Introduction
The mid twentieth century was characterized by unprecedented population growth rates in low income countries, and these were the direct result of sizable improvements in mortality that were not matched by declines in birth rates. In the midst of that period, where doomsday scenarios about excessive population growth widely circulated, Nathan Keyfitz (1971, 1985) further demonstrated that drastic and immediate declines in fertility rates would not suffice to halt population growth. Building on earlier work by Paul Vincent (1945), he called this phenomenon 'population momentum'.
Population momentum quantifies the future growth of a population following an instantaneous reduction of fertility rates to replacement level. It can be defined as the ratio of the future stationary equivalent population size (Ns) over that of the population (N) at the time that the replacement level fertility rates are adopted:
The stationary equivalent population is the population that would come about if the current mortality rates in combination with the replacement level fertility rates would prevail for a very long time (i.e., until the population stops growing and becomes stationary). Migration is usually ignored in discussions of population momentum. The notion of population momentum thus describes the inertia in natural population growth following a fertility decline – it can be thought of in much the same way as physical momentum that describes how a car does not immediately stop when shifting to neutral gear and lifting one's foot off the accelerator pedal (Schoen and Kim, 1991). In a car, the power that is transmitted from the engine is switched off, but the vehicle continues to go forward and the speed decreases gradually. In a population, the high fertility that forces the population to multiply no longer operates, but the population continues to grow, slowing down gradually.