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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. 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, and early mortality prior to the end of their fertile life.
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. 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.
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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.
The fact that more people are going to colleges and universities, and are working to obtain more post-graduate degrees there, along with the soaring costs of education, have contributed greatly to postponing marriage in many cases, and bearing children at all, or fewer numbers of children. And the fact that the number of women getting higher education has increased has contributed to fewer of them getting married younger, if at all. In the US, for example, females make up more than half of all college students, which is a reversal from a few decades back.
The growth of wealth and human development are related to this phenomenon (see the Demographic-economic paradox). High costs of living and job insecurity can make it difficult for young people to marry and start families.
The increase of urbanization around the world is considered by some a central cause. In recent times, residents of urban areas tend to have fewer children than people in rural areas,. The need for extra labour from children on farms does not apply to urban-dwellers. Cities tend to have higher property prices, making a large family more expensive, especially in those societies where each child is now expected to have his own bedroom, rather than sharing with siblings as was the case until recently. Rural areas also tend to be more conservative with less contraception and abortion than urban areas.
Changes in contraception are also an important cause, and one that has seen dramatic changes in the last few generations. Legalization, and widespread acceptance, of contraception in the developed world is a large factor in decreased fertility levels. A systematic review, however, came to the result that European fertility rates do not seem to be decreased significantly by availability of contraception. The other way around, the same review also stated that government support of assisted reproductive technology is beneficial for families, but its effect on total fertility rate in Europe is extremely small.
Growing female participation in the work force has led to many women delaying or deciding against having children, or to not have as many. A longer pursuit of education also delays marriages. Greater access to contraception and abortion, and greater proclivity of women to use them, also has reduced rates.
Other social changes both separate and related to feminism also have played a role. Bearing children is regarded as less of a social duty than it once was in many societies. Women's social status increasingly correlates with their work or behaviour as consumers rather than from their role as mothers. Indeed having a large family is often socially deprecated, being associated with lower status groups.
There are those who have pointed to the religiosity of the United States with its somewhat higher overall fertility rates as evidence of the influence of religion and human fertility. Some religious groups, such as Catholics, consider contraceptives to be sinful. Amish sects in the US, as well as among ultra orthodox Jews in Israel, very likely point to a high probability that the beliefs among certain religious groups, with their correspondingly higher fertility rates, indicate a causal effect among certain, though certainly not among all religious groups.
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In the conventionally reported measure of TFR, the period TFR (based on the level of fertility or number of births in a given year), there is a statistical effect called the tempo effect which makes it a misleading measure of overall (life cycle) fertility.
Specifically, if the age of childbearing increases – but assuming that the total number of births over a life cycle remains unchanged – then while this increase is happening, the measured TFR is lower (the births happen in a later year), but when the age of childbearing stops increasing the TFR increase, due to these births catching up. For illustration, if in the past women always had 1 child at the age of 20 (TFR of 1), but suddenly in the year 2000 all women born in 1980 or later postponed having children until age 30, there would suddenly be no births for 10 years (TFR of 0), and then 10 years later (2010) it would suddenly jump back up (TFR of 1) (assuming flat population structure, no deaths, etc.), even though the life cycle TFR was always 1.
Thus, period TFR reflects not only life cycle TFR, but also timing effects, and these effects are conflated in a simple period TFR number. Life cycle TFR is unambiguous, and strict measure of life cycle fertility are not affected by this effect (e.g., counting the average children that have been born to all women who cease child-bearing in a given year (via menopause, sterilization, death, etc.)), but are lagging statistics because they require women to cease child-bearing before they are counted. Thus, adjusted measures of TFR – period TFR, adjusted for timing – are proposed instead to give a more accurate measure of life cycle fertility, without needing to wait until women have definitively ceased bearing children.
Thus, if age of childbearing is increasing and life cycle fertility is decreasing, period TFR will initially overstate the decline, and then may have a spurious increase even if life cycle fertility is actually still declining. This is computed to be the case in Spain in the period 1980–2002, for instance.
John Bongaarts and Griffith Feeney have suggested that this tempo effect is driving the decline of measured fertility rate in the developed world. Taking tempo changes into account, adjusted birth rates for a number of European countries are higher than the conventional TFR. A particularly strong example is the Czech Republic in the period 1992–2002, which witnessed a steady rise in childbearing age, hence the period TFR dropped sharply, overstating the decline in life cycle fertility.
Another explanation for falling fertility could be a reduction in the frequency of sex in populations with low birth rates. For example, according to a survey published by the Japanese Family Planning Association in March 2007, a record 39.7 per cent of Japanese citizens aged 16–49 had not had sex for more than a month.  A study came to the result that instability of modern partnerships is a major cause of European sub-replacement fertility.
Also, a number of sociologists and demographers have pointed out that among those who co-habit, without marrying, are now usually likely to have fewer children than those who are married, due to the lack of commitment in the male/female relationship. This uncertainty induces a 'wait and see' approach in many cases, especially on the part of the female.
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Sub-replacement fertility does not immediately translate into a population decline because of population momentum: recently high fertility rates produce a disproportionately young population, and younger populations have higher birth rates. This is why some nations with sub-replacement fertility still have a growing population, because a relatively large fraction of their population are still of child-bearing age. But if the fertility trend is sustained (and not compensated by immigration), it results in population ageing and population decline. This is forecast for most of the countries of Europe and East Asia.
