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« The Future Starts Now | Main | RepRap: Machines building machine parts »

March 18, 2005

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jim moore

When people talk about developments in longevity they of often use a statistic called Life Expectancy (at Birth). This is the wrong statistic to focus on when talking about any developments in anti-aging.

To understand why, you have to understand how demographers calculate "Life Expectancy". A simple way to think about it is; you go out at the beginning of the year and count the number of people and how old they are. At the beginning of the next year you do the same thing. This allows you to calculate the probability that someone 1 years old will die, the probability that someone 2 years old will die, ... the probability that someone 121 years old would die. In other word you calculate what is called Age Specific Mortality Rates. What is commonly called Life Expectancy (at Birth) is a way to summarize all the Age Specific Mortality Rates for that year.

A hundred years ago in the US, if you graphed the individual Age Specific Mortality Rates you would get a U shaped curve. The probability of death was high when you were very young and high when you were old. The probability of death was low when you were a young adult. Today if you graph the Age Specific Mortality Rates it is a J shaped curve. You have a smaller (but still non zero) probability of death at a very young age, the probability falls and remains very low. After middle age the probability of death really starts to climb and continues to increase with age.

Now if you are interested in how fast peoples life span is growing using Life Expectancy at Birth will dramatically overestimate the change. The more appropriate statistic to use is Life Expectancy at 75 years. By using Life Expectancy at 75 you focus only on how Age Specific Mortality Rates for people 75 years old and older are changing. According to the US Census Bureau from 1995 - 2001 Life Expectancy at Birth in the US increased by 1.4 years, Life Expectancy at 75 years increased 0.5 years. In other words the more appropriate statistic is showing ~3 times less change.

michael vassar

No-one believes in near future radical longevity as a result of extrapolating historical trends. People believe in it because they are aware of specific technological solutions to specific physical problems associated with aging.

jim moore

Michael,
Ray Kurzweil and others do extrapolate from historical data on Life Expectancy at Birth.

My main point was: If you want to look for evidence that our life spans are actually increasing you should focus on how the Age Specific Mortality Rates of the old are changing. If radical longevity is possible the Age Specific Mortality Rates will not increase with age. If you graphed the probability of death verse age in a society with radical longevity you would get a flat line rather than the J shaped curve of today or the U shaped curve of non-industrial societies.

Brett Bellmore

Integrating age specific mortality rates, without taking into account the fact that medical technology is improving all along, is highly inaccurate in a society where medical progress is rapid.

Most of the people reaching retirement age today, for instance, would be long dead, if they'd really been subject through their entire life to the age specific mortality rates of their date of birth.

The application to life extending technologies is particularly obvious: An intervention that lengthens your life not only lengthens it, it enables you to last until better interventions have been developed. You don't need a pill that makes you immortal, just one that lets you last until the next pill is invented.

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