The most persistent imagery in literature, particularly in poetry, is human frailty, especially the inevitability of death. ‘Mortals’ is a term used to designate the human race. Demography, which focuses on the major events of our existence – birth, marriage, parenthood and death – has long been interested in measuring the force of mortality and in explaining its change.

This preoccupation is not as morbid as it sounds. In modern times the news has been almost entirely good. Western countries have doubled their life expectancies from around 40 years in the mid-nineteenth century1 to almost 80 years at the end of the twentieth century. The Third World’s life expectancy, taken as a whole, has climbed from 40 years in the mid-twentieth century to 65 years at its end3. If we were to enter one of those competitions to nominate the greatest advance of the latter part of the millennium, it would be difficult to overlook the pushing back of the frontiers of death and the guarantee that most people will live to old age. In Sweden, for which we have good statistics for the last two centuries, the 1800 mortality level would have meant only one-third of those born surviving until 60 years of age. In contrast, the present mortality level implies that such erosion does not take place until after 85 years.

The fact of the mortality decline is irrefutable. What is controversial is why it occurred. This lecture will draw upon research into the nature of the contemporary Third World mortality decline in order to throw light on the earlier Western decline. But, first, it will be useful to explain some of the mortality measures and their limitations.

The most frequently used measure is ‘expectation of life’, especially at birth, which is called ‘life expectancy’ in this lecture. It is the average length of life if mortality levels remain frozen.