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Robert S Kahn a Division of General and Community Pediatrics,
Children's Hospital Medical Center, CH-1, 3333 Burnet Avenue,
Cincinnati, OH 45229, USA, b Department of Pediatrics, Boston University School
of Medicine and Boston Medical Center, Boston, MA 02118, USA, c Division of
Public Health Practice, Harvard School of Public Health, Boston, MA
02115, d Department of Health and Social Behavior, Harvard School of
Public Health, Boston
Correspondence to: R Kahn kahnr0{at}chmcc.org
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Abstract |
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Objective:
To examine the association of state income inequality and individual household income with the mental and physical
health of women with young children.
While the importance of socioeconomic status for health is well
documented,
1 2
several recent studies suggest that
the relative distribution of income within a society is also an
important determinant of health.3-7 Large inequalities in
income between the rich and the poor correlate with lower overall life
expectancy and higher total age specific and cause specific mortality,
even after adjustment for absolute income differences among the 50 US
states.3-6 Explanations for the association include
the possibility that increased income inequality is associated with a
society's lack of investment in social goods such as public education
and accessible health care. This underinvestment, in turn, is
associated with poor health outcomes.
8 9
Alternatively, or perhaps in addition, wider disparities in income may
be associated with the erosion of social cohesion within communities,
leading to increased risk of social isolation, stress and, ultimately,
poor health outcomes.
8 9
Given US projections that
the income after tax in the richest fifth of households rose by 43%
from 1977 to 1999 while that of the bottom fifth fell
9%,10 the potential health impact of income inequality
warrants further evaluation. The need is further underscored by the
fact that the United States has among the highest levels of income
inequality of any industrialised nation.11
Three important gaps remain in our understanding of the relation
between income inequality and health. Firstly, concern remains that the
effects of income inequality evident in studies that used aggregated or
ecological data are simply an artefact of an underlying relation
between individual income and health.12 The four studies
with the ecological and individual level income data necessary to
examine this concern have shown mixed
results.
8 13-15
One study found that individual
income could account entirely for the mortality effects attributed to
income inequality.13 Others have found that an adverse
effect of income inequality on self rated health persisted after
adjustment for individual income.
14 15
Secondly,
among these multilevel studies, only two examined potential
interactions between individual income and income inequality. Daly et
al found a negative effect of income inequality on mortality only for
middle income individuals,8 while Kennedy et al found that
the adverse effect of income inequality on self rated health diminished
with rising individual income.14 Determining whether the
adverse health effects of income inequality are the same for
individuals with high and low income is critical for understanding the
mechanisms at work. Finally, while many have hypothesised that income
inequality operates via psychosocial processes, there have been no
studies using individual level data to examine mental health outcomes.
We examined the joint effects of state income inequality and individual
household income on maternal mental and physical health to examine
whether women with low household income are most vulnerable to high
income inequality. We focused on women with young children as they tend
to be a population particularly vulnerable to state economic and social
welfare policy.
16 17
Sources of data
Outcome measures of maternal morbidity
Independent variables
Table 1.
Design:
Cross sectional study. Individual level data (outcomes, income, and other sociodemographic covariates) from a 1991 follow up survey of a birth cohort established in 1988. State level
income inequality calculated from the income distribution of each state
from 1990 US census.
Setting:
United States, 1991.
Participants:
Nationally representative stratified
random sample of 8060 women who gave birth in 1988 and were
successfully contacted (89%) in 1991.
Main outcome measures:
Depressive symptoms (Center for
Epidemiologic Studies depression score >15) and self rated health
Results:
19% of women reported depressive symptoms, and 7.5% reported fair or poor health. Compared with women in the
highest fifth of distribution of household income, women in the lowest
fifth were more likely to report depressive symptoms (33%
v 9%, P<0.001) and fair or poor health (15% v
2%, P<0.001). Compared with low income women in states with low
income inequality, low income women in states with high income
inequality had a higher risk of depressive symptoms (odds ratio 1.6, 95% confidence interval 1.0 to 2.6) and fair or poor health (1.8, 0.9 to 3.5).
Conclusions:
High income inequality confers an
increased risk of poor mental and physical health, particularly among
the poorest women. Both income inequality and household income are important for health in this population.
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Introduction
Top
Abstract
Introduction
Methods
Results
Discussion
References
![]()
Methods
Top
Abstract
Introduction
Methods
Results
Discussion
References
The data used in this analysis are from the 1991 longitudinal
follow up to the 1988 National Maternal Infant Health Survey. The 1988 study was a national, population based, stratified random sample of
9953 women who gave birth to live babies in 1988. The stratified design
oversampled black infants and very low and low birthweight infants. In
1991, 8285 women (89%) with children aged 26 to 48 months were
successfully reinterviewed in the longitudinal follow up survey. This
analysis used data from the 8060 women whose children still lived with them.
