Income inequality and health: what have we learned so far?

Many developed countries have experienced a sharp rise in income inequality during the past three decades, and the United States is no exception (1). For example, the average annual salary in America in inflation-adjusted 1998 dollars increased from $32,522 in 1970 to $35,864 in 1999, that is, a modest 10 percent increase over three decades. By contrast over the same period, the average annual compensation of the top 100 chief executive officers rose from $1.3 million (or 39 times the pay of an average worker) to $37.5 million (or more than 1,000 times the pay of an average worker) (2). Recent trends in wealth inequality have been equally noteworthy. The net worth of families in the top decile rose by 69 percent, to $833,600 in 2001, from $493,400 in 1998. By contrast over the same period, the net worth of families in the lowest fifth of income earners rose 24 percent, to $7,900. The median accumulated wealth of families in the top 10 percent of the income distribution was 12 times that of lower-middle-income families through much of the 1990s, but in 2001, the median net worth of the top earners was about 22 times as great (3). It is by now widely accepted that income poverty is a risk factor for premature mortality and increased morbidity (4). It should also be noted that there exists persuasive evidence indicating the reverse pathway, from poor health status to persistent poverty and poorer economic growth (5). In this review, however, we focus on the question: Does the unequal distribution of income in a society pose an additional hazard to the health of the individuals living in that society? Earlier ecologic studies, summarized elsewhere (6, 7), suggested an association between income inequality and poor health status. However, these studies have been criticized because of their inability to disentangle the effects of individual income (and income poverty) from the contextual effects of income inequality (6). In other words, an ecologic association between income inequality (e.g., measured by the Gini coefficient of income distribution at the US state level) and poor health (e.g., measured by age-adjusted mortality rates within each state) may reflect either a contextual effect of income inequality on health, or a compositional effect of income-poor individuals residing in unequal states, or both. In attempts to overcome this methodological limitation of ecologic studies, researchers have published nearly two dozen multilevel studies of income inequality and health since 1997. Multilevel studies have the ability to simultaneously assess the associations of individual income and societal income inequality with individual health status. In this paper, we review the published multilevel studies of income inequality and health. Although the published evidence so far is by no means conclusive about the relation between income distribution and population health, our aim is to draw attention to some emerging patterns in the accumulated findings and to suggest future directions for research in this topic. We start, however, by briefly rehearsing the conceptual basis for the relation between income inequality and health. Since the most common statistic that is used to measure income inequality is the Gini coefficient, we also outline a brief description of this measure.