary
associations are contingent on the level and distribution of
other aspects of social resources. If income inequality is less linked
to investments in health related public infrastructure, the aggregate
level association between income inequality and health may break down.
In fact, recent evidence from Canada supports this view.40
This is in contrast to the psychosocial hypothesis, which implies a
universal association. Perceptions of relative position will always be
present, regardless of the actual living conditions for those at the
bottom of the social hierarchy. Evidence from animal studies on the
role of social hierarchy itself in generating health differences has
been used to support this aspect of the psychosocial
hypothesis.25 Health effects of social hierarchy in
animals are, however, contingent on relations between social position
and material living conditions such as availability of food, water, and
space. Sapolsky, an eminent primate
researcher, has recently proclaimed that "it seems virtually
meaningless to think about the physiological correlates of rank outside
the context of a number of other modifiers
the sort of society in
which the rank occurs."41
A metaphor
To appreciate how neo-material conditions can influence health, it
may be useful to consider the metaphor of airline travel. Differences
in neo-material conditions between first and economy class may produce
health inequalities after a long flight. First class passengers get,
among other advantages such as better food and service, more space and
a wider, more comfortable seat that reclines into a bed. First class
passengers arrive refreshed and rested, while many in economy arrive
feeling a bit rough. Under a psychosocial interpretation, these health inequalities are due to negative emotions engendered by perceptions of
relative disadvantage. Under a neo-material interpretation, people in
economy have worse health because they sat in a cramped space and an
uncomfortable seat, and they were not able to sleep. The fact that they
can see the bigger seats as they walk off the plane is not the cause of
their poorer health. Under a psychosocial interpretation, these health
inequalities would be reduced by abolishing first class, or perhaps by
mass psychotherapy to alter perceptions of relative disadvantage. From
the neo-material viewpoint, health inequalities can be reduced by
upgrading conditions in economy class. Of course, this simplistic
metaphor assumes that conditions in first class and economy class are
independent
in the real world, improvements in economy are often
resisted by those able to travel first class.
Examples from India and Britain
Cross nationally, higher levels of social expenditures
markers of
neo-material conditions
are associated with greater life expectancy,
lower maternal mortality, and a smaller proportion of low birthweight
babies.42 Thus, strategic social investment may be
important in determining health differences between countries.
Interpretation of health differences between and within countries
should be based on a historical view of social conditions and policies.
Consider, for example, the widely discussed favourable health situation
in Kerala state, India.43 Despite low individual income
the infant mortality, maternal mortality, childhood mortality, and
overall mortality in Kerala are better than in other Indian states and
approach levels in richer, industrialised countries. Greater
redistributive actions of the Kerala government over recent decades
have been viewed as the phenomenon underlying this. It is also the
case, however, that the social and cultural basis for these favourable
health outcomes can be traced to over a century of social activities
that have promoted greater gender equality, education, and general
public investment in human resources.44
0.76 for men aged 55-64 to
r=
0.86 for women aged 45-54 (fig 3). Understanding the
rapid decline in mortality in middle age against a background of
escalating income inequality in Britain may require consideration of
earlier social investments. Expansion of the welfare state, educational opportunities, and introduction of the NHS had positive influences in
early life for those cohorts in which mortality is currently declining,
and social circumstances in early life can have important long term
effects on later risk of death.
45 46
Such findings encourage a view that health in adulthood is the outcome of socially patterned processes acting across the entire life
course.47 This perspective would lead to attention being
paid to how income inequality
and the broader social processes which
income inequality indexes
influences health across the life course of
successive cohorts. In several countries, the burden of increased
income inequality has fallen disproportionately on poor households
containing young children, and this may lead to poor health outcomes in
the future.45-48
|
| |
Conclusions |
|---|
A combination of the individual income and neo-material
interpretations is a better fit to the available evidence on income inequality and health, is more comprehensive, and has greater potential
to inform interventions that advance public health and reduce
inequalities. The psychosocial environment interpretation focuses
attention on aspects of personal psychological functioning such as
trust, respect, and support. It is hard to understand how this emphasis
on psychological functioning and informal interpersonal relations would
serve as a basis for a public policy agenda to reduce health
inequalities. The neo-material interpretation is an explicit
recognition that the political and economic processes that generate
income inequality influence individual resources and also have an
impact on public resources such as schooling, health care, social
welfare, and working conditions. It is strategic investments in
neo-material conditions via more equitable distribution of public and
private resources that are likely to have the most impact on reducing
health inequalities and improving public health in both rich and poor
countries in the 21st century.
| |
Acknowledgments |
|---|
The idea for this paper arose from a meeting of invited participants who discussed the effects of income inequality on health in June 1998 at the University of Michigan School of Public Health. The meeting was sponsored by the University of Michigan Initiative on Inequalities in Health; the Survey Research Center at the University of Michigan; the Health Institute at the New England Medical Center; the Canadian Institute for Advanced Research Population Health Program, and the Population Health Program at the University of Texas at Houston, Health Sciences Center.
| |
Footnotes |
|---|
Funding: JSH was supported by a health investigator award from the Robert Wood Johnson Foundation. The other authors received no funding for this research.
Competing interests: None declared.
| |
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(Accepted 22 February 2000)
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