Introduction

Top Abstract Introduction Methods Results Discussion References

The "income inequality hypothesis" says that disparities in income among members of a community affect their health and, specifically, that economically egalitarian communities or societies have better health outcomes than more unequal communities.^1-3 Some proponents argue that inequality in incomes is a stronger determinant of health than the income of individuals or families.¹

Initial support for the income inequality hypothesis came from aggregate level studies of total mortality or cause specific mortality. ^{1 4-10} More recent studies show mixed results once individual characteristics are included in the analysis.^11-17 This study re-examines the income inequality hypothesis with measures of health that reflect the presence or absence of 17 chronic physical conditions and specific disorders of mental health, by using data from a survey carried out in 1997-8 in 60 metropolitan or economic areas across the United Sates.

	Methods

Top Abstract Introduction Methods Results Discussion References

Sources of data
"Healthcare for Communities" is a household telephone survey clustered in 60 randomly selected metropolitan areas or economic areas of the United States; it was carried out in 1997-8.¹⁸ This analysis focuses on 8235 respondents living in the 60 sites for which measures of income inequality are available (1337 respondents lived outside the 60 sites).

Outcome measures
For comparability with previous studies we analysed the self reported general health status of respondents and created an indicator for a response of poor or fair. ^{13 14 17} Our measure of mental health considered four psychiatric disordersmajor depressive disorder, dysthymic disorder, panic disorder, and generalised anxiety disorderwhich we assessed by using the composite international diagnostic interview, short form, plus role limitation for panic disorder.^19-21

We assessed physical health from answers to questions about 17 chronic health conditions: asthma; diabetes; hypertension; arthritis; a physical disability; trouble breathing; cancer; a neurological condition; stroke or paralysis; angina, heart failure, or coronary artery disease; chronic back problems; stomach ulcer; chronic liver disease; migraine or chronic severe headaches; chronic bladder problems; chronic gynaecological problems (women only); and unspecified chronic pain. We report results for the overall number of conditions and for the more common individual conditions or conditions that may have psychosocial components.

Income inequality, individual income, and other independent variables
We calculated income inequality at site level from the community tracking study. The results shown are based on the Gini coefficient,^22-24 which ranges from 0.38 to 0.54 across the 60 communities. This is higher than the 0.27-0.35 range found in a British mental health study, indicating higher levels of inequality.²⁵ Income at the individual level was measured as family income, which includes earnings from work, transfer income, and other sources.

Analyses
We grouped respondents by fifths of family income and by community level inequality and calculated a weighted mean for the prevalence of each condition in each group. We tested the association between prevalence of medical conditions and family income or inequality by using individual level logistic regressions with an indicator of a health condition as the dependent variable. We tested the association both with and without adjustment for other individual level sociodemographic variables such as age, sex, race or ethnicity, and size of family.

	Results

Top Abstract Introduction Methods Results Discussion References

The prevalence of most conditions decreased continuously across most of the income range (fig 1). However, the magnitude of the drop in the prevalence of health problems tended to be largest from the lowest fifth to the next fifth. The association between family income and prevalence was highly significant for almost all conditions (table 1).

View larger version (23K): [in this window] [in a new window]   Fig 1.   Adjusted prevalence of chronic conditions by fifths of family income

Consistent with previous studies, we found a highly significant (P<0.01) association between high income inequality and the probability that a person reports being in poor or fair health, although the finding was not robust to adjustment for other sociodemographic factors (table 2) Except for this self reported health measure, however, there was no discernible pattern in health outcomes by income inequality (fig 2). A third of the conditions were most prevalent in communities with average income inequality, and three health problems (depression, chronic pain, and asthma) were most prevalent in communities with low income inequality (bottom two fifths). With the exception of chronic gynaecological problems, we found no significant association between any specific health conditionchronic, mental, or otherwiseand income inequality (including conditions not shown). Even the significant result for gynaecological problems disappeared when individual sociodemographic variables were taken into account. In contrast, the highest prevalence for every condition occurred in one of the two poorest fifths as stratified by family income.

View larger version (28K): [in this window] [in a new window]   Fig 2.   Adjusted prevalence of chronic conditions by fifths of income inequality

	Discussion

Top Abstract Introduction Methods Results Discussion References

The relation between income inequality and health has been at the centre of a substantial amount of research, but the measures of health status that have been analysed to date have largely been limited to self reported health status or mortality in the case of physical health, and depressive symptoms or psychological distress for mental health. To our knowledge, this study is the first to explore the association between income inequality and several specific physical conditions as well as particular mental health disorders. Although our data confirm the association between income inequality and poor or fair self reported health, no similar relation exists between income inequality and depressive disorders or anxiety disorders or any of the medical conditions assessed, either at the population level or among people with lower incomes, wealthier people, women, or men. On the other hand, family income and education, which may reflect rank in the social hierarchy, are strongly related to health. Their effects are not confined to differences between the lowest income group and other groups (which would point towards material deprivation as an explanation) but show a gradient that flattens well above the median income level. This finding is similar to that of the Whitehall studies of British civil servants, where social gradients in morbidity and mortality ran from the bottom to the top of the hierarchy.^26-28

The sample size of this study provides good statistical power to detect differences between fifths of inequality up to 75% smaller than the estimated differences between fifths of family income. Smaller inequality effects (that is, more than 75% smaller than the estimated differences between fifths of income) may not be detectable, however. Measurement error in the site level inequality measure could also bias estimates downward, but the results were unchanged for alternative inequality measures at the state level.

