page_title: Sub-replacement fertility
section_title: Tempo effect
hierarchical_section_title: Sub-replacement fertility / Causes / Tempo effect
caption_attribution_description: Tokyo night view
context_page_description: Sub-replacement fertility is a total fertility rate that leads to each new generation being less populous than the older, previous one in a given area. In developed countries sub-replacement fertility is any rate below approximately 2.1 children born per woman, but the threshold can be as high as 3.4 in some developing countries because of higher mortality rates. Taken globally, the total fertility rate at replacement was 2.33 children per woman in 2003. This can be "translated" as 2 children per woman to replace the parents, plus a "third of a child" to make up for the higher probability of boys born and mortality prior to the end of a person's fertile life. In 2020, the average global fertility rate is around 2.4 children born per woman.
  Replacement-level fertility in terms of the net reproduction rate is exactly one, because the NRR takes both mortality rates and sex ratios at birth into account.
  As of 2010, about 48% of the world population lives in nations with sub-replacement fertility. Nonetheless most of these countries still have growing populations due to immigration, population momentum and increase of the life expectancy.
context_section_description: Total fertility rate (TFR) is affected by a phenomenon called the tempo effect, which describes "distortions due to changes in the timing of births."   John Bongaarts and Griffith Feeney have suggested that this tempo effect is driving the decline of measured fertility rate in the developed world.
caption: Japan, a highly developed country, has low fertility rates and a rapidly aging population
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