The twentieth century saw major improvements in the human condition, not only in the rich countries of the world but also in developing nations. Nothing has been more remarkable, however, than the extension of life expectancy, which has increased by about 30 years since 1900 in England, France, and the United States and in equal or larger amounts in such countries as India, China, and Japan. Among the nations of the Third World, the rate of improvement has been nearly twice as fast as among the nations in the Organization for Economic Cooperation and Development (OECD) (see Table 1.1).

What is responsible for this unanticipated extension of human life? That question has occupied some of the best minds of the past century in both the social sciences and the biomedical sciences, and it is also the central question of these chapters. The drive to explain the secular decline in mortality did not begin until about World War I because it was uncertain before that time whether such a decline was in progress. There was little evidence in the first four official English life tables covering the years 1831–80 of a downward trend in mortality. Although the signs of improvement in life expectancy became more marked when the fifth and sixth tables were constructed, covering the 1880s and 1890s, few epidemiologists or demographers recognized that England was in the midst of a secular decline in mortality that had begun about the second quarter of the eighteenth century and that would more than double life expectancy at birth before the end of the twentieth century.