Greater lifetime expectations

SHIRO HORIUCHI

Shiro Horiuchi is in the Laboratory of Populations, Rockefeller University, 1230 York Avenue, Box 20, New York, New York 10021-6399, USA.
e-mail: horiush@rockefeller.edu

Such a deceleration has not occurred yet. On page 789 of this issue, Tuljapurkar et al.¹ show that during the second half of the twentieth century, age-specific death rates in the G7 industrialized countries — the United States, Canada, Japan, France, Germany, Italy and the United Kingdom — continued to decline at a remarkably constant pace. There was no noticeable sign of a slowing down. This report follows one by Wilmoth², published two years ago, which indicated that the mortality reduction in the United States, measured by the age-standardized death rate, was even faster in the second half of the century than in the first half.

Assuming a further continuation of the stable pace of mortality decline, Tuljapurkar et al. forecast that the life expectancy at birth is likely to increase faster than predicted by the governments of the G7 countries (Fig. 1). This implies that the elderly population in the near future will be greater than in the official forecasts. Depending on the general state of health, the larger elderly population could entail higher medical costs and demands for long-term care and other services, and higher pension payments.

The findings give rise to two interrelated questions. Why has mortality decline not started to slow down? And will it continue into the future? Studies in demography, epidemiology and the biology of ageing and longevity provide clues to the answers. Underlying the steady decrease in mortality level were shifts in the pattern of mortality reduction³. In the second half of the nineteenth century and the first half of the twentieth century, there were large decreases in the number of deaths from infectious and parasitic diseases, and from poor nutrition and disorders associated with pregnancy and childbirth. The reduction was pronounced among infants, children and young adults, but modest among the elderly. This led to a view that the fall in death rates at young and middle ages to low levels would soon exhaust the potential to prolong life expectancy further.

But that view did not — and could not — take account of developments in the second half of the twentieth century. Mortality from degenerative diseases, most notably heart diseases and stroke, started to fall⁴. The reduction was pronounced among the elderly^{5, 6}, and some suspected that it might have been achieved through postponing the deaths of seriously ill people. But in the United States at least^{7, 8}, it seems that the health of the elderly greatly improved in the 1980s and 1990s, suggesting that the extended length of life in old age is mainly due to better health rather than prolonged survival in sickness.

These days, the existence of a biological limit to human longevity is considered questionable⁹. Biologists used to think that senescent processes might be programmed into the biological clock of the human body. But they have largely shifted to the view that senescence is mainly due to the body's imperfect systems of maintenance and repair, which allow the long-term accumulation of unrepaired damage in macromolecules, cells, tissues and organs¹⁰. Progress in ageing research may eventually lead to new medical approaches that lower the rates of damage accumulation¹¹.

Overall, the evidence supports the expectation that scientific, technological and economic developments will lead to more effective control of degenerative diseases and ageing processes, making it possible to sustain the rapid pace of mortality decline. However, this prospect is not unconditional. New threats to health and survival are arising, including the emergence and re-emergence of infectious diseases, increasing pollution, and the proliferation of nuclear, biological and chemical weapons. If we fail to control these hazards, some of the large gain in the life expectancy of the past 150 years may well be lost³.

What about the world outside the G7 nations? The prospect of life expectancy soon exceeding 80 years is limited to these and other countries with highly developed market economies. They are mostly in Western Europe, North America and Eastern Asia. In the rest of the world, the life expectancy is on average still under 65 years¹². In subSaharan Africa, it is under 50 years and may be falling because of the AIDS epidemic, as well as stagnated economic development and political conflicts. Some industrialized nations in Eastern Europe and the former Soviet Union have seen only slow increases, and even occasional reversals, in life expectancy.

It is not surprising that government forecasts of life expectancy and the size of the elderly population in G7 countries are much more conservative than those of Tuljapurkar et al.¹. In the past, national governments, as well as international organizations and academic researchers, have almost invariably underpredicted life expectancy in industrialized market-economy countries. For example, in 1984 the United Nations prepared international population projections based on the assumption that the maximal life expectancy in human populations is 75 years for males and 82.5 years for females. The assumption soon proved wrong: in Japan, for instance, the figures in 1998 were 77.2 years and 84 years respectively. The reason for the underestimates is partly because forecasters regard conservative prospects as less controversial and so 'safe'; and partly because they have often extrapolated past trends in death rates by cause of death or age (or both), missing future transitions in the cause-of-death pattern and age pattern of mortality reduction. It looks as if the same error is still being made.

References

Nature © Macmillan Publishers Ltd 2000 Registered No. 785998 England.