The new trends in survival improvement require a revision of traditional gerontological concepts.

In 1960, Strehler and Mildvan (SM) theoretically predicted that the parameters of the Gompertz approximation to a mortality curve are negatively correlated. This means that the changes in the human mortality rate resulting from improvement in living standards, progress in health care or the influence of other factors must follow certain regularities prescribed by dependence between the Gompertz parameters. Such dependence, called SM correlation, was then confirmed in a number of empirical studies using period data on human mortality. Since the SM theory was based on the cohort model of mortality, it was tacitly assumed that period and cohort SM correlation patterns are similar. The remarkable stability of the SM correlation pattern revealed in these studies was often regarded as manifestation of a universal demographic law regulating changes in the age pattern of mortality rates. In this paper, we investigated trends in mortality decline in France, Japan, Sweden and the United States. In contrast with traditional expectations, we found that the SM correlation pattern was relatively stable only in certain periods of a population's survival history. Recently, several new correlation patterns emerged and, despite some differences in the timing of the changes, the new patterns are remarkably similar in all four countries. Contrary to traditional expectations, the patterns are not the same for cohort and period mortality data when SM correlations are calculated for France, Sweden and the United States. We show that some changes in the patterns of SM correlation admit interpretation in terms of a biological mechanism of individual adaptation (survival trade off). Some other patterns, however, contradict basic postulates of the SM theory. This indicates the need for revision of traditional concepts establishing the relationship between physiological and demographic patterns of aging.

Have the oldest old adults ever been frail in the past? A hypothesis that explains modern trends in survival.

Three important results concerning the shape and the trends of the human mortality rate were discussed recently in demographic and epidemiological literature. These are the deceleration of the mortality rate at old ages, the tendency to rectangularization of the survival curve, and the decline of the old age mortality observed in the second part of the 20th century. In this paper we show that all these results can be explained by using a model with a new type of heterogeneity associated with individual differences in adaptive capacity. We first illustrate the idea of such a model by considering survival in a mixture of two subpopulations of individuals (called "labile" and "stable"). These subpopulations are characterized by different Gompertz mortality patterns, such that their mortality rates cross over. The survival chances of individuals in these subpopulations have different sensitivities to changes in environmental conditions. Then we develop a more comprehensive model in which the mortality rate is related to the adaptive capacity of an organism. We show that the trends in survival patterns experienced by a mixture of such individuals resemble those obtained in an analysis of empirical data on survival in developed countries. Lastly, we present evidence of the existence of subpopulations of phenotypes in both humans and experimental organisms, which were used as prototypes in our models. The existence of such phenotypes provides the possibility that at least part of today's centenarians originated from an initially frail part of the cohort.