Summary
Following Nathan Keyfitz' seminal work on the topic, we have defined population momentum as M=Ns/N; where N stands for the current population size, and Ns refers to the population that would ensue when projecting the population with current mortality rates and replacement level fertility rates until it becomes stationary. This projection approach was applied to the group of most, less and least developed countries to demonstrate the population dynamics that occur in the interim. These highlight the importance of the starting population age structure for the time-scale as well as size of momentum effects.
We then introduced a few methods for approximating population momentum from parameters describing the starting population only. This includes a formula by Goldstein & Stecklov that accommodates more gradual declines in fertility to replacement level. We subsequently applied Frauenthal’s (more conventional) method for estimating momentum following an instantaneous decline in fertility rates to obtain estimates for most countries in the world. These demonstrate that population momentum coefficients for 2010 vary from 0.83 (Germany) to 1.69 (Guatemala). More generally, they highlight that the momentum is largest in populations with relatively high birth rates as well as low mortality rates.