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Sub-replacement fertility
Sub-replacement fertility is a total fertility rate (TFR) that (if sustained) leads to each new generation being less populous than the 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.〔, Introduction and Table 1, p. 580〕 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 being born, infertility, individuals choosing not to have children, and early mortality prior to the end of their fertile life.〔For example in the United Kingdom in 2001 304,635 boys were born as opposed to 289,999 girls, and some of these girls will not survive to the end of their child bearing years. In future, therefore, the girls born in this year would have to have more than two children each to replace the total population. For a full explanation see ('Replacement Fertility, What has it been and What does it mean?' (PDF) )〕
Replacement level fertility in terms of the net reproduction rate (NRR) 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.〔("Figure 8: Population by Total Fertility (millions)" ) in ''World Population Prospects, the 2010 Revision''. United Nations, Department of Economic and Social Affairs, Population Division (2011)〕 Nonetheless most of these countries still have growing populations due to immigration, population momentum and increase of the life expectancy. This includes most nations of Europe, Canada, Australia, Brazil, Russia, Iran, Tunisia, China, and many others. The countries or areas that have the lowest fertility are Hong Kong, Macau, Singapore, Taiwan, Ukraine and Lithuania. Only a few countries have low enough or sustained sub-replacement fertility (sometimes combined with other population factors like emigration) to have population decline, such as Japan, Germany, Lithuania, and Ukraine.〔(【引用サイトリンク】title=International Data Base )〕
==Causes==
There have been a number of explanations for the general decline in fertility rates in much of the world, and the true explanation is almost certainly a combination of different factors.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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