آیا مقایسه اجتماعی ارتباط بین نابرابری درآمد و سلامت را توضیح می دهد؟: محرومیت نسبی و سلامت درک شده در میان ژاپنی های مرد و زن

Abstract Relative deprivation has been hypothesized as one of the pathways accounting for the link between income inequality and health. We tested this hypothesis in a large national sample of men and women in Japan. Our survey included a probability sample of 22,871 men and 24,243 women aged 25–64, from whom information was gathered on demographic variables, household income, occupation or employment status, and self-rated health. Our measure of relative deprivation was the Yitzhaki Index, which calculates the deprivation suffered by each individual as a function of the aggregate income shortfall for each person relative to everyone else with higher incomes in that person's reference group. We modeled several alternative reference groups, including others with the same occupation, others of the same age group, and others living in the same geographic area (prefecture), as well as combinations of these. Generalized estimating equations demonstrated that higher relative deprivation was associated with worse self-rated health. Even after controlling for absolute income as well as other sociodemographic factors, the odds ratio and its 95% confidence intervals (CI) for poor health ranged from 1.09 (95% CI: 1.02–1.16) to 1.18 (95% CI: 1.11–1.26) for men and from 1.10 (95% CI: 1.04–1.16) to 1.16 (95% CI: 1.09–1.23) for women per 1 million increase in the Yitzhaki Index. As such, relative income deprivation is associated with poor self-rated health independently of absolute income, and relative deprivation may be a mechanism underlying the link between income inequality and population health.

Introduction It is widely accepted that income poverty or lower absolute income adversely affects health (Lynch and Kaplan, 2000 and Marmot and Wilkinson, 2005). Although controversial, many papers have also suggested that income inequality or relative income affects health (Subramanian and Kawachi, 2004 and Wilkinson and Pickett, 2006). The empirical evidence linking income inequality to health outcomes is strongest in the case of U.S. state level analyses (Backlund et al., 2007 and Lochner et al., 2001). Outside the United States, the evidence showing a link between income inequality and population health is less secure, with some positive studies (Larrea and Kawachi, 2005, Subramanian et al., 2003 and Subramanian et al., 2007), but also several null studies (Blakely et al., 2004, Gerdtham and Johannesson, 2004 and Osler et al., 2002). In Japan, Shibuya, Hashimoto, and Yano (2002) previously reported that prefectural level income inequality in that country was not associated with poor self-rated health (“prefecture” refers to the geographical/administrative unit of local government in Japan). On the other hand, they found that a measure of relative income (calculated as the difference between an individual's income and median prefectural income) was associated with worse health status. The problem, however, is that this measure of relative income is collinear with absolute income. Until relatively recently, few studies have attempted to tease out the mechanisms underlying the relationship between income inequality and health. Two distinct pathways have been proposed through which income inequality is believed to affect population health: a macro policy-related pathway and an individual-level psychosocial pathway (Kawachi, Fujisawa, & Takao, 2007). At the societal level, income inequality is believed to erode social cohesion, cooperation, and support for the provision of public goods (Kawachi & Kennedy, 2006). Recent evidence from experimental economics – in which income inequality was manipulated in the context of trust games – supports this mechanism (Anderson, Mellor, & Milyo, 2004). Alternatively, the psychosocial pathway posits that income inequality will heighten individuals' sense of relative deprivation, resulting in frustration, shame, stress, and adverse health consequences (Wilkinson, 2001). The theory of social comparison, initially proposed by Festinger (1954), supports this hypothesized mechanism. Empirical support for this pathway was provided recently by studies in the United States, which examined individual relative deprivation as a predictor of increased risks of mortality, as well as smoking, obesity, and mental health services utilization (Eibner and Evans, 2005 and Eibner et al., 2004). However, few other studies have been reported on the association between relative deprivation and health outside the United States (Gravelle and Sutton, in press and Jones and Wildman, 2008). Evidence is particularly sparse among Asian countries, even though the region has experienced widening income inequalities since the 1990s (Khang et al., 2004 and Kondo et al., in press). In the present study, we sought to provide a test of the association between relative deprivation and health in Japan. Although previous studies have not found an association between aggregated measures of prefecture-level income inequality and health in Japan (Nakaya and Dorling, 2005 and Shibuya et al., 2002), this may be due to the timing of these studies, i.e., they may have presented a limited snapshot at an early stage of the surge in income inequality in Japan. The situation may change in the future. Meanwhile, we are not aware of a previous study that has formally tested the association between individual-level sense of relative deprivation and health in Japanese society.

