From: Robin Hanson (hanson@econ.Berkeley.EDU)
Date: Fri Apr 24 1998 - 11:44:49 MDT
On pp.395-396 of the 17Apr98 Science, John Wilmoth has a thoughtful
article, "The Future of Human Longevity: A Demographer's Perspective".
"The extrapolative approach to prediction is particularly compelling in
the case of human mortality. ... Preductions of future life expenctancy
by extrapolation yield values that are not too different from what is
observed today. ... An independent study, based on a purely extrapolative
technique, yielded more optimistic results (U.S. life expectancy at birth
in 2050 of 84.3 years for both sexes combined) ... Although short-term
fluctuations have been common, long-term mortality trends have been
remarkably stable. ... Quite simply, a projection based on less that 20
years of experience that extends 50 to 100 years into the future is
imprudent, if not foolhardy. ...
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 partly will increase rapidly in the near
future is the result of the acceration in rates of mortality decline
among the elderly in developed countries during the past few decades (5).
But from a historical perspective, this change is 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
the old trends have been replaced by new ones.
Another source of optimisim about future mortality rates likes in the
potential application of existing technology (for example, nutritional
supplememtns and reductions in smoking) or the unusual longevity of
certain groups (such as Mormons and Seventh Day Adventists) (6). Such
discussions may be a god way to improve health behaviors, but they are
not so good at informing predictions, largely because this same srot of
advocacy influenced past trends as well. For purposes of prediction,
we need to ask whether future positive reforms in life-style are likely
to be implemented faster of more effectively than were similar reforms
in the past.
From time to time, technological breakthroughs provide another source
of optimism about future mortality rates. Recently, 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 (7). 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 wonderous 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 anibiotic drug therapies
introduced in the 1940s. Extrapolations of past mortality trends assume,
implicitly, a continuation of social and technological advance on a part
with the earlier achievements."
Robin Hanson
hanson@econ.berkeley.edu http://hanson.berkeley.edu/
RWJF Health Policy Scholar, Sch. of Public Health 510-643-1884
140 Warren Hall, UC Berkeley, CA 94720-7360 FAX: 510-643-8614
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