(or momentum of population growth): The potential for future increases in population size. Even at the replacement level fertility rate in the U.S., the sheer number of Baby Boom women giving birth in the 1980s caused an increase in the number of annual births.
The tendency for a rapidly growing population to keep on growing, even after the implementation of policies designed to halt population growth.
The phenomenon whereby a population, even if it is below replacement-level fertility, will nonetheless continue to increase. This is due to the usual age distribution of a population, with many people in their child-bearing years and fewer older people.
The continued increase (or decrease) in population size that would occur if fertility immediately changed to replacement level. The momentum occurs because of the existing age structure. For example, zero population growth does not occur immediately a NRR of one is reached due to the comparatively high concentration of people in the childbearing and pre-childbearing years in populations that have previously had above replacement fertility. Momentum is thus built in to the current age structure. For example, if world fertility was to drop to replacement level immediately and remain at that level the relatively youthful present age structure would cause population to continue to increase for about 60 years. Note, momentum is usually positive, but it can also be negative. For instance, in Europe today fertility is below replacement level and the population has aged sufficiently so that if fertility rose to replacement level population size would continue to decline for a while.
The tendency for population growth to continue beyond the time that replacement-level fertility has been achieved because of the relatively high concentration of people in the childbearing years. (Also see momentum factors and decreasing population momentum.)
A potential for increased population growth as young members reach reproductive age.
Population momentum is an effect which causes population growth. This phenomenon refers to the percentage of the population that are in their child bearing years who have not yet had children, and thus are scheduled to eventually have children which add to the population through reproduction. The higher the percentage of people aged, for example 18 and under, the larger the population growth will be because there is such a large percentage of the population capable of having children.
