By Sara Gorman

In the late 1940s and 1950s, it became increasingly evident that mortality rates were falling rapidly worldwide, including in the developing world. In a 1965 analysis, economics professor George J. Stolnitz surmised that survival in the “underdeveloped world” was on the rise in part due to a decline in “economic misery” in these regions. But in 1975, Samuel Preston published a paper that changed the course of thought on the relationship between mortality and economic development.

In the Population Studies article “The changing relation between mortality and level of economic development,” Preston re-examined the relationship between mortality and economic development, producing a scatter diagram of relations between life expectancy and national income per head for nations in the 1900s, 1930s, and 1960s that has become one of the most important illustrations in population sciences. The diagram shows that life expectancy rose substantially in these decades no matter what the income level. Preston concluded that if income had been the only determining factor in life expectancy, observed income increases would have produced a gain in life expectancy of 2.5 years between 1938 and 1963, rather than the actual gain of 12.2 years. Preston further concluded that “factors exogenous to a country’s current level of income probably account for 75-90% of the growth in life expectancy for the world as a whole between the 1930s and the 1960s” and that income growth accounts for only 10-25% of this gain in life expectancy.

Preston’s next main task was to contemplate what these “exogenous factors” might be. Preston proposed that a number of factors aside from a nation’s level of income had contributed to mortality trends in more developed as well as in less developed countries over the previous quarter century.  These factors were not necessarily developed in the country that enjoyed an increase in lifespan but rather were imported and therefore are, according to Preston, less dependent on endogenous wealth. The exact nature of these “exogenous” factors differed according to the level of development of the nation in question.  Preston identified vaccines, antibiotics, and sulphonamides in more developed areas and insect control, sanitation, health education, and maternal and child health services in less developed areas as the main factors that contributed to increased life expectancy.

Preston’s paper continues to provide guidance in development theory and economics today. But there was and continues to be considerable resistance to Preston’s theory, mostly from economists. Shortly after Preston’s article appeared, Thomas McKeown published two books that argued essentially the opposite: that mortality patterns have everything to do with economic growth and standards of living. Pritchett and Summers argued in 1996 that national income growth naturally feeds into better education and health services, which in turn contribute to higher life expectancy.

How well does Preston’s analysis hold up today? For one thing, Preston did not foresee the seemingly intimate connection between development and the recent rapid increased incidence prevalence of some chronic diseases. As developing nations urbanize and become more affluent, noncommunicable diseases, such as cancer and heart disease, many secondary to “lifestyle” issues like obesity and lack of physical exercise, are now on the rise, with the potential to lower life expectancy significantly. So is wealthier healthier, to use the words of Pritchett and Summers? Not necessarily, as we are seeing increasingly.

Why is it so important to try to work out the relationship between health and wealth? If we assume that improvements in healthcare systems grow naturally out of increased wealth, then developing countries should be focusing primarily on economic growth in order to improve their healthcare. This must be true to a certain extent, but, as Preston is quick to point out, there are other factors that affect the health of a nation, and it is not sufficient to assume that economic growth will automatically lead to improved life expectancy. Preston’s analysis tends to emphasize instead that health systems strengthening and biological innovation must always take place beside economic growth to insure better health. Whether or not we can completely agree with Preston’s assertion that wealthier is not necessarily healthier, it is certainly the case that his landmark article stimulated an essential conversation about the relationship between economic development and mortality that continues avidly to the present day.

Sara Gorman is a PhD candidate at Harvard University. She has written extensively about HIV, TB, and women’s and children’s health for a variety of public health organizations, including Save a Mother and Boston Center for Refugee Health and Human Rights. She most recently worked in the policy division at the HIV Law Project.