JECH Online First, published on March 30, 2011 as 10.1136/jech.2010.127977 Research report
Mitigating effect of immigration on the relation between income inequality and mortality: a prospective study of 2 million Canadians Nathalie Auger,1,2,3 Denis Hamel,1 Je´roˆme Martinez,1 Nancy A Ross4 1 Institut National de Sante´ Publique du Que´bec, Montre´al, Canada, USA 2 Research Centre of the University of Montre´al Hospital Centre, Montre´al, Canada, USA 3 Department of Social and Preventive Medicine, University of Montre´al, Montre´al, Canada, USA 4 Department of Geography, McGill University, Montre´al, Canada, USA
Correspondence to Dr Nathalie Auger, Institut National de Sante´ Publique du Que´bec, 190, Boulevard Cre´mazie Est, Montre´al, Que´bec H2P 1E2, Canada, USA;
[email protected] Accepted 8 March 2011
ABSTRACT Background The relation between income inequality and mortality in Canada is unclear, and modifying effects of characteristics such as immigration have not been examined. Methods Using a cohort of 2 million Canadians followed for mortality from 1991e2001, we calculated HRs and 95% CIs for income inequality of 140 urban areas (Gini coefficient, Atkinson index, coefficient of variation; expressed as continuous variables) and working age (25e64 y) or post-working age ($65 y) mortality in men and women according to immigration status, accounting for individual and neighbourhood income, and sociodemographic characteristics. Major causes of mortality were examined. Results Relative to low income inequality, high inequality was associated with greater working age mortality in male (HRGini 1.08, 95% CI 1.04 to 1.13) and female (HRGini 1.12, 95% CI 1.06 to 1.18) nonimmigrants for all income inequality indictors. Results were similar for female post-working age mortality. There was no relation between income inequality and mortality in immigrants. Among Canadian-born individuals, associations were greater for alcohol-related mortality (both sexes) and smoking-related causes/ transport injuries (women). Conclusion Income inequality is associated with mortality in Canadian-born individuals but not immigrants.
INTRODUCTION Individual income is an established risk factor for mortality,1 2 but inequality in the distribution of income within countries may also influence health. Countries with high income inequality are those in which extremes of wealth and poverty are more pronounced. Understanding the relation between income inequality and health has been a priority in Europe and the USA since the 1990s.3 Income inequality is associated with mortality in several Western countries,4 but no such relation has been established for Canada.5e8 However, Canadian studies are limited by cross-sectional ecological designs, and multilevel prospective designs are preferred for evaluating income inequality and health.2 4 In fact, it was through prospective studies of populations from the USA9e12 and Europe13e15 that income inequality was more strongly established as a risk factor for mortality. Nonetheless, the relation between income inequality and mortality has been debated despite this evidence;9e15 in part because no associations were found in some studies accounting for
ethnicity and individual income.1 16e20 However, other underlying structural or demographic characteristics of populations may need to be addressed to accurately understand the relation between income inequality and mortality. Immigration patterns in particular may be important in places such as Canada where nearly 20% of the population is foreign-born.21 Immigrants may be generally healthier than native-born populations,22 23 and may inadvertently mask associations between income inequality and mortality. Immigration patterns have not been accounted for in previous Canadian cross-sectional analyses of income inequality and mortality.5e8 To clarify these issues, we evaluated whether the relation between income inequality and mortality in a large Canadian cohort was modified by immigration.
