Late-Life Mortality Deceleration,
Mortality Levelling-off, Mortality Plateaus

Here are some key references to pioneer scientific publications, which first described in detail the phenomenon of mortality leveling-off at advanced ages.

Mortality leveling-off in humans

Although the first notion on the deviation from the Gompertz law at advanced ages were made by Gompertz himself in 1825, the first article specifically devoted to the problem of mortality at advanced ages was published in 1939 by British researchers Greenwood and Irwin:

This 1939 article correctly describes and forestalls the main specific regularities of mortality at advanced ages.
The first important finding was formulated by Greenwood and Irwin in the following way: "…the increase of mortality rate with age advances at a slackening rate, that nearly all, perhaps all, methods of graduation of the type of Gompertz's formula over-state senile mortality" (Greenwood, Irwin, 1939, p.14).  This observation is known now as the "late-life mortality deceleration."

The authors also suggested "the possibility that with advancing age the rate of mortality asymptotes to a finite value" (Greenwood, Irwin, 1939, p.14).  Their conclusion that mortality at exceptionally high ages follows a first order kinetics (also known as the law of radioactive decay) was confirmed later by other researchers, including A.C. Economos (see below), who demonstrated the correctness of this law for humans and laboratory animals.  This observation is known now as the "mortality leveling-off" at advanced ages, and as the "late-life mortality plateau."

Moreover, Greenwood and Irwin made the first estimates for the asymptotic value of human mortality (one-year probability of death, qx) at extreme ages using data from the life insurance company.  According to their estimates, "… the limiting values of  qx  are 0.439 for women and 0.544 for men" (Greenwood and Irwin, 1939, p.21).  It is interesting that these first estimates are very close to estimates obtained later using more numerous and accurate human data including recent data on supercentenarians (those who survive to age 110).

Mortality leveling-off in animals and manufactured items

Angelos Economos was the first researcher who described mortality leveling-off in animals and manufactured products.   He demonstrated mortality leveling-off at advanced ages for invertebrates (including fruit flies and house flies), rodents, and several manufactured products:

After study of mortality kinetics for many biological species A. Economos came to the following conclusion: "...Gompertz's law is only an approximation, not valid over a certain terminal part of the lifespan, during which force of mortality levels off." (Economos, 1980, p.317, abstract).  The phenomenon of mortality leveling-off at advanced ages he called "a non-Gompertzian paradigm for mortality kinetics."