There is a very robust correlation between socioeconomic status and health. This correlation suggests that it is not only the poor who tend to be sick when everyone else is healthy, given that conditions such as heart disease, ulcers, type 2 diabetes, rheumatoid arthritis, certain types of cancer, and premature aging are present in all socioeconomic levels. Despite the reality of the SES Gradient, there is debate as to its cause. A number of researchers (A. Leigh, C. Jencks, A. Clarkwest—see also Russell Sage working papers) see a definite link between economic status and mortality due to the greater economic resources of the better-off, but they find little correlation due to social status differences. Other researchers such as Richard G. Wilkinson, J. Lynch, and G.A. Kaplan have found that socioeconomic status strongly affects health even when controlling for economic resources and access to health care. Most famous for linking social status with health are the Whitehall studies—a series of studies conducted on civil servants in London. The studies found that, despite the fact that all civil servants in England have the same access to health care, there was a strong correlation between social status and health. The studies found that this relationship stayed strong even when controlling for health-affecting habits such as exercise, smoking and drinking. Furthermore, it has been noted that no amount of medical attention will help decrease the likelihood of someone getting type 1 diabetes or rheumatoid arthritis—yet both are more common among populations with lower socioeconomic status. Lastly, it has been found that amongst the wealthiest quarter of countries on earth (a set stretching from Luxembourg to Slovakia) there is no relation between a country's wealth and general population health—suggesting that past a certain level, absolute levels of wealth have little impact on population health, but relative levels within a country do. The concept of psychosocial stress attempts to explain how psychosocial phenomena such as status and social stratification can lead to the many diseases associated with the SES gradient. Higher levels of economic inequality tend to intensify social hierarchies and generally degrades the quality of social relations—leading to greater levels of stress and stress related diseases. Richard Wilkinson found this to be true not only for the poorest members of society, but also for the wealthiest. Economic inequality is bad for everyone's health. Inequality does not only affect the health of human populations. David H.