In recent years, the extrapolative approach to mortality prediction has been challenged by assertions that future changes in average human life span may come more or less quickly than in the past. The more optimistic view that life span will increase rapidly in the near future is partly a result of the acceleration in rates of mortality decline among the elderly in developed countries during the past few decades. From a historical perspective, however, this change is relatively recent and should be extrapolated into the future with caution. If the new pattern persists for several more decades, it will then constitute strong evidence that old trends have been replaced by new ones.
Another source of optimism about future mortality rates lies in the potential application of existing technologies (e.g., nutritional supplements, reductions in smoking) or the unusual longevity of certain groups such as Mormons and Seventh Day Adventists (52,53). Such discussions may be a good way to improve health behaviors, but they are not so good at informing predictions, largely because this same sort of advocacy influenced past trends as well. For purposes of prediction, we need to ask whether future positive reforms in lifestyle are likely to be implemented faster or more effectively than were similar reforms in the past.
From time to time, technological breakthroughs provide another source of optimism about future mortality rates. In 1998, the manipulation of a gene that halts the shortening of telomeres during the replication of human cells in vitro was a source of great optimism in the popular media, provoking rather extraordinary claims about the possibility of surviving to unprecedented ages in the near future.c Talk of cures for cancer and vaccines against AIDS promotes similar hopes. Such discussions should not be dismissed as mere wishful thinking but should also be seen in historical perspective.
As wondrous as they may be, recent scientific advances should be compared, for example, to Koch's isolation of the tubercle bacillus in 1882, which provided confirmation of the germ theory of disease and led to a great flourishing of public health initiatives around the turn of the century, or to Fleming's discovery of the antibacterial properties of penicillin in 1928, an event that led to the antibiotic drug therapies introduced in the 1940s. Extrapolations of past mortality trends assume, implicitly, a continuation of social and technological advance on a par with these earlier achievements.
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 (21,55). Sometimes, evolutionary arguments are invoked in support of the notion that further extension of the human life span is impossible, even though existing theories say little about whether and to what degree the level of human mortality is amenable to manipulation (56).
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, as noted before, trends in death rates and in maximal ages at death show no sign of approaching a finite limit. Nevertheless, although claims about fixed limits to human longevity have little scientific basis, a life expectancy at birth of 85 years—the oft-postulated theoretical limit—is within the range of values predicted by extrapolative methods through the end of the twenty-first century (Fig.6). In contrast, more optimistic claims—a life expectancy at birth of 150 or 200 years or even more—are much farther afield and would require a much larger deviation from past trends.
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