Results Descriptive analysis showed that people in older age groups were more likely to report poor health. Perceived health status also varied across marital status and occupations (Table 1). The proportion of poor perceived health was higher in the lower absolute income categories (data not shown). Table 1. Percent reported poor health by the sociodemographic characteristics and summary of economic indicators in Japanese males and females, 2001 CLSC Variable Male (n = 22,871) Female (n = 24,243) Respondents (%) No. (%) of poor self-rated health or median [25%, 75%] Respondents (%) No. (%) of poor self-rated health or median [25%, 75%] Age group (years old) 25–34 4913 (23.0) 334 (6.8) 5305 (23.4) 474 (8.9) 35–44 4905 (23.0) 435 (8.9) 5235 (23.1) 534 (10.2) 45–55 5690 (26.6) 601 (10.6) 5914 (26.0) 792 (13.4) 55–64 5856 (27.4) 762 (13.0) 6259 (27.6) 959 (15.3) Marital status Married 17,295 (75.6) 1650 (10.1) 18,667 (77.0) 2126 (12.1) Never married 4691 (20.5) 369 (8.7) 3321 (13.7) 302 (9.8) Separated 244 (1.1) 28 (13.0) 992 (4.1) 135 (15.3) Divorced 641 (2.8) 85 (14.5) 1263 (5.2) 196 (17.0) Occupation/employment status Employed Professional/technician 4000 (17.5) 314 (8.2) 2328 (9.6) 245 (11.0) Manager/administrator 1967 (8.6) 179 (9.4) 343 (1.4) 36 (11.0) Sales/service/clerical 5879 (25.7) 501 (9.0) 7547 (31.1) 760 (10.7) Security/transportation/labour 6853 (30.0) 575 (9.1) 2560 (10.6) 232 (9.6) Farming/fishery/forestry 938 (4.1) 87 (10.2) 729 (3.0) 74 (11.3) Unknown Job 1680 (7.4) 179 (11.7) 2461 (10.2) 295 (13.0) Unemployed Homemakera _ _ 7502 (31.0) 971 (13.8) Unemployed 1554 (6.8) 297 (21.9) 773 (3.2) 146 (22.3) Household income (10,000 Japanese Yen) 351.1 [231.5, 509.1] 334.0 [214.7, 490.8] Relative income deprivation by the definition of reference group (10,000 Japanese yen) Prefecture of residence 121.1 [64.7, 195.1] 121.5 [66.0, 194.6] Occupation 116.4 [61.2, 188.4] 118.6 [63.5, 191.9] Age group 118.5 [61.9, 198.0] 120.3 [64.2, 196.4] Prefecture and age 114.6 [57.6, 192.2] 116.4 [60.6, 191.8] Occupation and age 109.6 [57.2, 184.3] 111.6 [57.5, 191.0] CSLC: Comprehensive Survey of Living Condition of People on Health and Welfare. a Male homemakers were categorized in unemployed due to the small number of male homemakers. Table options Univariate regression models demonstrated that higher relative deprivation was significantly associated with poor reported health (Table 2). ORs (95% CI) of poor health by 1 million increase in relative deprivation varied from 1.08 (1.03–1.13) to 1.27 (1.21–1.32) in men and from 1.08 (1.03–1.13) to 1.21 (1.16–1.26) in women, depending on the type of reference group that was fitted. Regardless of the reference group assumed, we found a statistically significant trend between increasing quintile of relative deprivation and higher ORs of poor self-rated health. When