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Community-level income inequality and mortality in Québec, Canada
Public Health 123 (2009) 438–443
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Original Research
Community-level income inequality and mortality in Que´bec, Canada N. Auger a, b, c, *, G. Zang b, M. Daniel b, c, d ´tudes et analyses de l’e´tat de sante´ de la population, Institut national de sante´ publique du Que´bec, Montre´al, Que´bec, Canada Unite´ E Centre de recherche du Centre hospitalier de l’Universite´ de Montre´al, Montre´al, Que´bec, Canada c De´partement de me´decine sociale et pre´ventive, Universite´ de Montre´al, Montre´al, Que´bec, Canada d School of Health Sciences, University of South Australia, Adelaide, South Australia, Australia a
b
a r t i c l e i n f o
s u m m a r y
Article history: Received 29 October 2008 Received in revised form 1 April 2009 Accepted 29 April 2009 Available online 30 May 2009
Objectives: Evidence of the association between income inequality and mortality for small rather than large areas is conflicting. This study evaluated community-level income inequality in relation to age- and cause-specific mortality. Study design: Ecological analysis.
Keywords: Income distribution Mortality Suicide Cardiovascular diseases Tobacco Alcohol-related disorders
Methods: Mortality data were extracted from the Que´bec, Canada registry for 1999–2003. For Que´bec communities (n ¼ 143), directly standardized mortality rates were calculated for all-cause (overall, working-age and post-working-age), suicide, alcohol, tobacco and cardiovascular mortality. Using 2001 Canada Census data, the tertiles of income inequality measured as the decile ratio, coefficient of variation and median share were calculated. The relative risk (RR) of death was determined using Poisson regression, accounting for median community income, family structure and rural–urban area. Results: Income inequality was most strongly associated with alcohol-related mortality (RRCoefficientVariation 0.85, 95% confidence interval 0.77–0.94), followed by statistically significant but weaker inverse associations with tobacco-related and age-specific all-cause mortality. Conclusions: Income inequality in Que´bec communities is inversely associated with mortality outcomes, particularly alcohol-related mortality. These associations contrast with positive or null associations observed in studies of larger US and Canadian metropolitan areas, respectively. Community-level studies accounting for individual-level covariates are necessary to clarify the relationship between income inequality and mortality. Ó 2009 The Royal Society for Public Health. Published by Elsevier Ltd. All rights reserved.
Introduction Income distribution in industrialized countries, measured using indicators of income inequality, has been linked with mortality. Income inequality indicators express the extent to which extremes of wealth and poverty are present within countries, as opposed to composite indices of deprivation which reflect central tendencies or the mean wealth of places. Associations between income inequality and mortality within countries are not fully understood. Income inequality in US states1–8 and metropolitan areas6,9,10 is associated with elevated mortality. Studies conducted in Canada,
* Corresponding author. Unite´ E´tudes et analyses de l’e´tat de sante´ de la population, Institut national de sante´ publique du Que´bec, 190, boulevard Cre´mazie Est, Montre´al, Que´bec H2P 1E2, Canada. Tel.: þ1 514 864 1600; fax: þ1 514 864 1616. E-mail address: [email protected] (N. Auger).
South America, Europe and Asia have demonstrated positive11–18 or null associations17,19–23 between income inequality and mortality. The bulk of research indicates more consistent associations for large areas such as states or metropolitan centres.24,25 The association between income inequality and mortality is less supported by studies of smaller areas.14–18,21–23,25 The influence of income inequality on morbidity has also been found to depend on level of geographic aggregation.26 In Canada, income inequality for small areas is associated with self-rated health27–29 and all-cause mortality,18 yet such associations have not been observed for larger metropolitan regions of Canada.6,27,30 Various factors may contribute to such discrepancies, including the possibility that income inequality expressed at the metropolitan level could mask relationships at smaller levels. Additional research evaluating local relationships between income inequality and mortality is necessary in order to compare results between small and large areas of Canada. Research on income
0033-3506/$ – see front matter Ó 2009 The Royal Society for Public Health. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.puhe.2009.04.012
N. Auger et al. / Public Health 123 (2009) 438–443
inequality in smaller areas is also useful to better understand how income inequality may contribute to or mediate area effects on health. The literature is further complicated by US data indicating that younger age groups may be more susceptible to income inequality5,6,9,24,31; a finding not observed for settings outside the USA.10 Whether income inequality within countries is more strongly related to specific causes of death, such as mental-healthrelated (e.g. suicide) or lifestyle-related (e.g. tobacco) mortality, is also unclear. In US states, income inequality has been linked to risk factors for cardiovascular mortality32,33 and alcohol dependence,34 but findings have been conflicting for mental health disorders.34–36 In contrast, income inequality in small areas of the USA and the UK has been linked to poor mental health37–39 and substance use.40,41 This study sought to evaluate community-level income inequality in relation to age effects and all-cause and cause-specific mortality in communities of Que´bec, Canada. It was hypothesized that community-level income inequality would be associated with elevated mortality, and that associations would be stronger for working-age individuals and for mortality related to mental health or health-related behaviours. Methods Data and study setting Non-infant deaths (1 year of age; n ¼ 271,068) were extracted from the Que´bec, Canada death file for the period 1999–2003. The total population in Que´bec in 2001 was 7,396,988. Seven mortality outcomes were considered: (1) (2) (3) (4)
all-cause overall mortality; all-cause working-age mortality (25–64 years); all-cause post-working-age mortality (65 years); suicide for population aged 10 years (ICD-9 codes E950–959 for 1999; ICD-10 codes X60–84/Y87.0 for 2000 onwards); (5) alcohol-related mortality for all ages [defined as oesophageal cancer (ICD-9 code 150; ICD-10 code C15), laryngeal cancer (161; C32), alcohol-dependency syndrome (303; F10), chronic liver disease and cirrhosis (571; K70/K73/K74/K76), and all external causes of death (E800–E999; V00–V99/W00–W99/ X00–X99/Y00–Y99)]; (6) tobacco-related mortality in the population aged 20 years [defined as oropharyngeal cancer (140–149; C00–C14), laryngeal cancer (161; C32), tracheobronchial/pulmonary cancer (162; C33/C34), oesophageal cancer (150; C15), ischaemic cardiopathy (410–414; I20–I25), cerebrovascular (430–438; I60–I69), and chronic lower respiratory tract disease (490–496; J40–J47)]; and (7) cardiovascular mortality in the population aged 20 years [defined as ischaemic cardiopathy (410–414; I20–I25), cerebrovascular (430–438; I60–I69), and arterial disease (440–448; I70–I78)].
