Population Research Institute, Social Science Research Institute, The Pennsylvania State University, USA.
Soc Sci Med. 2012 Jun;74(12):1900-10. doi: 10.1016/j.socscimed.2012.02.029. Epub 2012 Mar 17.
The U.S. has experienced a resurgence of income inequality in the past decades. The evidence regarding the mortality implications of this phenomenon has been mixed. This study employs a rarely used method in mortality research, quantile regression (QR), to provide insight into the ongoing debate of whether income inequality is a determinant of mortality and to investigate the varying relationship between inequality and mortality throughout the mortality distribution. Analyzing a U.S. dataset where the five-year (1998-2002) average mortality rates were combined with other county-level covariates, we found that the association between inequality and mortality was not constant throughout the mortality distribution and the impact of inequality on mortality steadily increased until the 80th percentile. When accounting for all potential confounders, inequality was significantly and positively related to mortality; however, this inequality-mortality relationship did not hold across the mortality distribution. A series of Wald tests confirmed this varying inequality-mortality relationship, especially between the lower and upper tails. The large variation in the estimated coefficients of the Gini index suggested that inequality had the greatest influence on those counties with a mortality rate of roughly 9.95 deaths per 1000 population (80th percentile) compared to any other counties. Furthermore, our results suggest that the traditional analytic methods that focus on mean or median value of the dependent variable can be, at most, applied to a narrow 20 percent of observations. This study demonstrates the value of QR. Our findings provide some insight as to why the existing evidence for the inequality-mortality relationship is mixed and suggest that analytical issues may play a role in clarifying whether inequality is a robust determinant of population health.
美国在过去几十年经历了收入不平等的复苏。关于这一现象对死亡率影响的证据一直存在分歧。本研究采用了死亡率研究中很少使用的方法，即分位数回归（QR），深入探讨了收入不平等是否是死亡率的决定因素这一持续存在的争论，并调查了不平等与死亡率在整个死亡率分布中的变化关系。本研究分析了一个美国数据集，其中将五年（1998-2002 年）的平均死亡率与其他县级协变量相结合，结果发现，不平等与死亡率之间的关系在整个死亡率分布中并非是固定不变的，而且不平等对死亡率的影响在 80 百分位之前一直稳步增加。当考虑到所有潜在的混杂因素时，不平等与死亡率呈显著正相关；然而，这种不平等与死亡率之间的关系并不适用于整个死亡率分布。一系列 Wald 检验证实了这种变化的不平等与死亡率之间的关系，特别是在尾部之间。基尼指数的估计系数存在很大差异，这表明不平等对死亡率约为每 1000 人 9.95 人（80 百分位）的县的影响最大，而对其他任何县的影响都较小。此外，我们的研究结果表明，传统的分析方法主要关注因变量的平均值或中位数，最多只能应用于 20%的观测值。本研究证明了 QR 的价值。我们的研究结果提供了一些见解，说明为什么关于不平等与死亡率关系的现有证据存在分歧，并表明分析问题可能在澄清不平等是否是人口健康的一个稳健决定因素方面发挥作用。
