Do social comparisons explain the association between income inequality and health?: Relative deprivation and perceived health among male and female Japanese individuals

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, Marmot and Wilkinson, 2005). Although controversial, many papers have also suggested that income inequality or relative income affects health (Subramanian and Kawachi, 2004, 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, 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, Subramanian et al., 2007), but also several null studies (Blakely et al., 2004, Gerdtham and Johannesson, 2004, 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, 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, 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, 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, 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.