Mortality was calculated for local community service centres (CLSC, n ¼ 166); areas of health service delivery corresponding to well-defined communities in Que´bec42 where variability in arealevel deprivation has been documented.43 Communities were linked to 11,612 census dissemination areas (DAs); the smallest census unit for which socio-economic data were available (mean 75 DAs per community; 120 DAs not fully nested in a community were assigned to the community containing the largest portion). Four Aboriginal communities in northern Que´bec were excluded as valid socio-economic census data were not available. Of the remaining 162 communities, 19 were merged following standard Que´bec
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Ministry of Health practice to correct small cell sizes and improve stability of health indicators. The final sample contained 143 communities. Communities had a mean population of 51,564 in 2001 (median 45,728; range 3700–142,025). Variables Income inequality may be sensitive to the choice of indicator.12 Therefore, three indicators of community income inequality were calculated: (1) decile ratio (DR),44 defined as the ratio of DAs with the highest to lowest mean population-weighted proportion of the population below the low income cut-off (income level at which economic families or unattached individuals spend 20% more than average on food, shelter and clothing, depending on family size and degree of urbanization)45; (2) coefficient of variation (CV) of mean household income (it was verified that household income was normally distributed)44; and (3) median share (MS), defined as the proportion of total household income received by the least well-off 50% of households.44 One community with an MS of 0.56 (an impossible value that arose due to sparse data) was recoded as 0.49. Analyses both including and excluding this community indicated no influence on results. Data required for calculation of the Gini coefficient were not available at the level of census DA units (these being nested within communities and the basis of inequality measures). Inferential analyses of the ecological association between income inequality and mortality may be biased by the average socio-economic status of communities.1,15 Therefore, median household income was used as a covariate in analyses. Other potential community confounders were also considered, including family structure (proportion of lone-parent families) and rural– urban area (calculated from the proportion of rural DAs). DAs were considered rural when they belonged to a municipality classified as such by Statistics Canada (municipalities are not equivalent to CLSCs).46 Three community rural–urban categories were created: urban (no rural DAs), rural (all rural DAs) and semi-urban (some rural DAs). Community predictors were examined as tertiles. Statistical analysis Directly standardized mortality rates were calculated for communities using 5-year age groups for all-cause mortality and 10-year age groups for cause-specific mortality. The 2001 Que´bec population served as the reference, and population counts were corrected for census under-enumeration.47 Preliminary genderspecific analyses indicated that associations were in similar directions for both genders. To conserve statistical power, analyses were not stratified by gender. Descriptive statistics were calculated for community-level predictors. An ecological analysis was then undertaken of community predictors in relation to mortality. Relative risks (RR) and 95% confidence intervals (CI) were computed using Poisson regression for univariate (crude) models, followed by multivariate models adjusting for covariates. Models contained a maximum of four independent variables given sample size considerations. An interaction term between income inequality and rural–urban status was tested to rule out an influence of income inequality that differed according to rural–urban area, and models stratified by rural–urban area were re-run. Due to the change in 2000 from ICD-9 to ICD-10 coding, it was verified that similar results were obtained in analyses restricted to 2000 onwards. The significance of parameter estimates was assessed with the Wald test. The GENMOD procedure in SAS Version 9.1 (SAS Institute Inc, Cary, NC, USA) was used, and results were corrected for overdispersion.
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Table 1 Characteristics of Que´bec communities,a 2001. n Population per community (n)
Median
Range
45,728
3700–142,025
Decile ratio Low inequality Moderate inequality High inequality
47 48 48
4.7 7.8 11.9
1.2–6.4 6.5–9.6 9.6–18.7
Coefficient of variation (%) Low inequality Moderate inequality High inequality
47 48 48
12.3 17.9 25.5
2.3–13.3 13.7–22.1 22.1–47.1
Median share (proportion) Low inequality Moderate inequality High inequality
48 48 47
0.42 0.40 0.36
0.41–0.49 0.38–0.41 0.21–0.38
Median household income ($) Low Moderate High
47 48 48
33,633 39,172 49,560
23,891–36,902 37,299–42,107 42,187–79,163
Lone-parent families (%) Low Moderate High
47 48 48
12.7 15.1 19.9
8.8–13.9 13.9–16.5 16.5–33.9
Rural–urban area (% rural DAs) Urban Semi-urban Rural
53 48 42
a
0 1.3–96.0 100
Refers to local community health centre regions.
Results Community-level income inequality had considerable heterogeneity, as indicated by the range of values for the DR (1.2–18.7), CV (2.3–47.1%) and MS (0.21–0.49) (Table 1). Median community income and the proportion of lone-parent families also ranged
widely. One-third (33.6%) of communities were semi-urban and 28.4% were rural. In general, all-cause mortality for all ages and for the workingage and post-working-age populations was lowest in areas with high income inequality, and this relationship was apparent for all three indicators of income inequality (Table 2). Suicide, alcoholand tobacco-related mortality were also lowest for areas with high income inequality for all three indicators. Relationships were inconsistent for cardiovascular mortality. A linear trend of reduced mortality with increasing income inequality was present for the DR across all mortality outcomes. This relation was only apparent in half of outcomes predicted by the CV and MS indicators. Moderate inequality was related to greatest mortality for the remaining outcomes. Mortality was lower for wealthier tertiles (indexed by median household income) and for tertiles with fewer lone parents (except suicide and alcohol-related deaths which were higher in areas with fewer lone parents). Mortality outcomes were also lower in urban areas. Tests for effect modification indicated no interaction between income inequality and rural–urban area. In univariate (unadjusted) models, the association between income inequality and mortality depended on the indicator (Table 3). With the DR, high relative to low income inequality was associated with a significantly lower risk of mortality for all outcomes except allcause post-working-age mortality and cardiovascular mortality. Risks were lowest for all-cause working-age (RRDR 0.83, 95% CI 0.77–0.91), suicide (RRDR 0.82, 95% CI 0.71–0.95) and alcohol-related mortality (RRDR 0.80, 95% CI 0.73–0.88). In addition, a linear trend suggesting greater risk with progressively lower income inequality was present for these outcomes. Income inequality was not associated with postworking-age mortality (RRDR 0.95, 95% CI 0.89–1.01). Income inequality measured with the CV and MS indicators was inversely related to mortality from suicide (RRCV 0.67, 95% CI 0.58–0.76; RRMS 0.75, 95% CI 0.65–0.86) and alcohol (RRCV 0.72, 95% CI 0.66–0.79; RRMS 0.76, 95% CI 0.69–0.83) alone.
