page_title: Total fertility rate
section_title: Population-lag effect
hierarchical_section_title: Total fertility rate / Population-lag effect
caption_attribution_description: Plot of population growth rate vs log(TFR/2), 2013 based on 137 countries in CIA WFB. Signature reflect population size.
context_page_description: The total fertility rate, sometimes also called the fertility rate, absolute/potential natality, period total fertility rate, or total period fertility rate of a population is the average number of children that would be born to a woman over her lifetime if:
  She was to experience the exact current age-specific fertility rates through her lifetime, and
  She was to survive from childbirth till the end of her reproductive life.
  It is obtained by summing the single-year age-specific rates at a given time. As of 2019, the total fertility rate varied from 0.92 in South Korea to 7.03 in Niger.
context_section_description: A population that maintained a TFR of 3.8 over an extended period without a correspondingly high death or emigration rate would increase rapidly (doubling period ~ 32 years), whereas a population that maintained a TFR of 2.0 over a long time would decrease, 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 childbearing years.
  This phenomenon carries forward for several generations and is called population momentum, population inertia or population-lag effect. This time-lag effect is of great importance to the growth rates of human populations.
  TFR (net) and long term population growth rate, g, are closely related. For a population structure in a steady state and with zero migration, g equals log(TFR/2)/Xm, where Xm is mean age for childbearing women and thus P(t) = P(0) exp(gt). At the left side is shown the empirical relation between the two variables in a cross-section of countries with most recent y-y growth rate. The parameter 1/b should be an estimate of the Xm; here equal to 1/0.02 = 50 years, way off the mark because of population momentum. E.g. for log(TFR/2) = 0, g should be exactly zero, which is seen not to be the case.
caption: A plot of population growth rate vs total fertility rate (logarithmic). Symbol radius reflect population size in each country