Stationary Fertility Trajectories and Population Momentum under Continually Falling Mortality

Carl P. Schmertmann, Florida State University
Roland Rau, University of Rostock

Low-fertility countries face population decline and aging. There is strong interest in demographic adjustments that might slow or halt these changes. We identify three stationary fertility trajectories for closed populations with constant mortality, and then then analyze the consequences of those trajectories if, instead, mortality continued to decline. Surprisingly, all three fertility paths can still lead to stationarity when mortality falls continuously. Convergence occurs over extremely long time spans, however, and stationary age structures are very old. We use formal methods and illustrate with German data. For an instantaneous transition to replacement fertility, negative momentum is partially offset by declining mortality, but population would still fall -- for about the same number of years as in the constant-mortality case. After that decline, a population with continually decreasing mortality would return to its initial size, but only after a century or more. Results are similar for less-familiar stationary fertility trajectories.