Table 2 All-cause and cause-specific mortalitya per 100,000 population, Que´bec communities, 1999–2003. All causes (1 year) n ¼ 271,068
All causes (25–64 years) n ¼ 58,550
All causes (65 years) n ¼ 208,789
Suicide n ¼ 6794
Alcohol n ¼ 23,356
Tobacco n ¼ 105,090
Cardiovascular n ¼ 66,771
Decile ratio Low inequality Moderate inequality High inequality
799.2 727.3 723.2
335.6 273.1 267.2
4533.6 4270.0 4270.5
24.7 19.9 19.7
77.3 60.9 60.0
405.5 366.8 364.0
250.7 233.8 234.0
Coefficient of variation Low inequality Moderate inequality High inequality
751.6 786.4 715.3
299.4 298.5 271.3
4297.1 4599.4 4201.7
27.8 22.9 18.1
81.1 69.7 57.0
373.1 401.1 360.1
225.9 251.3 233.3
Median share Low inequality Moderate inequality High inequality
762.5 760.7 720.7
302.1 288.6 272.8
4370.1 4460.5 4234.1
26.0 20.7 19.0
79.7 63.4 58.7
379.5 392.7 359.9
234.2 250.7 230.6
Median household income Low 792.6 Moderate 740.3 High 716.8
339.2 293.1 250.7
4468.9 4271.4 4300.4
26.0 23.1 16.8
78.7 67.6 54.4
403.3 375.2 357.3
249.2 238.3 231.1
Lone-parent families Low Moderate High
714.8 753.7 747.7
259.4 275.9 306.4
4217.7 4453.1 4297.7
21.8 21.3 19.7
66.0 64.4 62.3
355.8 382.4 378.7
224.7 241.9 242.0
Rural–urban area Urban Semi-rural Rural
720.4 767.3 760.4
275.1 287.4 307.2
4231.4 4495.3 4326.7
17.0 24.1 27.7
55.7 70.3 81.7
362.0 392.2 377.1
233.6 247.6 229.7
a
Directly standardized mortality rates, using the 2001 Que´bec population as the standard.
N. Auger et al. / Public Health 123 (2009) 438–443
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Table 3 Crude associations between income inequality and mortality outcomes, Que´bec communities, 1999–2003.a
Table 4 Adjusted associations between income inequality and mortality outcomes, Que´bec communities, 1999–2003.a
Mortality
Decile ratio
Coefficient of variation
Median share
Mortality
Decile ratio
Coefficient of variation
Median share
All causes, ‡1 year Low inequality Moderate inequality High inequality
1 0.93 (0.88–0.99) 0.92 (0.87–0.98)
1 1.02 (0.96–1.09) 0.94 (0.89–1.00)
1 1.02 (0.96–1.08) 0.97 (0.91–1.03)
All causes, ‡1 year Low inequality Moderate inequality High inequality
1 0.93 (0.88–0.99) 0.91 (0.85–0.98)
1 1.01 (0.95–1.09) 0.94 (0.86–1.02)
1 1.02 (0.95–1.10) 0.97 (0.89–1.05)
All causes, 25–64 years Low inequality 1 Moderate inequality 0.88 (0.81–0.95) High inequality 0.83 (0.77–0.91)
1 1.01 (0.92–1.09) 0.92 (0.84–1.00)
1 0.98 (0.90–1.07) 0.93 (0.85–1.01)
All causes, 25–64 years Low inequality 1 Moderate inequality 0.93 (0.87–1.00) High inequality 0.92 (0.85–1.01)
1 1.02 (0.93–1.11) 0.96 (0.87–1.06)
1 0.99 (0.91–1.08) 0.95 (0.86–1.05)
All causes, ‡65 years Low inequality Moderate inequality High inequality
1 0.95 (0.89–1.01) 0.95 (0.89–1.01)
1 1.03 (0.97–1.10) 0.96 (0.90–1.03)
1 1.04 (0.97–1.10) 0.99 (0.93–1.06)
All causes, ‡65 years Low inequality Moderate inequality High inequality
1 0.93 (0.87–0.99) 0.91 (0.84–0.98)
1 1.02 (0.95–1.10) 0.93 (0.86–1.02)
1 1.03 (0.96–1.11) 0.98 (0.90–1.07)
Suicide Low inequality Moderate inequality High inequality
1 0.87 (0.76–1.01) 0.82 (0.71–0.95)
1 0.84 (0.74–0.96) 0.67 (0.58–0.76)
1 0.86 (0.75–0.99) 0.75 (0.65–0.86)
Suicide Low inequality Moderate inequality High inequality
1 0.95 (0.84–1.08) 1.01 (0.87–1.16)
1 0.92 (0.80–1.05) 0.84 (0.71–0.99)
1 1.05 (0.92–1.21) 0.99 (0.84–1.17)
Alcohol Low inequality Moderate inequality High inequality
1 0.86 (0.78–0.94) 0.80 (0.73–0.88)
1 0.89 (0.82–0.97) 0.72 (0.66–0.79)
1 0.85 (0.78–0.93) 0.76 (0.69–0.83)
Alcohol Low inequality Moderate inequality High inequality
1 0.92 (0.85–0.99) 0.94 (0.86–1.02)
1 0.95 (0.88–1.03) 0.85 (0.77–0.94)
1 0.95 (0.88–1.04) 0.90 (0.81–0.99)
Tobacco Low inequality Moderate inequality High inequality
1 0.93 (0.87–1.00) 0.92 (0.86–0.99)
1 1.05 (0.98–1.13) 0.96 (0.89–1.04)
1 1.04 (0.96–1.12) 0.98 (0.90–1.05)
Tobacco Low inequality Moderate inequality High inequality
1 0.93 (0.86–1.01) 0.91 (0.83–0.99)
1 1.03 (0.95–1.13) 0.94 (0.85–1.04)
1 1.02 (0.94–1.12) 0.95 (0.86–1.05)
Cardiovascular Low inequality Moderate inequality High inequality
1 0.96 (0.89–1.03) 0.96 (0.90–1.04)
1 1.11 (1.03–1.19) 1.04 (0.97–1.12)
1 1.07 (0.99–1.15) 1.01 (0.94–1.09)
Cardiovascular Low inequality Moderate inequality High inequality
1 0.95 (0.88–1.03) 0.94 (0.86–1.02)
1 1.07 (0.99–1.16) 1.00 (0.91–1.11)
1 1.02 (0.94–1.11) 0.95 (0.86–1.05)
a
Relative risk (95% confidence interval).
In covariate-adjusted models, associations between income inequality and mortality were of lesser magnitude, but in the same direction as those from univariate analyses (Table 4). High relative to low income inequality was associated with a significantly lower risk of mortality for all-cause post-working-age and overall mortality with the DR alone. The CV and MS were associated with a progressively lower risk of alcohol-related mortality in adjusted models; for the DR, moderate (rather than high) income inequality was associated with alcohol-related mortality. Statistically significant associations also remained for the CV with suicide mortality and for the DR with tobacco-related mortality. Income inequality was not associated with a higher risk of cardiovascular mortality in any adjusted model. Discussion In this ecological study of Que´bec communities, high relative to low income inequality was not associated with a higher risk of mortality, but was inversely associated with several mortality outcomes (Tables 3 and 4). All three indicators of income inequality were associated with a significantly lower risk of alcohol-related mortality, accounting for community income, proportion of lowincome families and rural–urban area. A significantly lower risk of all-cause post-working-age and overall mortality was observed for the DR in adjusted models, while a lower risk of suicide mortality was observed with the CV. While null associations were observed for other mortality outcomes, including working-age mortality, relations were in the same inverse direction. In general, linear trends of progressively lower risk were observed with increasing inequality. Of the three income inequality indicators, the DR best
a Relative risk (95% confidence interval), adjusted for median income, rural–urban area and proportion of lone-parent families.