METHODS Data and variables Data were drawn from the Canadian Census Mortality Follow-up Study (CCMFS)da cohort containing a 15% sample of the Canadian population aged 25 years and over living in Canada on the day of the census (4 June 1991) who were not residents of a long-term institution and were followed for mortality for 10.6 years until 31 December 2001.24 The cohort contains individuals representative of the Canadian population who responded to a detailed census questionnaire. Individuals residing in all urban areas of Canada (n¼140 cities) who were part of the CCMFS cohort were included (N¼2 077 000). Urban areas consist of 25 census metropolitan areas (N¼1 637 900) and 115 smaller urban centres with populations over 10 000 known as census agglomerations (N¼439 000). Rural areas were not included as they were too small to allow meaningful calculations of income inequality. Mortality was evaluated for two separate age groups at the time of cohort inception, men and women of working age (25e64 y) and postworking age ($65 y) as previous research suggests relations depend on age and sex.10 Principal cause of death was assessed using the 9th and 10th revisions of the International Classification of Diseases (ICD)25 for deaths in 1991e1999 and 2000e2001, respectively. We considered general causes of death (cancer, cardiovascular, injury-related), specific causes (cancer of the lung, prostate/breast and colorectum, ischaemic heart disease, cerebrovascular disease, transport injuries, suicide) and behavioural or preventable causes (alcohol-related or smoking-related) used in previous research of
Auger N, Hamel D, Article Martinez J,author et al. J Epidemiol Community Health (2011). doi:10.1136/jech.2010.127977 Copyright (or their employer) 2011. Produced by BMJ
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Research report socioeconomic inequalities in mortality.25 ICD codes for these causes are available elsewhere.25 Urban-level income inequality was measured using the Gini coefficient, Atkinson index and coefficient of variation, since associations with health may vary with the indicator, and use of different indicators may help capture associations that depend on sensitivity to various parts of the income spectrum.26 Indicators were calculated for each of 140 urban areas using family income (before-tax, after-transfers), equivalised for family size and relationships between members according to the method of the Organisation for Economic Co-operation and Development (after-tax income was not available).27 For individuals not in a family (ie, those living alone), personal income was used instead. Income in dollars was obtained directly from the CCMFS cohort.28 The Gini coefficient measures deviation of the income distribution from perfect equality and ranges from 0 (perfect equality) to 1 (perfect inequality).7 26 The Atkinson Index also ranges from 0 to 1 and is interpreted similarly.26 The Gini and Atkinson indices were computed using a SAS macro from the Luxembourg Income Study (http://www.lisproject. org/key-figures/sasprograms/allkf_sas.txt) with a 3 value of 1 for the Atkinson Index. The coefficient of variation was obtained from the SD of the income distribution divided by its mean (we verified that income was normally distributed).26 Census respondents were asked whether they had immigrated and, if so, the year of immigration.28 In analyses, immigration status was assessed for the categories non-immigrant, long-term immigrant (>10 y) and recent immigrant (#10 y). A 10-year cut-point was used to differentiate recent from long-term immigrants as immigrant health in Canada tends to converge with non-immigrants after a decade.29 Finer categorisations based on source country or for narrower time periods since immigration were not considered as the relation between immigration and mortality was not the main focus of the study. Income was measured relative to Statistics Canada’s low income cut-off for families and communities grouped as quintiles (low, low-moderate, moderate, high-moderate, high).24 Covariates included individual age (25e34, 35e44, 45e54, 55e64 y for working age; and 65e74, 75e84, 85+ y for postworking age), education (no high-school graduation, high-school diploma and/or trade certificate, post-secondary non-university, university degree), employment (employed, not employed, not in labour force), marital status (legally married, common law, not married), visible minority (no, yes), provincial region (British Columbia and Territories, Prairies, Ontario, Québec, Atlantic) and mean neighbourhood household income (low, moderatelow, moderate-high, high). Neighbourhoods were specified as enumeration areasdthe smallest unit for which Statistics Canada disseminated 1991 socioeconomic data (containing 750 inhabitants on average).
Statistical analysis Directly standardised mortality rates were calculated using the 1991 population structure as the reference. We used Cox proportional hazards regression to calculate age-adjusted HRs and 95% CIs for the relation between income inequality and working age or post-working age mortality in men and women. The census day was specified as the start time. The proportional hazards assumption was verified with log(-log Survival) curves for all variables plotted against time. Models first evaluated income inequality alone and were subsequently adjusted for individual income, immigration and other covariates (employment, education, marital status, visible minority, mean neighbourhood household income and region). Effect modification was 2 of 7
tested with income inequality-by-immigration interaction terms, and fully adjusted models were run for non-immigrants, long term immigrants and recent immigrants, separately. Specific causes of mortality were evaluated separately in models with working age and post-working age groups combined, and with long-term and recent immigrants combined, to increase statistical power. For regression analyses, indicators of income inequality were expressed as continuous cumulative rank scores ranging from 0 to 1 to account for the population distribution across levels of income inequality30 (individuals in the same category were assigned the average rank).31 The cumulative rank score was used as a continuous variable in regressions after verifying that quartiles of income inequality were linear with the log-hazard of mortality. HRs obtained from regression of the cumulative rank score on mortality are interpreted as the relative difference in mortality between the hypothetical most and least unequal areas of the cumulative income inequality distribution.30 These HRs are equivalent (or equal) to the Relative Index of Inequalityda summary measure of inequality increasingly used in the literature.30 The data were hierarchically structured with individuals nested in 140 urban areas. Clustering in urban areas was accounted for with the robust sandwich estimator.32 We verified that results accounting for clustering in neighbourhoods were similar. We also verified that models run without small urban areas yielded similar results. Statistical analyses were undertaken using the TPHREG procedure of SAS 9.1 (SAS Institute Inc). The CCMFS was approved by the research ethics committee of the University of Toronto and the Statistics Canada Policy Committee.
