Overall levels of health are far worse in more economically unequal societies.
Research suggests that life expectancy is longer and rates of adult mortality, infant mortality, mental illness and obesity are lower in more equal societies. The strength of the relationship between inequality and health varies according to the health outcome in focus.
Overall levels of physical health and life expectancy are lower in more economically unequal developed countries. This relationship is seen in both international analyses of developed countries and analyses of US states.
A review of many different studies, including around 60 million participants in total, found that people living in regions with high income inequality have an excess risk of premature mortality and poor self-rated health, independent of their socio-economic status, age, and sex. The estimated excess mortality risk was 8% per 0.05 unit increase in the Gini coefficient.
Levels of adult obesity tend to be lower in countries where income differences are smaller. The link between income inequality and obesity is also seen internationally for children, but differences between countries are smaller.
- A much higher percentage of the population suffer from mental illness in more unequal countries; differences in inequality tally with more than triple the differences in the percentage of people with mental illness in different countries.
- Rates of depression in US states are associated with income inequality (after adjusting for income, proportion of population with a college degree and proportion over 65). The more unequal the state, the higher the prevalence of depression.
- People living within countries with a high level of inequality have an increased risk of schizophrenia.
- Infant mortality is higher in more unequal countries and more unequal US states.
- There is evidence to suggest that in developed countries, reduction of income inequality is likely to be more effective in lowering infant mortality rates than further increases in GNP per head would be. However, in developing countries a substantial reduction in infant mortality rate may be achievable by decreasing income inequality or increasing GNP per head.
The most plausible explanation for income inequality’s apparent effect on health and social problems is ‘status anxiety’. Income inequality is harmful because it places people in a steep hierarchy that increases status competition and causes stress, which in turn leads to poor health and other negative outcomes.
There is little consensus on how these mechanisms, particularly ‘status anxiety,’ work in practice, given different people’s different reference groups, their knowledge (or lack of knowledge) about social stratification and the complex nature of ‘status’ and self-esteem.
Other possible mechanisms put forward include stress in the womb and early life and the socioeconomic status of your parents.
Size of the Effect and Population
In studies showing that income inequality causes health and social problems, the size of this effect can look small in statistical terms. However, since these studies involve whole populations, the numbers of lives involved are significant.
More recent research has addressed the issue of lag effects of economic inequality on health. Studies analysing the link over a long period of time find that there is a stronger link between inequality and health than those studies that looked at the relationship at just one period of time.
Cause or Coincidence?
Not all research studies have shown an independent effect of income inequality on health and social problems. Some studies highlight the role of other factors such as material circumstances (individual income), culture/history, ethnicity and welfare state institutions/social policies:
- There are on-going controversies in interpreting the evidence on income inequality and public health. In particular, difficulties arise when looking at the effects of individual income; education; race and region. These factors may lessen the size of the effect of income inequality, while at the same time not fully accounting for the observed link between self-rated poor health and state income inequality .
- The relative stability of income inequality over time in most countries makes it difficult to test whether or not inequality is a cause of poor health or only associated with poor health and the evidence on what links inequality and health is inconclusive.
Research shows, however, that moving from a country with relatively high economic inequality to a country with relatively low inequality is linked with better health than moving from a more equal country to one which is more unequal. This suggests that economic inequality is a cause of poor health. A recent review of the literature has suggested that inequality meets many of the key criteria used in public health for showing causality.
 GNP per head <US$1000
 (Rowlingson 2011)
 (De Maio 2011)
 (De Maio 2011)
 (Babones 2008)