captured trends in mortality rates (Table 2) and unadjusted risks (Table 3) across inequality tertiles. What is already known The majority of community-level mortality studies conducted in developed nations indicate either positive15,16 or null associations21,22 with income inequality. However, a small number of community-level studies have documented negative associations, as observed in the present study.48,49 The first, a prospective study of death/hospitalization from ischaemic heart disease in municipalities of Denmark (similar in size to Que´bec communities), found an inverse association among women but not men.49 In a prospective study of all-cause mortality in Norwegian municipalities, also comparable in size to Que´bec communities, associations with income inequality were in the expected direction among men <50 years of age, but inverse associations were observed for those aged 50 years (null associations were present among women).48 Similarly, the present study found an inverse association with all-cause mortality among post-working-age persons aged 65 years, but this association was not robust across all indicators of income inequality. However, the present results cannot be compared directly with the Danish38 and Norwegian48 studies, as this study was not prospective or gender-specific (preliminary analyses indicated that relationships with income inequality did not vary according to gender). Researchers have documented inverse associations with self-rated health in census tract neighbourhoods of Canada27 and Chicago.50 Others have found inverse associations in analyses of larger areas such as metropolitan centres51 or states,52 but these are difficult to compare
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with community-level studies. Although a recent descriptive study found lower working-age mortality in certain Canadian metropolitan centres with greater income inequality,51 other studies using multivariate regression reported no association.10,30 Some authors reporting inverse associations between income inequality and health have discussed potential mechanisms.27,48,50 One explanation is that high income inequality may be a characteristic of generally affluent neighbourhoods having wide variations in household income, whereas low income inequality may be a characteristic of less affluent neighbourhoods with less variation in household income.27 What else can explain the inverse associations between community income inequality and mortality in these data? Three indicators of income inequality were used, each sensitive to inequality at different ends of the income spectrum. Associations were detected primarily for the DR and CV (indicators sensitive to the upper end of the income spectrum), and were weakest for the MS (sensitive to the middle part of the income spectrum).30 In contrast, studies on larger population areas report stronger associations for measures of inequality sensitive to the middle of the income spectrum.12,44,53 One possibility is that indicators sensitive to opposite ends of the income spectrum represent, in smaller areas, an aspect of inequality favourably associated with mortality. Another possibility is that community-level indicators of income inequality may be proxies for other unrelated environmental factors. Some research suggests that the positive relationship between income inequality and mortality in ecological studies is due to a statistical artefact.54–56 However, this study found statistically significant associations between higher income inequality and lower mortality; the opposite to what would be expected if the results were due to a statistical artefact. Other possible mechanisms Age-specific analyses indicated that the magnitude of associations between income inequality and mortality were similar in both the working-age and post-working-age populations. This finding can be considered in light of studies performed in larger areas that document steeper relations between income inequality and mortality in younger populations, albeit in the opposite direction.5,6,9,24,31 Some authors maintain that the influence of income inequality is linked to relative mechanisms involving psychosocial comparisons and subsequent chronic stress and biological effects24; a process which may be more common in younger populations. The present results suggest that these mechanisms may apply to both the young and old populations. The tendency towards stronger associations for mentalhealth-related mortality (i.e. alcohol and, to a lesser extent, suicide) as opposed to lifestyle-related mortality (i.e. cardiovascular) further supports psychosocial mechanisms. The protective associations observed in the present study also suggest that psychosocial comparisons, if present, might be favourable for health in small (but not large) areas. Such a situation might arise if, for example, small inequalities in communities generate an environment in which striving for better life conditions is perceived positively. The absolute deprivation hypothesis, also identified in the literature, posits that underinvestment in material resources in unequal areas translates into unfavourable lifestyles that influence health.57 However, the absolute deprivation hypothesis does not rule out the possibility that inequality in communities (as opposed to larger areas) might be associated with greater investment in material resources, which is another possible explanation for these results. The present results for cause-specific mortality may be useful to better understand processes of social selection versus social causation.24 The former refers to selection of individuals into neighbourhoods based on individual-level characteristics, while the latter refers to causal relations between places and people. This study found that
protective associations were more consistent with income inequality for mortality related to alcohol use (Table 4). If social selection was operating, the data suggest that individuals at risk of alcohol mortality may self-select out of areas with high income inequality. That is, individuals with severe alcohol dependency may not have the means required for residence in areas with high income inequality, assuming such areas represent affluence. The weaker associations observed for cardiovascular and, to a lesser extent, suicide and tobacco-related mortality indicate that selection may be less important for risk factors related to these outcomes. Thus, the stronger association with alcohol-related mortality lends support to processes of social selection rather than causation. Nonetheless, more research is necessary to determine the processes underpinning these associations. This study was subject to limitations. The analysis was ecological and could not account for factors such as individual income that might confound or moderate relationships. Inference cannot extend to the individual level. Ecological estimates of contextual effects may themselves be subject to error.58 The unit of analysis was CLSCs with administratively defined borders, rather than borders defined by residents themselves; the latter may be more meaningful for estimating contextual effects. The sample contained 143 communities; a relatively small number of units which limited statistical power and thus the number of covariates that could be considered. It was not possible to account for health services utilization, which may influence relations between income inequality and mortality.59 Healthcare insurance, however, is universal in Que´bec, and equality of access and service provision independent of station in life is theoretically ensured. Census data (rather than tax return data) were used to calculate income inequality, and the census may not be the best source of income data.60 It was not possible to evaluate other indicators such as the Gini index, which may better represent income inequality in small communities. Last, some of the study outcomes were inter-related and thus not independent (e.g. tobacco-related and cardiovascular mortality). This analysis of administrative communities in a Canadian province found an inverse association between community income inequality and several mortality outcomes, accounting for community income, family structure and rural–urban area. Associations were strongest for alcohol-related mortality. These findings suggest that the link between income inequality and mortality is not clear-cut, and that spatial scale must be considered. As mortality differentials in Que´bec are not explained by income inequality, it is concluded that narrowlyfocused polices intending to reduce income inequality may be insufficient to attenuate social differentials in mortality. Acknowledgements The authors wish to thank Karine Le´ger, Geomatics Specialist, for calculating community-level measures of income inequality, and Robert Choinie`re for helpful comments on the manuscript.