Current estimates expect the world's total fertility rate to fall below replacement levels by 2050, although population momentum will continue to increase global population for several generations beyond that. The promise of eventual population decline helps reduce concerns of overpopulation, but many[who?] believe the Earth's carrying capacity has already been exceeded and that even a stable population would not be sustainable.
|“||"In our time all Greece was visited by a dearth of children and generally a decay of population, owing to which the cities were denuded of inhabitants, and a failure of productiveness resulted, though there were no long-continued wars or serious pestilences among us…. For this evil grew upon us rapidly, and without attracting attention, by our men becoming perverted to a passion for show and money and the pleasures of an idle life, and accordingly either not marrying at all, or, if they did marry, refusing to rear the children that were born, or at most one or two out of a great number, for the sake of leaving them well off or bringing them up in extravagant luxury."||”|
|“||"We liberate slaves chiefly for the purpose of making out of them as many citizens as possible. We give our allies a share in the government that out numbers may increase; yet you, Romans of the original stock, including Quintii, Valerii, Iulii, are eager that your families and names at once shall perish with you."||”|
Some believe that not only the Great Recession, but the Great Depression, may have been the result of a decline in birthrates overall. Clarence L. Barber, an economist at the University of Manitoba, pointed out how demand for housing in the US, for example, began to decline in 1926, due to a decline in 'household formation' (marriage), due, he believed, to the effects of World War I upon society. In early 1929, US housing demand declined precipitously. And, of course, the stock market crash followed in October of that same year.
Even though the overall world population continues to increase, it is more at the 'back end' than the 'front end' that this is occurring. That is, more people are kept alive than in the past due to improved nutrition, more refrigeration and better sanitation worldwide, as well as health care advances, from vaccines to antibiotics, and many other advances in medications and in different improvements in health care. Certainly, in advanced nations, few groups would be considered to be "breeding like rabbits". The 'baby boom' (1946–1964) in the US, was likely, if Barber's contentions are correct, more of a return to birthrates closer to historical norms, like those of the first decade of the 20th century (but the 'baby boom' of 1946–1964 were still lower than the 1900–1910 period), with birth dearths both before and since making the so-called "baby boom" appear so big.
Sub-replacement fertility can also change social relations in a society. Fewer children, combined with lower infant mortality has made the death of children a far greater tragedy in the modern world than it was just fifty years ago. Having many families with only one or two children also reduces the number of siblings, aunts and uncles and other extended family members.
Population aging may pose an economic challenge to governments as the number of retired citizens drawing public pensions rises in relation to the number of workers. This has been raised as a political issue in France, Germany, and the United States where many people have advocated policy changes to encourage higher birth and immigration rates. In France, payments to couples who have children have increased birth rates.[not in citation given]
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Some European governments, fearful of a future pensions crisis, have developed natalist policies to attempt to encourage more women to have children. Measures include increasing tax allowances for working parents, improving child-care provision, reducing working hours/weekend working in female-dominated professions such as healthcare and a stricter enforcement of anti-discrimination measures to prevent professional women's promotion prospects being hindered when they take time off work to care for children. Over recent years, the fertility rate has increased to around 2.0 in France and 1.9 in Britain and some other northern European countries, but the role of population policies in these trends is debated.
Attempts to increase the fertility rate among working women bring difficult political dilemmas: how far to alter traditional working practices so that women who are juggling work and child-raising responsibilities are not disadvantaged in their careers compared with men (for example, by legislating for compulsory paternity leave, flexible working and/or limiting total weekly working hours for men as well as women) and above all the question of whether the problem of sub-replacement fertility is so serious that unmarried women should also now be encouraged to have more children.
Giving women paid maternity leave can have the unintended negative consequence of dissuading employers from hiring women because they may fear having to pay a pregnant woman wages for a job she isn't doing. This may increase the gender-wage gap, the income disparity between men and women in the labor force. This disincentive can be ameliorated by giving parental leave to both men and women.
European analysts hope, with the help of government incentives and large-scale change towards family-friendly policies, to stall the population decline and reverse it by around 2030, expecting that most of Europe will have a slight natural increase by then. C. D. Howe Institute, for example, tries to demonstrate that immigration can not be used to effectively counter population ageing.
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|This section possibly contains original research. (October 2008)|
While much of the world has experienced declining fertility rates over the last twenty years, the total fertility rate in the United States has remained relatively stable. This is largely due to the high fertility rate among communities such as Hispanics, but it is also because the fertility rate among non-Hispanic whites in the US, after falling to about 1.6 in the 1970s and early 1980s, had increased and is now around 1.89, or slightly below replacement level, rather than collapsing to the 1.3–1.5 level common in Europe.
New England has a rate similar to most Western European countries, while the South, Midwest, and border states have fertility rates considerably higher than replacement. States where The Church of Jesus Christ of Latter-day Saints has a strong presence, most notably Utah, also have higher-than-replacement fertility rates, especially among the LDS population. Heaton and Goodman (1985) found that LDS women average about one child more than women in other religious groups.
Some other developed countries are also experiencing an increase in their birth rate, including France, which recorded a TFR of over 2.00 in 2008; the United Kingdom where TFR increased from 1.64 in 2000 to 1.98 in 2010; Australia, where the birth rate rose from 1.73 in 2001 to 1.93 in 2007 and New Zealand, where the TFR was 2.2 in 2008.
Israel is the only developed country that has never had sub-replacement fertility; a declining Arab and Bedouin fertility rate is countered by religious Jewish groups (mostly Haredim) with higher than average fertility rates. In addition, the (mostly non-religious) aliyah Jews from the former USSR shifted from a 1 child per woman fertility rate to an average fertility rate close to 2.2 children per woman. As of 2008, Israel's Jewish fertility rate is the highest among the industrial nations.