Together with economic growth and technological advances, improvements in health and longevity are the typical hallmarks of a population’s transition to modern society. Among the earliest countries to undergo such experiences were England and France, where mortality rates began declining steadily during the eighteenth century. Elsewhere in western and northern Europe, health and longevity began to improve during the nineteenth century. In the twentieth century, this pattern has been replicated in developing countries throughout the world.

Understanding the causes that underlie this pattern of mortality decline is important not only as a matter of historical interest but also because of the practical implications for policies that aim to improve life in developing countries, and for forecasting changes in mortality in developed countries. Accordingly, there has been much interest in identifying the causes of patterns of mortality decline and measuring their impact. By the 1960s, a consensus had emerged that the factors underlying mortality trends could be delineated within four categories, as reported in a study by the United Nations (UN) (1953): (1) public-health reforms, (2) advances in medical knowledge, (3) improved personal hygiene, and (4) rising income and standards of living. A later UN study (1973) added as an additional category “natural causes,” such as a decline in the virulence of pathogens.