Ethical approval This study was conducted as part of the Que´bec Population Health Surveillance Plan mandated by the Health Ministry and approved by the Public Health Ethics Committee. Funding This research was funded by MD through a Canada Research Chair award from the Canadian Institutes of Health Research, and a grant from the Canada Foundation for Innovation (#201252). MD is supported by a University of South Australia Research Chair for Social Epidemiology. The funding sources did not participate in
N. Auger et al. / Public Health 123 (2009) 438–443
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Original Research
Community-level income inequality and mortality in Que´bec, Canada N. Auger a, b, c, *, G. Zang b, M. Daniel b, c, d ´tudes et analyses de l’e´tat de sante´ de la population, Institut national de sante´ publique du Que´bec, Montre´al, Que´bec, Canada Unite´ E Centre de recherche du Centre hospitalier de l’Universite´ de Montre´al, Montre´al, Que´bec, Canada c De´partement de me´decine sociale et pre´ventive, Universite´ de Montre´al, Montre´al, Que´bec, Canada d School of Health Sciences, University of South Australia, Adelaide, South Australia, Australia a
b
a r t i c l e i n f o
s u m m a r y
Article history: Received 29 October 2008 Received in revised form 1 April 2009 Accepted 29 April 2009 Available online 30 May 2009
Objectives: Evidence of the association between income inequality and mortality for small rather than large areas is conflicting. This study evaluated community-level income inequality in relation to age- and cause-specific mortality. Study design: Ecological analysis.
Keywords: Income distribution Mortality Suicide Cardiovascular diseases Tobacco Alcohol-related disorders
Methods: Mortality data were extracted from the Que´bec, Canada registry for 1999–2003. For Que´bec communities (n ¼ 143), directly standardized mortality rates were calculated for all-cause (overall, working-age and post-working-age), suicide, alcohol, tobacco and cardiovascular mortality. Using 2001 Canada Census data, the tertiles of income inequality measured as the decile ratio, coefficient of variation and median share were calculated. The relative risk (RR) of death was determined using Poisson regression, accounting for median community income, family structure and rural–urban area. Results: Income inequality was most strongly associated with alcohol-related mortality (RRCoefficientVariation 0.85, 95% confidence interval 0.77–0.94), followed by statistically significant but weaker inverse associations with tobacco-related and age-specific all-cause mortality. Conclusions: Income inequality in Que´bec communities is inversely associated with mortality outcomes, particularly alcohol-related mortality. These associations contrast with positive or null associations observed in studies of larger US and Canadian metropolitan areas, respectively. Community-level studies accounting for individual-level covariates are necessary to clarify the relationship between income inequality and mortality. Ó 2009 The Royal Society for Public Health. Published by Elsevier Ltd. All rights reserved.
Introduction Income distribution in industrialized countries, measured using indicators of income inequality, has been linked with mortality. Income inequality indicators express the extent to which extremes of wealth and poverty are present within countries, as opposed to composite indices of deprivation which reflect central tendencies or the mean wealth of places. Associations between income inequality and mortality within countries are not fully understood. Income inequality in US states1–8 and metropolitan areas6,9,10 is associated with elevated mortality. Studies conducted in Canada,
* Corresponding author. Unite´ E´tudes et analyses de l’e´tat de sante´ de la population, Institut national de sante´ publique du Que´bec, 190, boulevard Cre´mazie Est, Montre´al, Que´bec H2P 1E2, Canada. Tel.: þ1 514 864 1600; fax: þ1 514 864 1616. E-mail address: [email protected] (N. Auger).
South America, Europe and Asia have demonstrated positive11–18 or null associations17,19–23 between income inequality and mortality. The bulk of research indicates more consistent associations for large areas such as states or metropolitan centres.24,25 The association between income inequality and mortality is less supported by studies of smaller areas.14–18,21–23,25 The influence of income inequality on morbidity has also been found to depend on level of geographic aggregation.26 In Canada, income inequality for small areas is associated with self-rated health27–29 and all-cause mortality,18 yet such associations have not been observed for larger metropolitan regions of Canada.6,27,30 Various factors may contribute to such discrepancies, including the possibility that income inequality expressed at the metropolitan level could mask relationships at smaller levels. Additional research evaluating local relationships between income inequality and mortality is necessary in order to compare results between small and large areas of Canada. Research on income
0033-3506/$ – see front matter Ó 2009 The Royal Society for Public Health. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.puhe.2009.04.012
N. Auger et al. / Public Health 123 (2009) 438–443
inequality in smaller areas is also useful to better understand how income inequality may contribute to or mediate area effects on health. The literature is further complicated by US data indicating that younger age groups may be more susceptible to income inequality5,6,9,24,31; a finding not observed for settings outside the USA.10 Whether income inequality within countries is more strongly related to specific causes of death, such as mental-healthrelated (e.g. suicide) or lifestyle-related (e.g. tobacco) mortality, is also unclear. In US states, income inequality has been linked to risk factors for cardiovascular mortality32,33 and alcohol dependence,34 but findings have been conflicting for mental health disorders.34–36 In contrast, income inequality in small areas of the USA and the UK has been linked to poor mental health37–39 and substance use.40,41 This study sought to evaluate community-level income inequality in relation to age effects and all-cause and cause-specific mortality in communities of Que´bec, Canada. It was hypothesized that community-level income inequality would be associated with elevated mortality, and that associations would be stronger for working-age individuals and for mortality related to mental health or health-related behaviours. Methods Data and study setting Non-infant deaths (1 year of age; n ¼ 271,068) were extracted from the Que´bec, Canada death file for the period 1999–2003. The total population in Que´bec in 2001 was 7,396,988. Seven mortality outcomes were considered: (1) (2) (3) (4)
all-cause overall mortality; all-cause working-age mortality (25–64 years); all-cause post-working-age mortality (65 years); suicide for population aged 10 years (ICD-9 codes E950–959 for 1999; ICD-10 codes X60–84/Y87.0 for 2000 onwards); (5) alcohol-related mortality for all ages [defined as oesophageal cancer (ICD-9 code 150; ICD-10 code C15), laryngeal cancer (161; C32), alcohol-dependency syndrome (303; F10), chronic liver disease and cirrhosis (571; K70/K73/K74/K76), and all external causes of death (E800–E999; V00–V99/W00–W99/ X00–X99/Y00–Y99)]; (6) tobacco-related mortality in the population aged 20 years [defined as oropharyngeal cancer (140–149; C00–C14), laryngeal cancer (161; C32), tracheobronchial/pulmonary cancer (162; C33/C34), oesophageal cancer (150; C15), ischaemic cardiopathy (410–414; I20–I25), cerebrovascular (430–438; I60–I69), and chronic lower respiratory tract disease (490–496; J40–J47)]; and (7) cardiovascular mortality in the population aged 20 years [defined as ischaemic cardiopathy (410–414; I20–I25), cerebrovascular (430–438; I60–I69), and arterial disease (440–448; I70–I78)].
