A typical consequence of the demographic transition-a population's shift from high mortality and high fertility to low mortality and low fertility-is a period of robust population growth. This growth occurs once survival has improved but before fertility has fallen to or below replacement level, so that the birth rate substantially exceeds the death rate. During the second half of the twentieth century, the world experienced unprecedented population growth as developing countries underwent a demographic transition. It was during this period that Nathan Keyfitz demonstrated how an immediate drop to replacement fertility in high-fertility populations could still result in decades of population growth. Building on work by Paul Vincent (1945), he calledthis outcome "population momentum." Keyfitz wrote, "The phenomenon occurs because a history of high fertility has resulted in a high proportion of women in the reproductive ages, and these ensure high crude birth rates long after the age-specific rateshave dropped" (Keyfitz 1971: 71).
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