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	Population Momentum	Net Reproduction Rate
分野	人口学	人口学
系統	Process / pipeline	Process / pipeline
提唱年	1971	2001
提唱者	Nathan Keyfitz	Richard Böckh and Robert Kuczynski (formalized in Preston, Heuveline & Guillot)
種類	Measure of latent growth from age structure after fertility reaches replacement	Period measure of generational replacement combining fertility and mortality
原典	Keyfitz, N. (1971). On the momentum of population growth. Demography, 8(1), 71–80.	Preston, S. H., Heuveline, P., & Guillot, M. (2001). Demography: Measuring and Modeling Population Processes. Blackwell. ISBN: 9781557864512
別名	Demographic Momentum, Momentum of Population Growth, Keyfitz Momentum	NRR, Net reproduction ratio, Net reproductive rate, Net Üreme Hızı
関連	3	4
概要	Population momentum is the tendency of a growing population to keep growing for decades even after fertility falls to the replacement level, simply because its age structure is heavily weighted toward young people who have yet to reach childbearing age. Introduced by Nathan Keyfitz in 1971, the momentum factor measures how much larger (or smaller) a population will ultimately become if fertility instantly drops to exact replacement. It explains why ending rapid population growth is not immediate: the built-in youthfulness of a fast-growing population carries growth forward long after birth rates stabilize.	The net reproduction rate (NRR) is the demographic measure of generational replacement: the average number of daughters a woman would bear who survive to the age their mother was when she bore them, given the period's age-specific fertility rates and female mortality. By combining fertility with survival, the NRR answers the fundamental question of whether a population is replacing itself — an NRR of one means each generation of women exactly reproduces the next, below one signals long-run decline, and above one signals growth.