Mortality was calculated for local community service centres (CLSC, n ¼ 166); areas of health service delivery corresponding to well-defined communities in Que´bec42 where variability in arealevel deprivation has been documented.43 Communities were linked to 11,612 census dissemination areas (DAs); the smallest census unit for which socio-economic data were available (mean 75 DAs per community; 120 DAs not fully nested in a community were assigned to the community containing the largest portion). Four Aboriginal communities in northern Que´bec were excluded as valid socio-economic census data were not available. Of the remaining 162 communities, 19 were merged following standard Que´bec
439
Ministry of Health practice to correct small cell sizes and improve stability of health indicators. The final sample contained 143 communities. Communities had a mean population of 51,564 in 2001 (median 45,728; range 3700–142,025). Variables Income inequality may be sensitive to the choice of indicator.12 Therefore, three indicators of community income inequality were calculated: (1) decile ratio (DR),44 defined as the ratio of DAs with the highest to lowest mean population-weighted proportion of the population below the low income cut-off (income level at which economic families or unattached individuals spend 20% more than average on food, shelter and clothing, depending on family size and degree of urbanization)45; (2) coefficient of variation (CV) of mean household income (it was verified that household income was normally distributed)44; and (3) median share (MS), defined as the proportion of total household income received by the least well-off 50% of households.44 One community with an MS of 0.56 (an impossible value that arose due to sparse data) was recoded as 0.49. Analyses both including and excluding this community indicated no influence on results. Data required for calculation of the Gini coefficient were not available at the level of census DA units (these being nested within communities and the basis of inequality measures). Inferential analyses of the ecological association between income inequality and mortality may be biased by the average socio-economic status of communities.1,15 Therefore, median household income was used as a covariate in analyses. Other potential community confounders were also considered, including family structure (proportion of lone-parent families) and rural– urban area (calculated from the proportion of rural DAs). DAs were considered rural when they belonged to a municipality classified as such by Statistics Canada (municipalities are not equivalent to CLSCs).46 Three community rural–urban categories were created: urban (no rural DAs), rural (all rural DAs) and semi-urban (some rural DAs). Community predictors were examined as tertiles. Statistical analysis Directly standardized mortality rates were calculated for communities using 5-year age groups for all-cause mortality and 10-year age groups for cause-specific mortality. The 2001 Que´bec population served as the reference, and population counts were corrected for census under-enumeration.47 Preliminary genderspecific analyses indicated that associations were in similar directions for both genders. To conserve statistical power, analyses were not stratified by gender. Descriptive statistics were calculated for community-level predictors. An ecological analysis was then undertaken of community predictors in relation to mortality. Relative risks (RR) and 95% confidence intervals (CI) were computed using Poisson regression for univariate (crude) models, followed by multivariate models adjusting for covariates. Models contained a maximum of four independent variables given sample size considerations. An interaction term between income inequality and rural–urban status was tested to rule out an influence of income inequality that differed according to rural–urban area, and models stratified by rural–urban area were re-run. Due to the change in 2000 from ICD-9 to ICD-10 coding, it was verified that similar results were obtained in analyses restricted to 2000 onwards. The significance of parameter estimates was assessed with the Wald test. The GENMOD procedure in SAS Version 9.1 (SAS Institute Inc, Cary, NC, USA) was used, and results were corrected for overdispersion.
440
N. Auger et al. / Public Health 123 (2009) 438–443
Table 1 Characteristics of Que´bec communities,a 2001. n Population per community (n)
Median
Range
45,728
3700–142,025
Decile ratio Low inequality Moderate inequality High inequality
47 48 48
4.7 7.8 11.9
1.2–6.4 6.5–9.6 9.6–18.7
Coefficient of variation (%) Low inequality Moderate inequality High inequality
47 48 48
12.3 17.9 25.5
2.3–13.3 13.7–22.1 22.1–47.1
Median share (proportion) Low inequality Moderate inequality High inequality
48 48 47
0.42 0.40 0.36
0.41–0.49 0.38–0.41 0.21–0.38
Median household income ($) Low Moderate High
47 48 48
33,633 39,172 49,560
23,891–36,902 37,299–42,107 42,187–79,163
Lone-parent families (%) Low Moderate High
47 48 48
12.7 15.1 19.9
8.8–13.9 13.9–16.5 16.5–33.9
Rural–urban area (% rural DAs) Urban Semi-urban Rural
53 48 42
a
0 1.3–96.0 100
Refers to local community health centre regions.
Results Community-level income inequality had considerable heterogeneity, as indicated by the range of values for the DR (1.2–18.7), CV (2.3–47.1%) and MS (0.21–0.49) (Table 1). Median community income and the proportion of lone-parent families also ranged
widely. One-third (33.6%) of communities were semi-urban and 28.4% were rural. In general, all-cause mortality for all ages and for the workingage and post-working-age populations was lowest in areas with high income inequality, and this relationship was apparent for all three indicators of income inequality (Table 2). Suicide, alcoholand tobacco-related mortality were also lowest for areas with high income inequality for all three indicators. Relationships were inconsistent for cardiovascular mortality. A linear trend of reduced mortality with increasing income inequality was present for the DR across all mortality outcomes. This relation was only apparent in half of outcomes predicted by the CV and MS indicators. Moderate inequality was related to greatest mortality for the remaining outcomes. Mortality was lower for wealthier tertiles (indexed by median household income) and for tertiles with fewer lone parents (except suicide and alcohol-related deaths which were higher in areas with fewer lone parents). Mortality outcomes were also lower in urban areas. Tests for effect modification indicated no interaction between income inequality and rural–urban area. In univariate (unadjusted) models, the association between income inequality and mortality depended on the indicator (Table 3). With the DR, high relative to low income inequality was associated with a significantly lower risk of mortality for all outcomes except allcause post-working-age mortality and cardiovascular mortality. Risks were lowest for all-cause working-age (RRDR 0.83, 95% CI 0.77–0.91), suicide (RRDR 0.82, 95% CI 0.71–0.95) and alcohol-related mortality (RRDR 0.80, 95% CI 0.73–0.88). In addition, a linear trend suggesting greater risk with progressively lower income inequality was present for these outcomes. Income inequality was not associated with postworking-age mortality (RRDR 0.95, 95% CI 0.89–1.01). Income inequality measured with the CV and MS indicators was inversely related to mortality from suicide (RRCV 0.67, 95% CI 0.58–0.76; RRMS 0.75, 95% CI 0.65–0.86) and alcohol (RRCV 0.72, 95% CI 0.66–0.79; RRMS 0.76, 95% CI 0.69–0.83) alone.
