Survival Convergence and the Preceding Mortality Crossover for Two Population Subgroups

Xian Liu, Uniformed Services University of the Health Sciences (USUHS)
Charles Engel, Jr., Uniformed Services University of the Health Sciences (USUHS)
Han Kang, Department of Veterans Affairs

In this paper, we present and develop the argument that if the survival functions for two population subgroups converge in later life, mortality crossover must precede the occurrence of this convergence. Specifically, two survival curves, S1(x) and S2(x), associated with two distinct population subgroups (one "advantaged" and one "disadvantaged"), tend to converge before all members die out, as often observed and anticipated. This convergence leads to an increased mortality acceleration for the "advantaged" group, and eventually foster the occurrence of a mortality crossover. We present a mathematical proof for this relationship and offer several optional explanations for the mechanisms involved in the process of survival convergence and the preceding mortality crossover. This new presentation demonstrates that mortality crossover is a highly observable demographic event given the trend of survival convergence in later life.