The succinct version of the actuarial viewpoint on limits to the healthy human life span is as follows:

• limits to life span presently exist
• these limits are increasing at a modest rate, something like 1 year every 5 years
• there appears to be no ceiling to life span in the future

This viewpoint is derived from an examination of mortality rates in various population groups over time, but still comes to the same answer as a consideration of the operation of our biology from first principles, such as the reliability theory of aging and longevity. Our bodies are enormously complex machines and, like all machines, the life span and failure rates are determined by the degree and capabilities of maintenance.

So: more knowledge, better medicine and better biotechnology means longer, healthier lives. This will happen slowly as a matter of course, since better medicine reduces the rate at which damage accumulates over a lifetime. It could happen much faster if we put our minds to it and there was a much greater level of support for moving directly and deliberately to extend healthy life span by the most efficient means possible.

Here is the abstract for another recently published paper giving support to the actuarial viewpoint outlined above; hopefully you recall a little of statistics lessons from past years. If not, then skip to the last line and the explanation below:

Increase in common longevity and the compression of mortality: The case of Japan

This study shows a strong increase in the modal age at death (M) in Japan over a period of 50 calendar years, accompanied by a clear decrease in the standard deviation of ages at death above M (SD(M+)) until the 1990s for men and the mid-1980s for women. For the most recent periods SD(M+) appears to have stopped decreasing, even though M has continued to increase linearly. This stagnation in SD(M+) has been accompanied by stagnation in q(M). The number of deaths at M (d(M)) and the number of deaths at and above M (d(M+)) have increased, but significantly more slowly since the period 1975-79. Since the 1980s an acceleration in the increase of M+kSD(M+), our indicator of the longest life durations, has been essentially due to the pause in SD(M+). Our data do not suggest that we are approaching an upper limit in human longevity.

Translated for those of us far past our school days: the most common age of death is increasing with no signs of slowing down. If there was a compression of mortality - in other words, if medicine was only succeeding in pushing all our life spans ever closer up to some absolute limit - then we would see the distribution of ages at death compressed down into a smaller span of years. However, this distribution of age of death for the longest lived people continues to expand ahead in years. Compression of mortality is not what we are seeing here, but rather something much more like the scenario of better maintenance leading to longer-lasting machinery.

This bodes well for the future, should we get our act together and push harder for the development of better medicine aimed at the repair of damage that causes aging.

Technorati tags: aging, statistics

Posted by Reason at March 25, 2007 1:40 PM | TrackBack (1)