The future of human longevity: A demographer's perspective
Demographers are criticized for basing predictions on past trends without taking mechanisms into account, but the criticism is only valid if we understand and can incorporate these mechanisms into alternative predictive methods. Wilmoth thinks that life expectancy gets too much emphasis. Life expectancy is deceptive in that it reflects age patterns of morbidity/mortality as well as underlying declining death rates. When put into a historical perspective, recent medical advancements (nutritional supplements, smoking) do not compare to the vast influence of past advancements like germ theory or penicillin. However, there seems to be little evidence that populations are approaching any sort of life expectancy limit (net of unforseen disaster). It seems that e0=85 in 2050 is within the realm of extrapolation from past trends. The appeal of extrapolation lies in the stability of long-term trends which reflect complex mechanisms rather than single-variable improvements in mortality. Responses: Gavrilov and Gavrilova point out that ASDRs aren't quite as steady as the aggregate death rates used by Wilmoth, and that underlying mechanisms must be researched. Olshansky and colleagues also assert that biological mechanisms must come into play when making mortality predictions. They state that extrapolation is only valid for short time windows. Miguel's summary: Demographers claim some expertise in predicting future mortality levels usually through extrapolation of past trends. Biologists and others are often critical of this approach because it seems to ignore underlying mechanisms. However, our understanding of the complex interactions of social and biological factors that determine mortality levels is still imprecise. The extrapolative approach to prediction is particularly compelling in the case of human mortality: (1) mortality decline is driven by a widespread desire for a longer, healthier life; (2) mortality has been falling steadily, and lifespan increasing, for more than 100 years in economically advanced societies; (3) these gains in longevity are the result of a complex array of changes; (4) much of this decline can be attributed to the directed actions of individuals and institutions, which is likely to continue in the future. Predictions of future life expectancy by extrapolation yield values that are not too different from what is observed today. An important issue for consideration in forecasting mortality is the time frame both the time frame of the data that form the input to an extrapolation and the time horizon of the projection itself. Although short-term fluctuations have been common, long-term mortality trends in industrialized countries have been remarkably stable. Errors result from extrapolating farther into the future than is warranted. Another common error results from an undue emphasis on trends in life expectancy. Although the increase in life expectancy has slowed down, the decline in death rates has quickened. In recent years, the extrapolative approach to mortality prediction has been challenged by assertion that future changes in average human life-span may come more or less quickly than in the past. Moreover, there are potential applications of existing technologies that may elongate life. It is even possible that technological breakthroughs will provide another source of optimism about future mortality rates. However, such scientific advances should be compared with past ones when forecasting their effects on mortality. More pessimistic scenarios of the future course of human longevity are based on notions of biological determinism or arguments about practicality, yielding the now-familiar claim that life expectancy at birth cannot exceed 85 years. Such scenarios arise from evolutionary theory, which predicts a sharp rise in death rates in post-reproductive years, because deleterious genes operating at these ages have evolved with no opposition from the forces of natural selection. However, current patterns of survival indicate that death rates in later life can be altered considerably by environmental influences, and there is little conclusive evidence that further reductions are impossible. Furthermore, trends in death rates and in maximal ages at death show no sign of approaching a finite limit. Extrapolation rides the steady course of past mortality trends, whereas popular and scientific discussions of mortality often buck these historical trends, in either an optimistic or pessimistic direction. Of course, extrapolation is not without its flaws. It could not predict the rise in mortality in the former Soviet Union after 1990 or the emergence of AIDS in Africa and elsewhere. However, such observations are less an indictment of extrapolation than a demonstration that the greatest uncertainties affecting future mortality trends derive from social and political, rather than technological, factors.