From the paperlinked in my last post. This should give some idea of what all this means:
We define lowest-low fertility as a total fertility rate below 1.3. TFR levels below 1.3 are clearly not a demographic equilibrium, and sustained lowest-low fertility implies far-reaching demographic, economic and social consequences. For instance, a TFR of 1.3 implies an annual decline of the population size by 1.5% in a stable population with an overall mean age at birth of 30 years. A TFR of 1.3 also implies a reduction of the birth cohort by 50% and a halving of the stable population size every 45 years.2 If the TFR further declines and persists at a level of one, the annual rate of decline in the stable population rises to 2.4% and the halving-times of population size and birth cohorts are merely 30 years. This substantially faster decline of the population also reveals that the precision of demographic measures becomes increasingly important in lowest-low fertility contexts: a difference in the TFR between 1.0 and 1.3 is equivalent to the difference between 3.2 and 4.2 in terms of stable population growth rates.
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