Population-lag Effect
A population that maintained a TFR of 3.8 over an extended period of time without a correspondingly high death or emigration rate would increase rapidly, whereas a population that maintained a TFR of 2.0 over a long time would decline (unless it had a large enough immigration). However, it may take several generations for a change in the total fertility rate to be reflected in birth rate, because the age distribution must reach equilibrium. For example, a population that has recently dropped below replacement-level fertility will continue to grow, because the recent high fertility produced large numbers of young couples who would now be in their child-bearing years. This phenomenon carries forward for several generations and is called population momentum or population-lag effect. This time-lag effect is of great importance to the growth rates of human populations. Life has been increasing at a steady pace and with the growing feasibility of radical life extension the death rate may continue to drop requiring an ever decreasing fertility rate in order to maintain a sustainable and stable population number.
Read more about this topic: Total Fertility Rate
Famous quotes containing the word effect:
“In his very rejection of art Walt Whitman is an artist. He tried to produce a certain effect by certain means and he succeeded.... He stands apart, and the chief value of his work is in its prophecy, not in its performance. He has begun a prelude to larger themes. He is the herald to a new era. As a man he is the precursor of a fresh type. He is a factor in the heroic and spiritual evolution of the human being. If Poetry has passed him by, Philosophy will take note of him.”
—Oscar Wilde (1854–1900)