Table 2 All-cause and cause-specific mortalitya per 100,000 population, Que´bec communities, 1999–2003. All causes (1 year) n ¼ 271,068
All causes (25–64 years) n ¼ 58,550
All causes (65 years) n ¼ 208,789
Suicide n ¼ 6794
Alcohol n ¼ 23,356
Tobacco n ¼ 105,090
Cardiovascular n ¼ 66,771
Decile ratio Low inequality Moderate inequality High inequality
799.2 727.3 723.2
335.6 273.1 267.2
4533.6 4270.0 4270.5
24.7 19.9 19.7
77.3 60.9 60.0
405.5 366.8 364.0
250.7 233.8 234.0
Coefficient of variation Low inequality Moderate inequality High inequality
751.6 786.4 715.3
299.4 298.5 271.3
4297.1 4599.4 4201.7
27.8 22.9 18.1
81.1 69.7 57.0
373.1 401.1 360.1
225.9 251.3 233.3
Median share Low inequality Moderate inequality High inequality
762.5 760.7 720.7
302.1 288.6 272.8
4370.1 4460.5 4234.1
26.0 20.7 19.0
79.7 63.4 58.7
379.5 392.7 359.9
234.2 250.7 230.6
Median household income Low 792.6 Moderate 740.3 High 716.8
339.2 293.1 250.7
4468.9 4271.4 4300.4
26.0 23.1 16.8
78.7 67.6 54.4
403.3 375.2 357.3
249.2 238.3 231.1
Lone-parent families Low Moderate High
714.8 753.7 747.7
259.4 275.9 306.4
4217.7 4453.1 4297.7
21.8 21.3 19.7
66.0 64.4 62.3
355.8 382.4 378.7
224.7 241.9 242.0
Rural–urban area Urban Semi-rural Rural
720.4 767.3 760.4
275.1 287.4 307.2
4231.4 4495.3 4326.7
17.0 24.1 27.7
55.7 70.3 81.7
362.0 392.2 377.1
233.6 247.6 229.7
a
Directly standardized mortality rates, using the 2001 Que´bec population as the standard.
N. Auger et al. / Public Health 123 (2009) 438–443
441
Table 3 Crude associations between income inequality and mortality outcomes, Que´bec communities, 1999–2003.a
Table 4 Adjusted associations between income inequality and mortality outcomes, Que´bec communities, 1999–2003.a
Mortality
Decile ratio
Coefficient of variation
Median share
Mortality
Decile ratio
Coefficient of variation
Median share
All causes, ‡1 year Low inequality Moderate inequality High inequality
1 0.93 (0.88–0.99) 0.92 (0.87–0.98)
1 1.02 (0.96–1.09) 0.94 (0.89–1.00)
1 1.02 (0.96–1.08) 0.97 (0.91–1.03)
All causes, ‡1 year Low inequality Moderate inequality High inequality
1 0.93 (0.88–0.99) 0.91 (0.85–0.98)
1 1.01 (0.95–1.09) 0.94 (0.86–1.02)
1 1.02 (0.95–1.10) 0.97 (0.89–1.05)
All causes, 25–64 years Low inequality 1 Moderate inequality 0.88 (0.81–0.95) High inequality 0.83 (0.77–0.91)
1 1.01 (0.92–1.09) 0.92 (0.84–1.00)
1 0.98 (0.90–1.07) 0.93 (0.85–1.01)
All causes, 25–64 years Low inequality 1 Moderate inequality 0.93 (0.87–1.00) High inequality 0.92 (0.85–1.01)
1 1.02 (0.93–1.11) 0.96 (0.87–1.06)
1 0.99 (0.91–1.08) 0.95 (0.86–1.05)
All causes, ‡65 years Low inequality Moderate inequality High inequality
1 0.95 (0.89–1.01) 0.95 (0.89–1.01)
1 1.03 (0.97–1.10) 0.96 (0.90–1.03)
1 1.04 (0.97–1.10) 0.99 (0.93–1.06)
All causes, ‡65 years Low inequality Moderate inequality High inequality
1 0.93 (0.87–0.99) 0.91 (0.84–0.98)
1 1.02 (0.95–1.10) 0.93 (0.86–1.02)
1 1.03 (0.96–1.11) 0.98 (0.90–1.07)
Suicide Low inequality Moderate inequality High inequality
1 0.87 (0.76–1.01) 0.82 (0.71–0.95)
1 0.84 (0.74–0.96) 0.67 (0.58–0.76)
1 0.86 (0.75–0.99) 0.75 (0.65–0.86)
Suicide Low inequality Moderate inequality High inequality
1 0.95 (0.84–1.08) 1.01 (0.87–1.16)
1 0.92 (0.80–1.05) 0.84 (0.71–0.99)
1 1.05 (0.92–1.21) 0.99 (0.84–1.17)
Alcohol Low inequality Moderate inequality High inequality
1 0.86 (0.78–0.94) 0.80 (0.73–0.88)
1 0.89 (0.82–0.97) 0.72 (0.66–0.79)
1 0.85 (0.78–0.93) 0.76 (0.69–0.83)
Alcohol Low inequality Moderate inequality High inequality
1 0.92 (0.85–0.99) 0.94 (0.86–1.02)
1 0.95 (0.88–1.03) 0.85 (0.77–0.94)
1 0.95 (0.88–1.04) 0.90 (0.81–0.99)
Tobacco Low inequality Moderate inequality High inequality
1 0.93 (0.87–1.00) 0.92 (0.86–0.99)
1 1.05 (0.98–1.13) 0.96 (0.89–1.04)
1 1.04 (0.96–1.12) 0.98 (0.90–1.05)
Tobacco Low inequality Moderate inequality High inequality
1 0.93 (0.86–1.01) 0.91 (0.83–0.99)
1 1.03 (0.95–1.13) 0.94 (0.85–1.04)
1 1.02 (0.94–1.12) 0.95 (0.86–1.05)
Cardiovascular Low inequality Moderate inequality High inequality
1 0.96 (0.89–1.03) 0.96 (0.90–1.04)
1 1.11 (1.03–1.19) 1.04 (0.97–1.12)
1 1.07 (0.99–1.15) 1.01 (0.94–1.09)
Cardiovascular Low inequality Moderate inequality High inequality
1 0.95 (0.88–1.03) 0.94 (0.86–1.02)
1 1.07 (0.99–1.16) 1.00 (0.91–1.11)
1 1.02 (0.94–1.11) 0.95 (0.86–1.05)
a
Relative risk (95% confidence interval).
In covariate-adjusted models, associations between income inequality and mortality were of lesser magnitude, but in the same direction as those from univariate analyses (Table 4). High relative to low income inequality was associated with a significantly lower risk of mortality for all-cause post-working-age and overall mortality with the DR alone. The CV and MS were associated with a progressively lower risk of alcohol-related mortality in adjusted models; for the DR, moderate (rather than high) income inequality was associated with alcohol-related mortality. Statistically significant associations also remained for the CV with suicide mortality and for the DR with tobacco-related mortality. Income inequality was not associated with a higher risk of cardiovascular mortality in any adjusted model. Discussion In this ecological study of Que´bec communities, high relative to low income inequality was not associated with a higher risk of mortality, but was inversely associated with several mortality outcomes (Tables 3 and 4). All three indicators of income inequality were associated with a significantly lower risk of alcohol-related mortality, accounting for community income, proportion of lowincome families and rural–urban area. A significantly lower risk of all-cause post-working-age and overall mortality was observed for the DR in adjusted models, while a lower risk of suicide mortality was observed with the CV. While null associations were observed for other mortality outcomes, including working-age mortality, relations were in the same inverse direction. In general, linear trends of progressively lower risk were observed with increasing inequality. Of the three income inequality indicators, the DR best
a Relative risk (95% confidence interval), adjusted for median income, rural–urban area and proportion of lone-parent families.
