Will the twenty-first century witness as large an increase in the average life expectancy of the rich countries–thirty to forty years–as occurred during the twentieth century? Most experts believe it will not. The middle estimate of the U.S. Census Bureau, for example, is that the increase in life expectancy between 2000 and 2050 will be only about 7 years, and the estimated increase for the entire twenty-first century is just 13 years. This is less than half the increase that occurred during the twentieth century. The same conservatism is evident in the projections of the UN, OECD, and other national and international agencies.

These pessimistic projections rest on several propositions. Perhaps the most widely accepted is the proposition that opportunities for large reductions in mortality rates are possible only when death rates under age 5 are very high. Proponents of this view argue, for example, that the sharp decline in U.S. mortality rates during the twentieth century was the result of a unique opportunity that cannot be replicated by those nations that have already experienced it: the opportunity to wipe out the majority of deaths due to acute infectious diseases, which were concentrated in infancy and early childhood. Whereas more than a third of all deaths at the turn of the twentieth century were of children under 5, today infant and childhood deaths are less than 2 percent of the annual total.