RESULTS Long term and recent immigrants constituted approximately 20% and 6% of the population, respectively (table 1). There were 202 354 deaths (9.7%) over the follow-up period. Age-adjusted mortality rates per 100 000 person-years were 530 and 310 for working age men and women versus 6600 and 3900 for postworking age men and women, respectively. Death was more frequent among low-income individuals (table 2). Among postworking age individuals, deaths were proportionately higher for non-immigrants than for immigrants. However, among working age individuals, higher death proportions were present for long-term immigrants relative to non-immigrants. Median (IQR) values for income inequality were 0.32 (0.31 to 0.34) for the Gini index, 0.21 (0.19 to 0.23) for the Atkinson index and 0.68 (0.63 to 0.75) for the coefficient of variation. Although death proportions varied slightly across levels of income inequality, a directional trend was not apparent. Median (IQR) proportions for the immigrant population per urban area were 11.5% (3.5% to 18.4%). Table 3 shows HRs for the relation between income inequality and mortality. Contrary to what may be expected, models adjusted only for age suggested that high income inequality was associated with a lower hazard of mortality relative to low inequality. Adjustment for individual income did not change these associations. However, further adjustment for immigration tended to nullify or reverse associations to the expected direction in both men and women, especially for working age mortality. Most indicators of income inequality in fully adjusted models were associated with a higher risk of working age mortality, but not with post-working age mortality. Tests for interaction between income inequality and immigration were statistically significant in both sexes and age groups
Auger N, Hamel D, Martinez J, et al. J Epidemiol Community Health (2011). doi:10.1136/jech.2010.127977
Research report Table 1
Distribution of population according to baseline characteristics
Age Working age, 25e64 y Post-working age, $65 y Income inequality* Gini coefficient Low Moderate High Atkinson index Low Moderate High Coefficient of variation Low Moderate High Income Low Low-moderate Moderate Moderate-high High Education No high-school graduation High-school6trade certificate Post-secondary non-university University degree Employment Employed Not employed Not in labour force Immigrant status Non-immigrant Immigrant, >10 y Recent immigrant, #10 y Visible minority No Yes Marital status Legally married Common law Not married Mean area household income Low Moderate-low Moderate-high High Region British Columbia and Territories Prairies Ontario Que´bec Atlantic Total population
Men, %
Women, %
85.8 14.2
83.7 16.3
32.7 34.2 33.2
32.1 34.3 33.6
31.7 34.0 34.2
31.3 34.0 34.7
32.7 32.1 35.2
32.4 32.3 35.3
14.2 19.3 21.2 22.3 22.9
19.8 19.8 20.1 20.1 20.1
30.8 38.0 13.5 17.7
32.1 35.6 19.0 13.3
72.8 7.1 20.2
59.0 6.1 34.9
74.3 19.7 5.9
75.8 18.4 5.9
90.4 9.6
90.5 9.5
74.2 6.7 19.2
64.5 6.1 29.4
24.4 25.2 25.2 25.3
24.9 24.8 24.9 25.4
13.5 15.1 39.9 25.8 5.8 1 019 000
13.4 15.0 39.8 26.1 5.7 1 058 100
*Expressed as population-weighted tertiles for descriptive statistics.
except for the coefficient of variation in men. When models were run for non-immigrants alone, a statistically significant relation was present between income inequality and mortality among both working age and post-working age men and women except for the coefficient of variation in post-working age men (table 4). The magnitude of the hazard tended to be greater for working age than post-working age men and women. Among long-term
immigrants, associations were either null (for working age mortality) or protective (for post-working age mortality). HRs tended to be even lower among recent immigrants, although not statistically significant with the exception of male post-working age mortality. Tables 3 and 4 demonstrate that all three indicators of income inequality tended to yield similar results and, therefore, that associations were not dependent on one particular part of the income spectrum. Among non-immigrant men, colorectal cancer, alcohol-related and ‘other’ causes were most strongly related to the Gini coefficient (table 5). The relation between income inequality and colorectal mortality was also present for immigrant men. Patterns were different among non-immigrant women for whom cancer (lung in particular), smoking-related causes and transport injuries were important causes associated with income inequality. Like men, income inequality among non-immigrant women was associated with alcohol-related and ‘other’ causes of mortality. Among immigrant women, income inequality was associated with a lower hazard of cardiovascular mortality.