captured trends in mortality rates (Table 2) and unadjusted risks (Table 3) across inequality tertiles. What is already known The majority of community-level mortality studies conducted in developed nations indicate either positive15,16 or null associations21,22 with income inequality. However, a small number of community-level studies have documented negative associations, as observed in the present study.48,49 The first, a prospective study of death/hospitalization from ischaemic heart disease in municipalities of Denmark (similar in size to Que´bec communities), found an inverse association among women but not men.49 In a prospective study of all-cause mortality in Norwegian municipalities, also comparable in size to Que´bec communities, associations with income inequality were in the expected direction among men <50 years of age, but inverse associations were observed for those aged 50 years (null associations were present among women).48 Similarly, the present study found an inverse association with all-cause mortality among post-working-age persons aged 65 years, but this association was not robust across all indicators of income inequality. However, the present results cannot be compared directly with the Danish38 and Norwegian48 studies, as this study was not prospective or gender-specific (preliminary analyses indicated that relationships with income inequality did not vary according to gender). Researchers have documented inverse associations with self-rated health in census tract neighbourhoods of Canada27 and Chicago.50 Others have found inverse associations in analyses of larger areas such as metropolitan centres51 or states,52 but these are difficult to compare
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with community-level studies. Although a recent descriptive study found lower working-age mortality in certain Canadian metropolitan centres with greater income inequality,51 other studies using multivariate regression reported no association.10,30 Some authors reporting inverse associations between income inequality and health have discussed potential mechanisms.27,48,50 One explanation is that high income inequality may be a characteristic of generally affluent neighbourhoods having wide variations in household income, whereas low income inequality may be a characteristic of less affluent neighbourhoods with less variation in household income.27 What else can explain the inverse associations between community income inequality and mortality in these data? Three indicators of income inequality were used, each sensitive to inequality at different ends of the income spectrum. Associations were detected primarily for the DR and CV (indicators sensitive to the upper end of the income spectrum), and were weakest for the MS (sensitive to the middle part of the income spectrum).30 In contrast, studies on larger population areas report stronger associations for measures of inequality sensitive to the middle of the income spectrum.12,44,53 One possibility is that indicators sensitive to opposite ends of the income spectrum represent, in smaller areas, an aspect of inequality favourably associated with mortality. Another possibility is that community-level indicators of income inequality may be proxies for other unrelated environmental factors. Some research suggests that the positive relationship between income inequality and mortality in ecological studies is due to a statistical artefact.54–56 However, this study found statistically significant associations between higher income inequality and lower mortality; the opposite to what would be expected if the results were due to a statistical artefact. Other possible mechanisms Age-specific analyses indicated that the magnitude of associations between income inequality and mortality were similar in both the working-age and post-working-age populations. This finding can be considered in light of studies performed in larger areas that document steeper relations between income inequality and mortality in younger populations, albeit in the opposite direction.5,6,9,24,31 Some authors maintain that the influence of income inequality is linked to relative mechanisms involving psychosocial comparisons and subsequent chronic stress and biological effects24; a process which may be more common in younger populations. The present results suggest that these mechanisms may apply to both the young and old populations. The tendency towards stronger associations for mentalhealth-related mortality (i.e. alcohol and, to a lesser extent, suicide) as opposed to lifestyle-related mortality (i.e. cardiovascular) further supports psychosocial mechanisms. The protective associations observed in the present study also suggest that psychosocial comparisons, if present, might be favourable for health in small (but not large) areas. Such a situation might arise if, for example, small inequalities in communities generate an environment in which striving for better life conditions is perceived positively. The absolute deprivation hypothesis, also identified in the literature, posits that underinvestment in material resources in unequal areas translates into unfavourable lifestyles that influence health.57 However, the absolute deprivation hypothesis does not rule out the possibility that inequality in communities (as opposed to larger areas) might be associated with greater investment in material resources, which is another possible explanation for these results. The present results for cause-specific mortality may be useful to better understand processes of social selection versus social causation.24 The former refers to selection of individuals into neighbourhoods based on individual-level characteristics, while the latter refers to causal relations between places and people. This study found that
protective associations were more consistent with income inequality for mortality related to alcohol use (Table 4). If social selection was operating, the data suggest that individuals at risk of alcohol mortality may self-select out of areas with high income inequality. That is, individuals with severe alcohol dependency may not have the means required for residence in areas with high income inequality, assuming such areas represent affluence. The weaker associations observed for cardiovascular and, to a lesser extent, suicide and tobacco-related mortality indicate that selection may be less important for risk factors related to these outcomes. Thus, the stronger association with alcohol-related mortality lends support to processes of social selection rather than causation. Nonetheless, more research is necessary to determine the processes underpinning these associations. This study was subject to limitations. The analysis was ecological and could not account for factors such as individual income that might confound or moderate relationships. Inference cannot extend to the individual level. Ecological estimates of contextual effects may themselves be subject to error.58 The unit of analysis was CLSCs with administratively defined borders, rather than borders defined by residents themselves; the latter may be more meaningful for estimating contextual effects. The sample contained 143 communities; a relatively small number of units which limited statistical power and thus the number of covariates that could be considered. It was not possible to account for health services utilization, which may influence relations between income inequality and mortality.59 Healthcare insurance, however, is universal in Que´bec, and equality of access and service provision independent of station in life is theoretically ensured. Census data (rather than tax return data) were used to calculate income inequality, and the census may not be the best source of income data.60 It was not possible to evaluate other indicators such as the Gini index, which may better represent income inequality in small communities. Last, some of the study outcomes were inter-related and thus not independent (e.g. tobacco-related and cardiovascular mortality). This analysis of administrative communities in a Canadian province found an inverse association between community income inequality and several mortality outcomes, accounting for community income, family structure and rural–urban area. Associations were strongest for alcohol-related mortality. These findings suggest that the link between income inequality and mortality is not clear-cut, and that spatial scale must be considered. As mortality differentials in Que´bec are not explained by income inequality, it is concluded that narrowlyfocused polices intending to reduce income inequality may be insufficient to attenuate social differentials in mortality. Acknowledgements The authors wish to thank Karine Le´ger, Geomatics Specialist, for calculating community-level measures of income inequality, and Robert Choinie`re for helpful comments on the manuscript.
Ethical approval This study was conducted as part of the Que´bec Population Health Surveillance Plan mandated by the Health Ministry and approved by the Public Health Ethics Committee. Funding This research was funded by MD through a Canada Research Chair award from the Canadian Institutes of Health Research, and a grant from the Canada Foundation for Innovation (#201252). MD is supported by a University of South Australia Research Chair for Social Epidemiology. The funding sources did not participate in
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