The authors also suggested “the possibility that with advancing age the rate of
mortality asymptotes to a finite value” (Greenwood and 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 with exponential decline in survival probabilities)
was confirmed later by other researchers, including A.C. Economos (1979; 1980), who
demonstrated the correctness of this law for humans and laboratory animals (linear
decrease for the logarithm of the number of survivors). This observation is known now as
the “mortality leveling-off” at advanced ages, and as the “late-life mortality plateau”
(Curtsinger et al., 2006).