Mortality Deceleration and Mortality Selection: Three Unexpected Implications of a Simple Model

Elizabeth Wrigley-Field, University of Wisconsin-Madison

Mortality deceleration is commonly attributed to mortality selection: as cohorts age, the frailest die first. Drawing on formal analysis and mortality simulations, I dispute some common assumptions about how population heterogeneity produces deceleration. I show that even in a very simple model—one composed of just two subpopulations with Gompertz mortality—aggregate mortality can decelerate even while a majority of the population is frail; multiple decelerations are possible; and mortality selection can produce acceleration as well as deceleration. Simulations show that these counter-intuitive patterns occur widely in model populations that in the aggregate resemble those in the Human Mortality Database. I argue that these results: challenge conventional heuristics for understanding the relationship between selection and deceleration; suggest that standard parametric models, assumed to plateau at most once, may sometimes mislead analysts as to deceleration timing; and make problematic certain inferences from deceleration timing to patterns of social inequality.