Robert Lucas once famously said regarding the determinants of economic growth, "once one starts to think about them, it is hard to think of anything else." The same could be said for the determinants of mortality, since the length of life is as critical a measure of our wellbeing as is our income.
In "Determinants of Mortality" (NBER Working Paper 11963), David Cutler, Angus Deaton, and Adriana Lleras-Muney explore many aspects of this important topic, including the decline in mortality rates over time, differences in mortality across countries, and differences in mortality across groups within countries.
For most of human history, life expectancy has been short - perhaps 25 years for our hunter-gatherer ancestors and only 37 years for residents of England in 1700. Dramatic changes began in the 18th century, with life expectancy in England rising to 41 years by 1820, 50 years by the early 20th century, and 77 years today. The decline in mortality rates was particularly sharp among children. This can be explained by the near elimination of deaths from infectious diseases, formerly the most common cause of death, since the young are most susceptible to infection.
Weighing the various explanations for these mortality reductions, the authors see three phases. From the mid-18th century to the mid-19th century, improved nutrition and economic growth played a large role, as did emerging public health measures. From the mid-19th century to the early 20th century, the delivery of clean water, removal of waste, and advice about personal health practices all led to lower mortality rates, though urbanization had the opposite effect, due to high mortality rates in cities. Since the 1930s, mortality reductions have been driven primarily by medicine, first by vaccination and antibiotics and later by the expensive and intensive interventions that characterize modern medicine.
Looking across countries, there are vast differences in life expectancy, as illustrated in Figure 1. There are also sharp differences in who dies and from what. Deaths among children account for 30 percent of deaths in poor countries but less than 1 percent of deaths in rich countries. Most deaths in rich countries are from cancers and cardiovascular disease, while most deaths in poor countries are from infectious diseases.
Though differences persist, many poorer countries have recently experienced large improvements in life expectancy. In India and China, life expectancy has risen by 30 years since 1950. Even in Africa, life expectancy rose by 13 years from the early 1950s until the late 1980s, when the spread of HIV/AIDS reversed the trend.
What factors explain these reductions in mortality rates? Some of the leading candidates are changes in income, literacy (particularly among women), and the supply of calories, as well as public health interventions such as immunization campaigns, improvements in water supply, and the use of antibiotics. Although it seems logical that economic growth should improve health, the authors point out that the evidence for this is mixed at best. This may be because urbanization often goes along with growth, or because growth must be accompanied by effective public health measures in order to bring about mortality reductions.
Within developed countries such as the U.S., there are well documented differences in mortality rates by race, income, education, occupation, or urban/rural status, with the low socioeconomic status groups exhibiting higher mortality rates. Some explanations for these inequalities include differences in access to medical care, in access to the resources needed to buy food and shelter, in health related behaviors such as drinking and smoking, or in levels of "psychosocial stress." While the link between social status and health is likely not due to any single factor, it does seem to be primarily a result of health affecting income rather than the reverse. Education seems to have a positive effect on health, which may result from differential use of health knowledge and technology.
Is there a universal theory of mortality that can explain improvements over time, differences across countries, and differences across groups? The authors argue that "knowledge, science, and technology are the keys to any coherent explanation," perhaps controversially downplaying the role of income. As for the future, they predict that an acceleration in the production of new knowledge and treatments is likely to increase inequality in health outcomes in the short run, but the silver lining is that "help is on the way, not only for those who receive it first, but eventually for everyone."
The authors gratefully acknowledge financial support from the National Institute on Aging through the NBER (grant P01 AG05842-14) and directly (grant R01 AG015911 and R01 AG20275-01).