Mathematically, aging can be quantified using mortality curves such
as those of Figure 1. There are several mathematical functions that
can be used (
, pp. 103-124). From these
equations we can derive the initial mortality rate (IMR), which is the
mortality rate independent of aging, often calculated from the
mortality rate prior to its exponential increase with age; in this case,
IMR = 0.0002/year since that is the mortality rate at ages 10-20.
Another important variable taken from the Gompertz equation is the
mortality rate doubling time (MRDT) given by MRDT = 0.693/
, pp. 22-24). Hence, MRDT = 0.693/0.0800 = 8.66
years. In fact, human populations have a MRDT of about 8 years.
This means that after our sexual peak, or roughly age 30, our
chances of dying double about every 8 years.
Demographic measurements of aging, such as the MRDT, may then
serve as estimates of the rate of aging. Changes in the MRDT are
expected to reflect changes in the rate of aging, but the same is not
true for the IMR (
Finch and Pike, 1996
et al., 2005a
). For example, the life expectancy at birth increased
considerably in the past 100 years: in the US, the life expectancy at
birth jumped from 47.3 years in 1900 to 77.3 years in 2002
National Center for Health Statistics, Data Warehouse on Trends in
Health and Aging
). Still, the rate of aging and the MRDT have
remained unaltered for thousands of years (
). What happened was that the IMR, which is not affected by
the aging rate, was lowered due to breakthroughs in different areas,
such as in the war against infectious diseases, thus lowering mortality
rates across the entire lifespan and increasing the life expectancy.
As is common knowledge, women have a higher life expectancy