DISCUSSION This study of a large prospective cohort is the first to demonstrate that income inequality is associated with mortality in Canada independent of individual income. Furthermore, the relation between income inequality and mortality is restricted to non-immigrant Canadians. Income inequality was most strongly associated with mortality among non-immigrant working age men and women. Elevated risks were also observed for post-working age women, but associations were borderline for post-working age men. In contrast, associations among immigrants were either absent or protective. Unadjusted models suggesting a protective effect of income inequality on mortality were, therefore, in part explained by the modifying effect of immigration. Though precision was not high, the causes of death primarily associated with income inequality among nonimmigrant Canadians were colorectal cancer in men, lung cancer/smoking-related/transport injuries in women and alcohol-related in both sexes. Among immigrants, protective associations were observed for prostate cancer (men) and cardiovascular mortality (men and women). The literature cites two general pathways through which income inequality can influence health. The first involves direct effects though individual factors such as poverty that are characteristic of unequal societies.3 33 Under this hypothesis, income redistribution would be sufficient to elevate the health status of the poorest up to the standard of the wealthiest. However, the extent to which income inequality might operate through individual factors is unclear, given that individual income did not confound the associations we observed between income inequality and mortality. Our results are in fact in line with previous research refuting statistical artefact as an explanation of the relation between income inequality and mortality.34 Since neighbourhood income did not influence the associations in our data, it is unlikely that urban-level income inequality operates through neighbourhood (or contextual) material pathways in Canada. The other pathway identified in the literature involves the potential for inequality to generate divisive conditions in society impacting social status, friendships, sense of control and social capital.3 35 These routes are possible, although our data could not be used to explore their role. Pathways may be different among immigrants for whom elevated HRs were generally absent. Immigrants to Canada come from diverse countries, including Europe in early decades
Auger N, Hamel D, Martinez J, et al. J Epidemiol Community Health (2011). doi:10.1136/jech.2010.127977
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Research report Table 2
Population counts and proportion of working age and post-working age deaths according to characteristics of individuals* Working age
Post-working age
Men
Income inequalityy Gini coefficient Low Moderate High Atkinson index Low Moderate High Coefficient of variation Low Moderate High Income Low Low-moderate Moderate Moderate-high High Education No high-school graduation High-school +/- trade certificate Post-secondary non-university University degree Employment Employed Not employed Not in labour force Immigrant status Non-immigrant Long term immigrant, >10 y Recent immigrant, #10 y Visible minority No Yes Marital status Legally married Common law Not married Mean area household income Low Moderate-low Moderate-high High Provincial region British Columbia and Territories Prairies Ontario Que´bec Atlantic Total
Women
Men
Women
Total N
% death
Total N
% death
Total N
% death
Total N
% death
284 900 296 600 292 700
5.5 5.4 5.5
284 700 301 000 299 700
3.1 3.2 3.0
48 000 51 700 45 100
48.4 47.5 45.8
55 200 62 000 55 500
34.3 35.1 32.1
271 500 300 200 302 500
5.6 5.4 5.6
273 100 302 600 309 700
3.2 3.0 3.0
