Population momentum

Population momentum is a consequence of the demographic transition. Population momentum explains why a population will continue to grow even if the fertility rate declines. Population momentum occurs because it is not only the number of children per woman that determine population growth, but also the number of women in reproductive age. Eventually, when the fertility rate reaches the replacement rate and the population size of women in the reproductive age bracket stabilizes, the population achieves equilibrium and population momentum comes to an end. Population momentum is defined as the ratio of the size of the population at that new equilibrium level to the size of the initial population. Population momentum usually occurs in populations that are growing. Assume that a population has three generations: First (oldest), Second (child bearing), and Third (children). Further assume that this population has a fertility rate equal to four (4). That is, each generation is twice the size of the previous. If the population of the first generation is arbitrarily set at 100, the second is then 200, and the third is 400. The spreadsheet below shows the initial population in the first row. First note that the second and third generation of the initial population are each twice the size of the previous. The total of the initial population is 700 = 100 + 200 + 400. Then assume that at the end of the third generation, fertility falls to replacement (for simplicity assume that to be two). Now take the population forward in time to the next generation, line two of the spreadsheet. The first generation dies, and the new generation, the fourth, is equal to the third (because now fertility is replacement). Repeat the process again to reach the fifth generation (line 3 in the spreadsheet). The fifth generation is again equal to the fourth and now the population’s three generations are equal, and the population has reached equilibrium. The initial population has grown from 700 to 1,200 even though fertility dropped from four to replacement (two) at the end of the third generation.