Abstract
“One of the problems with posing a 'bold new plan' is that you can't just extrapolate from previous plans.”
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You would not know this, observing the history of expert attempts to predict future life expectancy. Over the past 50 years, actuaries and demographers have lined up to pronounce on the trajectory which they believe longevity will follow — and every single time, they have been wildly incorrect. By and large, the rate of increase of life expectancy has far exceeded what was predicted.
Why? What's so difficult about this? Surely we can just look back at past trends and extrapolate them into the future? Well, yes, that's a great starting hypothesis — and indeed, it's exactly what everyone has always done. Why doesn't it work? There are, broadly, three reasons.
The first is that the past trajectory is somewhat complex: for a start, when viewed over the very long term longevity has clearly been increasing at an accelerating rate, having hardly risen at all from the dawn of civilization until 1800 or so and roughly trebled since then. This means that the trend must be described by a function with more than two parameters — and unfortunately there are many ways to do so that are pretty similar in terms of simplicity and plausibility.
The second problem is that, whatever method you may choose to do your extrapolation, the answer you get depends a lot on the details of what question you ask. A classic case in point is Oeppen and Vaupel's 1 celebrated report from a decade ago that “best practice” longevity, defined as the world record national life expectancy in any given year, has risen uncannily linearly since 1840 or so. Few were the commentators who looked under the hood at the supplementary material and noted that the strength of this correlation relies greatly on fine details, such as the use of just the right type of data for New Zealand in the period 1900–1950, or the fact that Japan transitioned to having negligible infant mortality a couple of decades later than the West.
But ultimately, the third difficulty eclipses those two in terms of its severity. It is that those who perform these extrapolations almost invariably feel the need to come up with answers that coincide with their intuition, and the quality of their intuition is limited by their command of the relevant knowledge. Unfortunately, that knowledge is generally technological, especially biomedical, and demographers and actuaries don't generally know much about medical research.
Thus it is, again picking just one example for illustration, that Olshansky and colleagues 2 have found it possible to predict an imminent and permanent plateauing of life expectancy, on the basis of the diminishing returns that would result from continuing to decrease mortality rates at each age at the proportional rate that they have declined in the past century. Few were the commentators who looked more closely at the statistics and noted that that decline has accelerated for the older ages where mortality is high, and that if the extrapolation is refined to take that into account then one instead derives the prediction that most people alive today will live to at least 200. Of course, this prediction is every bit as worthless as the plateau prediction, being based on the same flawed method, but the point is that the plateau prediction is the one that gets published and believed, simply because it is the answer that most people, including most demographers, were already making off the top of their heads before the study was even conceived.
I have been gratified that those with a financial interest in the accuracy of long-term longevity predictions, such as pension funds and life insurance companies, have been inviting me regularly for nearly a decade to offer my thoughts on how this parlous situation can be improved. Until quite recently, however, my pleasure at this was tempered by the niggling suspicion that the enthusiastic reception I always received would transmute by the following day into a distant memory of a bad dream, and my audience would continue as before. This was, of course, because my own predictions of the trajectory of longevity are dramatically more optimistic than those on the basis of which such professionals have already placed very, very large bets.
It is thus a source of unconfined joy to me that, within only the past year or so, one very respected forecasting company has altered the playing field entirely. Risk Management Solutions (RMS) recently began to sell a system that they have developed over the past five years, in which future life expectancies are calculated on the basis of expert opinion concerning the likely time frame for the development and subsequent dissemination of new technologies that might substantially disrupt prior trends. Regenerative medicine and more general postponement of age-related ill-health feature front and centre in their model; check it out at <