The dynamics of population momentum.

Population momentum is generally defined in terms of fertility, as the tendency for population growth to continue beyond the time that replacement level fertility has been achieved because of a high concentration of women in the childbearing years. However, population momentum can be defined more generally as the influence that a population's age structure has on its rate of natural increase. Changes in the age structure affect both birth and death rates. The birth rate is influenced by the relative number of women of reproductive age and by the rate at which they give birth. The maturing of the age structure acts to slow down the decline in the birth rate. The extent of further declines in the rate of natural increase depends on the extent to which the crude death rate increases and/or the crude birth rate decreases. Age structure variations can have a substantial effect on the crude death rate, which can reach a very low level in populations with relatively few elderly people. Yet, as the percentage of elderly increases due to longterm declines in fertility, crude death rates in these countries rise. If the birth rate continues to fall as the death rate rises, the rate of natural increase may soon reach zero or turn negative. In summary, population momentum tends at 1st to slow down declines in rates of natural increase. Eventually, however, continued aging of the population accelerates declines in rates of natural increase and population growth.