Although we found no empirical support for the hypothesis that income inequality affects mortality or self rated health status through higher rates of specific medical conditions, the results do not necessarily contradict previously reported associations between income inequality and self rated health status or mortality. Factors linking income inequality to health may include the severity of disorder, the probability that a person receives a diagnosis conditional on having a disorder, and the way in which having a disorder determines people's perceptions of their health. But some of these factors are likely to be influenced by environmental factors other than income inequality, including state policies and healthcare infrastructure, that may be unrelated to income distribution. It seems premature to conclude that income inequality itself is an important risk factor for poor health, and the results highlight the need to better understand the psychological and physiological pathways through which the social environment affects health.

	Acknowledgments

We thank Michael Schoenbaum and Jürgen Unützer for comments, Jennifer Mellor and Jeff Milyo for providing their measures of income inequality, and Lingqi Tang and Fuan-Yue Kung for assistance with programming.

	Footnotes

Funding: Robert Wood Johnson Foundation, which funded the healthcare for communities survey, and the National Institute of Mental Health (R01-MH62124).

Competing interests: None declared.

The full version of this article appears on bmj.com

	References

Top Abstract Introduction Methods Results Discussion References

1.	Wilkinson RG. Unhealthy societies: the affliction of inequality. London: Routledge, 1996.
2.	Lynch JW, Smith GD, Kaplan GA, House JS. Income inequality and mortality: importance to health of individual income, psychosocial environment, or material conditions. BMJ 2000; 320: 1200-1204[Full Text].
3.	Marmot M, Wilkinson RG. Psychosocial and material pathways in the relation between income and health. BMJ 2001; 322: 1233-1236[Full Text].
4.	Rodgers GB. Income and inequality as determinants of mortality: an international cross-section analysis. Population Studies 1979; 33: 343-351.
5.	Flegg A. Inequality of income, illiteracy and medical care as determinants of infant mortality in developing countries. Population Studies 1982; 36: 441-458.
6.	Le Grand J. Inequality in health: some international comparisons. Eur Econ Rev 1987; 31: 182-191.
7.	Wilkinson RG. Income distribution and life expectancy. BMJ 1992; 304: 165-168[Medline].
8.	Kaplan GA, Pamuk E, Lynch JW, Cohen RD, Balfour JL. Income inequality and mortality in the United States: analysis of mortality and potential pathways. BMJ 1996; 312: 99-1003.
9.	Kennedy BP, Kawachi I, Prothrow-Stith D. Income distribution and mortality: cross-sectional ecological study of the Robin Hood index in the United States. BMJ 1996; 312: 1004-1007[Abstract/Full Text].
10.	Mellor JM, Milyo J. Reexamining the evidence of an ecological association between income inequality and health. J Health Polit Policy Law 2001; 26: 487-522[Medline].
11.	Fiscella K, Franks P. Poverty or income inequality as predictor of mortality: longitudinal cohort study. BMJ 1997; 314: 1724-1727[Abstract/Full Text].
12.	Daly MC, Duncan GJ, Kaplan GA, Lynch JW. Macro-to-micro links in the relation between income inequality and mortality. Milbank Q 1998; 76: 303-304, 315-39.
13.	Kennedy BP, Kawachi I, Glass R, Prothrow-Stith D. Income distribution, socioeconomic status, and self rated health in the United States. BMJ 1998; 317: 917-921[Abstract/Full Text].
14.	Soobader MJ, LeClere FB. Aggregation and the measurement of income inequality: effects on morbidity. Soc Sci Med 1999; 48: 733-744[Medline].
15.	Fiscella K, Franks P. Individual income, income inequality, health, and mortality: what are the relationships? Health Serv Res 2000; 34: 307-318.
16.	Mellor JM, Milyo J. Income inequality and health status in the United States: evidence from the current population survey. J Hum Resources (in press).
17.	Kahn RS, Wise PH, Kennedy BP, Kawachi I. State income inequality, household income, and maternal mental and physical health: cross sectional national survey. BMJ 2000; 321: 1311-1315[Abstract/Full Text].
18.	Sturm R, Gresenz C, Sherbourne CD, Minnium K, Klap R, Bhattacharya J, et al. The design of healthcare for communities: a study of health care delivery for alcohol, drug abuse, and mental health conditions. Inquiry 1999; 36: 221-233[Medline].
19.	Kessler RC, Andrews G, Mroczek D, Ustun B, Wittchen HU. The World Health Organization composite international diagnostic interview short-form (CIDI-SF). Int J Methods Psychiatric Res 1998; 7: 171-185.
20.	Ware JE, Kosinski M, Keller SD. A 12-item short-form health survey: construction of scales and preliminary tests of reliability and validity. Med Care 1996; 34: 220-233[Medline].
21.	World Health Organization. Composite international diagnostic interview (version 2.0). Geneva, Switzerland: WHO, 1995.
22.	Sen A. On economic inequality. Oxford: Oxford University Press, 1973.
23.	Cowell FA. Measuring inequality. Oxford: Allan, 1977.
24.	Jones AF, Weinberg DH. The changing shape of the nation's income distribution: 1947-1998. Current Population Reports P60-204, Bureau of the Census, 2000.
25.	Weich S, Lewis G, Jenkins SP. Income inequality and the prevalence of common mental disorders in Britain. Br J Psychiatry 2001; 178: 222-227[Abstract/Full Text].
26.	Marmot MG, Shipley MJ, Rose G. Inequalities in deathspecific explanations of a general pattern? Lancet 1984; 1: 1003-1006[Medline].
27.	Marmot MG, Shipley MJ. Do socioeconomic differences in mortality persist after retirement? 25 year follow up of civil servants from the first Whitehall study. BMJ 1996; 313: 1177-1180[Abstract/Full Text].
28.	Marmot MG, Davey Smith G, Stansfeld S, Patel C, North F, Head J, et al. Health inequalities among British civil servants: the Whitehall II study. Lancet 1991; 337: 1387-1393[Medline].

(Accepted 17 September 2001)