  We wish to verify that the mortality deceleration (or decrease) is a
consequence of the bending of the shape parameter at old ages. This
investigation is based upon the Weon model (the Weibull model with an
age-dependent shape parameter) for human survival and mortality curves.
According to the Weon model, we are well able to describe the mortality
decrease after the mortality plateau, including the mortality deceleration.
Furthermore, we are able to simply define the mathematical limit of longevity
by the mortality decrease. From the demographic analysis of the historical
trends in Switzerland (1876-2001) and Sweden (1861-2001), and the most recent
trends in the other eleven developed countries (1996-2001), we confirm that the
bending of the shape parameter after characteristic life is correlated with the
mortality deceleration (or decrease). As a consequence, this bending of the
shape parameters and the mortality deceleration is associated with the
mathematical limit on longevity. These results suggest that the mathematical
limit of longevity can be induced by the mortality deceleration (or decrease)
in nature. These findings will give us a breakthrough for studying the
mortality dynamics at the highest ages.
