Tuesday, June 7, 2011

Historical increase in human longevity

The word "longevity" is sometimes used as a synonym for "life expectancy" in demography or to connote "long life". Reflections on longevity have usually gone beyond acknowledging the brevity of human life and have included thinking about methods to extend life.

Life expectancy during much of pre-modern history averaged just below 30 years. Part of the reason for such a low figure is that many children died at a very young age, pulling down the average life expectancy. Those who didn’t die young had a good chance of surviving to what we now call “middle age”.

After the Industrial Revolution many more children survived into adulthood and by the beginning of the 20th century average life expectancy in the developed world was close to 50, whereas for the world as a whole it was only around 40 years. The figures now are 78 and 67 respectively.

Let’s review the numbers in more details. Average lifespan at birth:
·         Neanderthals (30,000 years ago) – 30 years.

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·         Neolithic (8,500 BC to 3,500 BC) – 38 years.

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·         Classic Greece and Rome (500 BC to 500 AD) – 35 years.

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·         Early Medieval – 48 years.
·         Late Medieval – 38 years.

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·         Sweden, 1750 – 36 years.
·         General Victorian (1850 – 1900) – 40 years. 

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·         India, 1880 – 25 years.
·         USA, 1900 – 48 years.
·         France, 1950 – 66 years.
·         Japan, 2007 – 83 years.

As you see, from the beginning of the human race, the life expectance significantly improved, however, the substantial changes occurred only recently. This graph shows the rapid and sudden improvement after centuries of stagnation:

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The reason for this sudden improvement during and after the industrial revolution is a combination of improved medical technology and higher wealth.

The historical perspective

Demographers who look at life expectancy trends over time, and give considerable weight to the findings from historical study, expect there to be continued improvements to life expectancy. Historical analysis undertaken collaboratively in the Department of Geography in Cambridge and at the Max Planck Institute for Demographic Research in Rostock, Germany, has focused on life expectancy by looking at records dating back to the 1840s in a large number of countries for which estimates can be made. These data have been used to chart trends over a period from 1840 to the present day.

In 1840, life expectancy was highest among Swedish women, who lived on average 45 years. Today, as mentioned earlier, the longest life expectancy is to be found among Japanese women, whose lives on average exceed 85 years. This improvement appears to have been steady, with an average increase in life expectancy of three months every year until the present day. Record male longevity has risen rather more slowly yet still shows the same linear rise, with Japanese men now holding the record for the longest male survivors at an average age of just over 78 years.

In England, female adult mortality has been estimated from Anglican parish registers and genealogies relating to the Peerage, which reveal an increase in adult survivorship that began as early as 1700. Interestingly, this rise was experienced by both high- and low-income members of society, suggesting that at least initially the process derived from health-related changes rather than from a rise in economic well-being.

Future longevity

Historical demographic analysis has exposed a line of challenging enquiry: will life expectancy continue to rise, as predicted from the previous trends, or are we reaching a biologically determined ceiling? It would be rash on the basis of the historical trends to promote the idea of the attainment of eternity among humans or even an untrammelled route to a life expectancy at birth of 100 years by 2060, as some enthusiasts have done. Nonetheless, it is certain that centenarians will soon become commonplace individuals in our midst.

A significant part of the improvement in average life expectancy, certainly before 1950, was achieved by improvements in survivorship among the youngest age group; behavioral changes associated with health enhancement, such as a reduction in smoking, will also have contributed in recent decades. However, looking to the future, a fundamental finding of the historical analysis is that there is no sign of a leveling off of rates of improvement at the oldest ages. In fact, these rates of improvement would appear to be rising rather than leveling. It is perhaps noteworthy that, following a sequence of years when they had to raise their maximum life expectancy repeatedly, the United Nations has at last abandoned the practice of imposing such limits in their population projections.


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