Symptoms of depression and self reported health were used as
measures of maternal morbidity. Depressive symptoms were measured with
the Center for Epidemiologic Studies Depression scale. Twenty questions
determine how many days in the past week a respondent felt each of a
range of positive and negative symptoms. A summary score of 16 or
higher is correlated with the presence of clinical
depression.18 Maternal self report of health status was
determined in response to the question "In general, would you say
your health is excellent, very good, good, fair, or poor?" For this
analysis, we collapsed maternal general health status to form a
dichotomous variable (0=excellent, very good, good; 1=fair, poor.)
Previous work has shown self reported health status to be highly
predictive of mortality, independent of health behaviours, comorbid
conditions, and access to health care.19
We used the Gini coefficient derived from the Lorenz curve of
household income within a state to measure income distribution. The
Lorenz curve plots the cumulative share of income held by the
cumulative tenths of households ranked from the poorest to the most
affluent. If each successive 10% of households held 10% of total
income, then the curve would be a diagonal line at 45°. The greater
the income disparity between tenths of households, the greater the
bowing of the curve away from the diagonal line and the higher the Gini
coefficient. The Gini coefficient ranges theoretically from 0.0 (perfect equality) to 1.0 (perfect inequality). The coefficient for
each of the fifty states was calculated from data in the 1990 United
States census population and housing summary tape file 3A. As some
states surveyed only a limited number of women, we grouped states into
approximate thirds of "low," "medium," or "high" inequality
(see table w1 on the BMJ 's website). Each woman
was assigned a categorical value based on her state of residence. Annual household income reported by the survey respondents was categorised into fifths ($10 000, 10-19 999, 20-34 999, 35-49 999,
>50 000). Covariates included categorical variables for age (<20,
20-29,
30 years), marital status (never married, married, divorced/separated), education (below high school degree, high school
degree, above high school degree), race/ethnicity (non-Hispanic white,
non-Hispanic black, Hispanic), and the number of people living in the
household.
Analysis
Initial bivariate analyses examined the associations between
maternal morbidity and state income inequality, individual income,
race/ethnicity, education, and marital status. The independent contributions of these variables were then determined in multivariate logistic regression analyses. Given our hypothesis that the effect of
income disparity would vary by individual income, the multivariate analyses were stratified and run separately for each income group.
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Results |
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Table 1 shows the demographic and health characteristics of a nationally representative sample of women who gave birth in 1988. About 19% of women had symptoms of depression and 7.5% of women reported themselves as in fair or poor health. Women who were young, unmarried, less educated, or not white were significantly more likely to have symptoms of depression and to rate themselves as in fair or poor health. A strong gradient was found between lower income and both depressive symptoms and fair or poor health. Women living in states with high income inequality were somewhat more likely to report worse mental and physical health.
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Figures 1 and 2 show the associations of fifth of household income and state income inequality with the two maternal outcomes. Compared with women with low income living in more egalitarian states, those living in high inequality states were significantly worse off in terms of depressive symptoms (34.8% v 24.3%, P<0.05) and self rated health (16.9% v 8.9%, P<0.05).
We used multivariate models to determine the independent associations of household income and income inequality with maternal health, adjusted for other individual level sociodemographic factors, including race/ethnicity, education, marital status, age, and household size (table 2). The three models presented respond to the common critique that household income and demographic characteristics probably confound estimates of the effects of income inequality. Model 1 is the effect of income inequality alone, model 2 estimates the effect of state income inequality independent of individual income and vice versa, and model 3 estimates their independent effects adjusted for other key sociodemographic characteristics. The estimates of the association between income inequality and both maternal health outcomes were modestly attenuated by the addition of individual income to the models. Full adjustment for covariates further attenuated the odds ratios, though income inequality remained a significant negative predictor of depressive symptoms.
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The effects of income inequality stratified by household income were also examined. Among the poorest women, both high and medium income inequality were associated with an increased risk of depressive symptoms (table 3). A similar relation was observed between income inequality and fair or poor health, though this association was not significant. High income inequality was most strongly associated with depressive symptoms (odds ratio 2.3, 95% confidence interval 1.2 to 4.4) and fair or poor health (2.4, 1.0 to 5.8) among women in the lowest tenth of income distribution.
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To estimate the joint association of income inequality and household
income, the reference group used was women in the top fifth of income
distribution who were living in states with low income inequality.
Compared with this group, women in the lowest fifth of income
distribution who also lived in high inequality states were
significantly more likely to have depressive symptoms (3.6, 1.8 to 7.3)
but not more likely to be in fair or poor health (2.3, 0.8 to 6.5)
after adjustment for covariates. Consistent with the bivariate results
in figure 2, certain health advantages could conceivably accrue to
women in the top fifth of income distribution in high inequality
states, particularly if the mechanisms at work were based more on
structural than psychosocial processes. In an exploratory analysis,
we used this group (top fifth, high inequality) as the reference group.