the models were further adjusted for potential confounding variables including absolute income, the trend between increasing relative deprivation and worse health remained statistically significant across all reference group comparisons. When prefecture of residence was used as a reference group, relative deprivation showed the strongest association with poor self-rated health. Adjusted OR (95% CI) of the deprivation by the increase in one million Japanese yen per person was 1.18 (1.11–1.26) in men and 1.16 (1.09–1.23) in women, while the ORs with other reference groups ranged from 1.11 to 1.16 in men and from 1.10 to 1.14 in women. We did not find clear gender differences. Table 2. Crude and adjusted odds ratio (95% confidence intervals) for reporting poor health by the level of relative deprivation in Japanese men and women, 2001 CLSC Reference group defined by Prefecture of residence Occupation Age group Prefecture and age Occupation and age Male Crude Continuousa 1.21 (1.15–1.27) 1.08 (1.03–1.13) 1.27 (1.21–1.32) 1.26 (1.21–1.32) 1.11 (1.07–1.17) Top vs bottom quintile 1.53 (1.33–1.75) 1.20 (1.04–1.38) 1.75 (1.52–2.02) 1.73 (1.50–1.99) 1.35 (1.17–1.56) p for trend <0.0001 0.005 <0.0001 <0.0001 <0.0001 Model 1 Continuousa 1.23 (1.17–1.29) 1.11 (1.06–1.16) 1.20 (1.15–1.26) 1.20 (1.15–1.26) 1.07 (1.02–1.12) Top vs bottom quintile 1.62 (1.41–1.87) 1.30 (1.13–1.50) 1.54 (1.33–1.78) 1.53 (1.33–1.77) 1.22 (1.06–1.41) p for trend <0.0001 <0.0001 <0.0001 <0.0001 0.003 Model 2 Continuousa 1.18 (1.11–1.26) 1.11 (1.04–1.19) 1.16 (1.08–1.24) 1.16 (1.09–1.23) 1.09 (1.02–1.16) Top vs bottom quintile 1.37 (1.12–1.67) 1.22 (1.01–1.49) 1.25 (1.00–1.58) 1.25 (1.02–1.53) 1.26 (1.04–1.54) p for trend 0.0008 0.006 0.04 0.01 0.01 Female Crude Continuousa 1.17 (1.12–1.22) 1.08 (1.03–1.13) 1.20 (1.15–1.25) 1.21 (1.16–1.26) 1.13 (1.09–1.18) Top vs bottom quintile 1.39 (1.23–1.58) 1.15 (1.02–1.30) 1.61 (1.42–1.82) 1.58 (1.39–1.79) 1.43 (1.26–1.62) p for trend <0.0001 0.0008 <0.0001 <0.0001 <0.0001 Model 1 Continuousa 1.16 (1.11–1.21) 1.08 (1.03–1.13) 1.13 (1.08–1.18) 1.15 (1.10–1.20) 1.07 (1.02–1.11) Top vs bottom quintile 1.39 (1.23–1.58) 1.18 (1.04–1.34) 1.38 (1.21–1.57) 1.37 (1.20–1.56) 1.23 (1.08–1.40) p for trend <0.0001 0.0003 <0.0001 <0.0001 0.002 Model 2 Continuousa 1.16 (1.09–1.23) 1.12 (1.06–1.19) 1.12 (1.05–1.19) 1.14 (1.08–1.21) 1.10 (1.04–1.16) Top vs bottom quintile 1.30 (1.10–1.54) 1.24 (1.05–1.48) 1.28 (1.06–1.54) 1.27 (1.07–1.51) 1.33 (1.12–1.58) p for trend 0.0004 0.0002 0.01 0.005 0.001 Model 1 was adjusted for age group and marital status and model 2 was adjusted for age group, marital status, absolute income, and employment status. CSLC: Comprehensive Survey of Living Condition of People on Health and Welfare. a Odds ratios by the increase in 1 million Japanese yen per person of relative deprivation.