A new biodemographic model to explain the trajectory of mortality.

Since Buffon's time (1749), biologists and demographers have repeatedly stated that a man in good health will live to be 90, 100 or 110 years old but not longer. For demographers, mortality measures essentially the current conditions: the quality of the ecological and social environment. For biologists, mortality measures mainly the ageing process. Can a biodemographic approach measure the current conditions (i.e. the quality of the environment) and the ageing schedule together, taking into account that human beings spend the greater part of their time improving the quality of their physical and social environment, making it more and more favourable to the realisation of their potential longevity? I propose two measures of the quality of this environment: first at the age when individuals (in average by cohort), in the course of their development, are the most robust and the most resistant to environmental hazards, indicated by the lowest mortality rate recorded; second at the age when individuals (in average by cohort) become frail because of the passage of time, with no or extremely little resistance to environmental hazards, indicated by a constant mortality rate among the oldest old. Between these two measures of the quality of the environment, mortality measures the ageing process leading young vigorous individuals into frail senile elders.