Compared with this reference group the poorest women in high inequality
states were more likely to be in fair or poor health (4.3, 2.0 to 9.3).
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Discussion |
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High state income inequality is associated with poorer mental and physical health in women with young children, these associations being most pronounced in women with low incomes. In our study high state income inequality was associated with a 60% greater risk of depressive symptoms and an 80% greater risk of fair or poor health among women already substantially worse off because of low household income.
Our findings show the joint influence of high income inequality and low individual income on health. Studies of income inequality that use ecological data have not had the capacity to discern the unique effects of individual income,12 while studies of income that use individual level data often fail to examine broader contextual influences. 21 22 Among multilevel studies, only one reported that both income inequality and individual income increase the risk of poor health. Kennedy et al found that high income inequality significantly increased the odds of fair or poor health among those adults already at two to threefold greater risk because of low individual income14; however, high income inequality was not associated with adverse health among those in the top income bracket in that study (>$35 000 (about £23 000)). Fiscella and Franks found no independent effect of income inequality on mortality but did not explore potential interaction effects.13 Daley et al found an adverse effect of income inequality on adult mortality among middle income adults but did not report the effects of individual income.8 Our results suggest the need for more concerted efforts to explore the combined effects of income inequality and individual income.
The need for a more balanced approach is underscored by the fact that the poorest fifth of families in the United States face both relative and absolute poverty. From 1968 to 1997 their relative income position worsened as the average income of the richest fifth of households rose from 10.2 to 13.6 times their own.23 During the same period their mean income remained essentially equal to the US poverty line. Given that the US poverty threshold is not updated for rising standards of living, the living conditions of the poor often reflect absolute deprivation: 11% report not having enough food to eat, 39% report living conditions with rats, mice, or cockroaches, and 20% report not seeking needed medical care. 24 25 Ultimately, the suggestion that relative inequality matters for health should not detract from the continuing importance of absolute deprivation for young families.
Possible mechanisms
The mechanisms by which income inequality influences maternal
mental and physical health are not clear, although several pathways are
possible. Consistent with the hypothesis that income inequality is
associated with systematic underinvestment in social infrastructure,
Kaplan et al found high correlations between income inequality and such
indicators as state per capita medical care expenditure and
unemployment.5 Medical care and labour markets that poorly
accommodate women with young children might offer mechanisms by which
income inequality operates in this population. More generally, income
inequality has been correlated with women's level of political
participation (for example, voter turnout, representation in elected
office) and economic autonomy (for example, access to health insurance,
business ownership) at the state level.26 Truncated
political and economic opportunities for women in states with high
income inequality may ultimately lead to the poor health of women,
including low income mothers with young children.
Interpretation
Several factors should be considered in the interpretation of
these results. Firstly, the data are cross sectional and therefore
limit any inferences regarding causation. For example, income and
health might both be associated with migration between states. Poor
women in good health might be better able to move to states with low
inequality and more generous family benefits. Furthermore, poor
maternal mental health might lead to lower household income. Studies
that used more longitudinal data found limited evidence for income
drift.29 Further studies examining trends in income
inequality, individual income, and health are needed. Secondly, the
data on income inequality are not adjusted for taxes, benefits, and
household size. Previous work, however, suggests such adjustment of US
income data makes little difference in the relation between income
inequality and health.
30 31
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What is already known on this topic
Among the few studies of income inequality, individual income, and health, the two that considered an interaction between income inequality and individual income found mixed results Though income inequality is commonly thought to operate via psychosocial mechanisms, no previous studies have used individual level data to examine such mental health outcomes What this study addsWomen with young children in the lowest fifth of distribution of household income were at substantially higher risk of depression and poor health; the risk being further increased if women also lived in states with high income inequality Household income and income inequality operated jointly to influence maternal mental and physical health, suggesting a more integrated understanding of the two may help to focus research on the mechanisms at work |
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Acknowledgments |
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Contributors: RSK conceived the study, analysed the data, and participated in writing the paper. IK, BPK, and PHW assisted with the design, the interpretation of the data, and the editing of the paper. RSK is the guarantor of this paper.
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Footnotes |
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Funding: Ichiro Kawachi and Bruce P Kennedy are recipients of the RWJF Investigator Awards in Health Policy Research. Ichiro Kawachi is a core member of the MacArthur Foundation Network on Socioeconomic Status and Health.
Competing interests: None declared.
An extra table on state income
disparity can be found on the BMJ's website
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References |
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(Accepted 11 September 2000)
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