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
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“We all have—to put it as nicely as I can—our lower centres and our higher centres. Our lower centres act: they act with terrible power that sometimes destroys us; but they don’t talk.... Since the war the lower centres have become vocal. And the effect is that of an earthquake. For they speak truths that have never been spoken before—truths that the makers of our domestic institutions have tried to ignore.”
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