51 900 46 400 46 400
48.4 47.3 45.9
57 700 57 500 57 500
35.2 34.3 32.2
286 400 277 900 309 900
5.5 5.8 5.2
287 600 281 600 316 200
3.0 3.1 3.1
46 800 49 400 48 600
48.8 48.2 44.8
55 400 60 300 56 900
34.4 33.5 33.8
113 100 155 300 189 200 205 900 210 700
8.8 5.5 4.9 4.7 4.9
147 600 167 400 186 500 191 600 192 300
4.6 3.0 2.7 2.6 2.9
32 000 41 600 27 100 21 600 22 400
56.2 50.5 43.9 41.3 38.3
62 100 42 000 26 700 21 300 20 600
40.5 32.4 29.8 28.8 27.8
233 300 346 900 128 100 165 900
8.8 4.8 3.5 3.9
237 900 331 800 183 300 132 400
4.8 2.7 2.3 2.3
80 700 40 000 9400 14 600
51.2 44.3 39.9 38.1
101 900 44 900 18 000 7900
36.1 31.3 30.6 27.7
723 600 70 700 79 900
4.1 5.6 17.7
614 200 64 000 207 200
2.2 2.5 5.9
17 900 1200 125 600
30.5 28.4 49.8
9600 700 162 400
18.1 20.0 34.9
657 300 159 700 57 200
5.5 6.0 3.9
675 600 151 600 58 200
3.1 3.4 2.6
100 000 41 400 3400
48.9 44.7 30.1
126 000 42 900 3700
34.2 33.8 26.1
783 200 91 000
5.7 3.4
791 900 93 400
3.2 2.3
137 800 7000
48.0 33.3
165 300 7400
34.3 24.2
638 400 66 000 169 800
5.5 3.7 6.2
617 600 63 600 204 100
2.9 1.7 4.2
117 200 2000 25 500
45.0 46.6 57.4
64 900 1300 106 500
24.4 26.8 39.8
213 900 222 900 221 000 216 300
6.4 5.4 5.1 5.1
217 200 223 800 222 500 221 900
3.5 2.9 2.9 3.0
34 300 33 700 35 400 41 400
51.6 48.4 46.1 43.7
46 400 39 000 40 600 46 700
37.6 34.2 32.9 30.9
114 900 133 900 346 700 228 000 50 800 874 200
5.2 5.1 5.6 5.7 5.3 5.5
116 400 134 700 352 000 230 700 51 600 885 400
3.1 3.2 3.3 2.8 3.0 3.1
22 600 19 900 59 800 34 500 8000 144 800
46.0 46.9 46.7 48.6 50.0 47.2
25 200 23 900 69 000 45 300 9200 172 700
34.5 34.6 34.7 31.1 38.3 33.9
*Population counts are rounded to the nearest 100 to conform to requirements of the Health Information and Research Division branch of Statistics Canada. yExpressed as population-weighted tertiles for descriptive statistics.
of the last century and, increasingly, Asian countries in recent decades (with consequent challenges related to knowledge of official languages and visible minority status).21 The majority settle in urban areas, especially large metropolitan centres like Toronto and Vancouver that have even higher rates of foreignborn populations than do Miami and Los Angeles.21 Indictors of income inequality may potentially be proxies for other factors 4 of 7
influencing where immigrants live rather than income inequality per se. Despite being highly skilled, immigrants face an income gap that improves with length of time in Canada;36 sense of community may also change over time. These factors may, in part, be related to why associations between income inequality and mortality for long-term immigrants tended to approach those of non-immigrants.
Auger N, Hamel D, Martinez J, et al. J Epidemiol Community Health (2011). doi:10.1136/jech.2010.127977
Research report Table 3
HR and 95% CI for the relation between income inequality and mortality, men and women*
Working age mortality Men Gini coefficient Atkinson index Coefficient of variation Women Gini coefficient Atkinson index Coefficient of variation Post-working age mortality Men Gini coefficient Atkinson index Coefficient of variation Women Gini coefficient Atkinson index Coefficient of variation
Age-adjusted HR (95% CI)
Age and income adjusted HR (95% CI)
Age, immigration and income adjusted HR (95% CI)
Fully adjusted* HR (95% CI)
0.86 (0.83 to 0.89) 0.89 (0.86 to 0.92) 0.79 (0.76 to 0.81)
0.85 (0.83 to 0.88) 0.88 (0.85 to 0.91) 0.79 (0.77 to 0.82)
1.03 (1.00 to 1.06) 1.05 (1.02 to 1.09) 0.99 (0.96 to 1.02)
1.05 (1.01 to 1.08) 1.05 (1.01 to 1.09) 1.06 (1.02 to 1.10)
0.002 0.0004 0.09
0.85 (0.81 to 0.88) 0.85 (0.81 to 0.89) 0.84 (0.81 to 0.88)
0.85 (0.82 to 0.89) 0.85 (0.81 to 0.89) 0.87 (0.83 to 0.91)
0.99 (0.95 to 1.04) 0.98 (0.94 to 1.03) 1.05 (1.00 to 1.10)
1.05 (1.00 to 1.10) 1.05 (1.00 to 1.10) 1.03 (0.98 to 1.08)
0.0002
