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Some countries have [[sub-replacement fertility]] (that is, below 2.1–2.2 children per woman). Replacement fertility is generally slightly higher than 2 (the level which replaces the two parents, achieving equilibrium) both because boys are born more often than girls (about 1.05–1.1 to 1), and to compensate for deaths prior to full reproduction. Many European and East Asian countries now have higher death rates than birth rates. [[Population aging]] and [[population decline]] may eventually occur, assuming that the fertility rate does not change and sustained mass immigration does not occur.
Using data through 2005, researchers have suggested that the negative relationship between development, as measured by the [[Human Development Index]] (HDI), and birth rates had reversed at very high levels of development. In many countries with very high levels of development, fertility rates were approaching two children per woman in the early 2000s.<ref name="Nature" /><ref>{{Citation | title= The best of all possible worlds? | url = https://www.economist.com/science-and-technology/2009/08/06/the-best-of-all-possible-worlds | newspaper = The Economist | date = 6 August 2009}}.</ref> However, fertility rates declined significantly in many very high development countries between 2010 and 2018, including in countries with high levels of [[gender parity]]. The global data no longer support the suggestion that fertility rates tend to broadly rise at very high levels of national development.<ref name=":1">{{Cite journal|last=Gaddy|first=Hampton Gray|date=2021-01-20|title=A decade of TFR declines suggests no relationship between development and sub-replacement fertility rebounds|url=https://www.demographic-research.org/volumes/vol44/5/|journal=Demographic Research|language=en|volume=44|pages=125–142|article-number=5 |doi=10.4054/DemRes.2021.44.5|issn=1435-9871|doi-access=free}}</ref>
From the point of view of [[evolutionary biology]], wealthier people having fewer children is unexpected, as [[natural selection]] would be expected to favor individuals who are willing and able to convert plentiful resources into plentiful fertile descendants. This may be the result of a departure from the [[Evolutionary psychology#Environment of evolutionary adaptedness|environment of evolutionary adaptedness]].<ref name=bbc_sure /><ref>{{cite journal | last1 = Clarke | first1 = Alice L. | last2 = Low | first2 = Bobbi S. | year = 2001 | title = Testing evolutionary hypotheses with demographic data |journal = Population and Development Review | volume = 27 | issue = 4| pages = 633–660 | doi=10.1111/j.1728-4457.2001.00633.x| url = https://deepblue.lib.umich.edu/bitstream/2027.42/74296/1/j.1728-4457.2001.00633.x.pdf | hdl = 2027.42/74296| hdl-access = free }}</ref><ref>{{cite journal |last1=Daly |first1= Martin |last2=Wilson |first2=Margo I |title=Human evolutionary psychology and animal behaviour |journal= Animal Behaviour |url= http://courses.washington.edu/evpsych/Daly%26Wilson-HEP-AB1999.pdf |publisher= Department of Psychology, McMaster University |access-date=14 November 2018 |date=26 June 1998|volume= 57 |issue= 3 |pages= 509–519 |doi= 10.1006/anbe.1998.1027 |pmid= 10196040 |s2cid= 4007382 }}</ref>
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