Models, such as fertility projection models, are usually based on the record of countries that have earlier experienced fertility decline and on assumptions about what is reasonable in the future. There is sometimes a trade-off between a good statistical fit and factors substantively related to fertility change. The most authoritative and influential fertility projections are those by the United Nations Population Division, which in the last decade has changed their fertility projection assumptions three times.
Before 2004, the UN Population Division had long projected TFR to asymptotically reach replacement level, TFR=2.07. This assumed that all countries would eventually have low mortality, low fertility stationary populations.
In the 1990s, many countries had sustained below replacement fertility (TFR<2.07), sometimes falling to lowest-low fertility (TFR<=1.3). There were various theories about why sustained very low fertility occurred, the most influential being Second Demographic Transition Theory. After extensive consultation, fertility projection assumptions were changed in 2004, and all countries were then projected to asymptotically approach TFR=1.86, which implies long term population decline. This was a major departure from the earlier eventual stationary population assumption.
In the 2000s, TFR increased across at least two five-year periods (such as 1995-2000 to 2000-2005 and 2000—2005 to 2005-2010) in twenty-one below replacement fertility countries. Based on fertility increases in those countries, in 2010 assumptions were again changed, so that in the new model TFR in below replacement fertility countries increased toward replacement, with the pace of increase more rapid the farther TFR was below replacement. For countries with above replacement fertility in 2005-2010 such as Algeria, TFR was projected to fall below replacement and then increase toward replacement. This marked a return to the eventual stationary population assumption.
Analyses found that in some below replacement fertility countries, such as Sweden, the dip in period TFR was the consequence of a shift in fertility to older ages, resulting in little change in cohort TFR. Low incompatibility between women having children and working for pay and relatively high gender equity were positively related to TFR in low fertility countries. In some low fertility countries, such as Singapore and the Republic of Korea, there was no indication of TFR increase by 2005-2010 and no indication of an increase in fertility at older ages. Some found the projected rapid increase in TFR in such countries implausible.
In 2012, TFR projection assumptions were again changed. By 2012, TFR had increased across at least three five-year periods in 25 low fertility countries. The new low fertility projection model for many individual countries was based both on the experience of these 25 countries and on the TFR record of the individual country. The 2012 projections resulted in less extreme departures from earlier projected TFRs than occurred between the 2004 and 2010 projections. In some countries, such as Sweden, the 2010 and 2012 projections differ little and are consistent with earlier empirical trends.
It would be prudent:
1) To make clearer the substantive reasoning behind changes in fertility assumptions in addition to the degree of fit to recent empirical patterns in some countries.
2) To consider whether conditions and relationships in countries that have experienced very low fertility are likely to be the same in all other countries in the future.
3) To have a longer observation period of new empirical patterns before major fertility projection assumptions are changed.
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