The Momentum of Mortality Decline

Michel Guillot, University of Wisconsin at Madison

In Keyfitz' classical definition, population momentum is the growth that would occur in a previously growing stable population if fertility were immediately reduced to replacement level. Since population momentum is associated with an abrupt change in fertility while mortality rates remain constant, it is not well recognized that mortality itself can play a significant role in the value for the population momentum. This paper shows that population momentum is not only due to the disparity between actual and replacement fertility. It is also due to the fact that if mortality has been previously declining in a population, cohort mortality will continue improving even after period mortality reached constancy. An equation for estimating the population momentum directly attributable to mortality decline is developed and applied to various populations.