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The progress of physiological aging processes in human populations is often studied by analyzing the age trajectory of total and cause specific mortality rates under the assumption that the rate of increase in the risk of death indexes the average rate of change in the physiology of the individuals comprising the population. Unfortunately, such efforts have usually assumed that the age trajectory of the average mortality rate is the same as the age trajectory of any given individual’s risk of death—equivalent to the assumption that all individuals in the population have the identical rate of aging. In this paper, we present a model that permits the evalua­tion of the aging process of individuals from the age trajectory of mortality rates in the population by a. positing a model of the distribution of individual constitutional differences in the age trajectory of mortality risks, and b. adjusting the population mortality rates under the model of heterogeneity to retrieve the individual risks.This model is applied to human mortality data from the U.S. Black and White populations for the period 1935 to 1975. This example was selected because of the observation of a mortality crossover (Blacks having relatively lower mortality rates) about age 75. The crossover could be explained under the proposed model of population heterogeneity and differential mortality selection. The implications of the model for estimating the heterogeneity of aging processes for individuals from human population data are discussed as well as implications for our existing perception of the human aging mechanism.