The Quiescent Phase in Human Mortality: When Do Populations Start to Age?

Michal Engelman, University of Chicago
Ravi Varadhan, Johns Hopkins University
Christopher L. Seplaki, University of Rochester

Demographic studies of mortality hazards tend to emphasize the two ends of the lifespan, focusing on the declining pattern of child mortality or the increasing trajectory of deaths at older ages. We call attention to the relatively quiescent phase in between, representing the ages when we are least vulnerable to the force of mortality. We describe the quiescent phase theoretically (by reviewing mortality trajectory models and demonstrating the phase's consistency across populations), mathematically (via life table functions and the Siler model), and empirically (using data for cohorts born 1800-1919). We also trace its beginning (representing the end of childhood vulnerability), end (the age when the hazard begins its exponential rise), and changes in these inflection points over time. As survival improved, the quiescent phase has gotten longer and more pronounced, reflecting the retreat of external and "premature" causes of death and potentially revealing an underlying biological hazard trajectory.