Sub-replacement fertility is a total fertility rate (TFR) that (if sustained) leads to each new generation being less populous than the older, 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 born and 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% (3.3 billion people) 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, the United States and many others. In 2015, all European Union countries had a sub-replacement fertility rate, ranging from a low of 1.31 in Portugal to a high of 1.96 in France. The countries or areas that have the lowest fertility are in developed parts of East and Southeast Asia: Singapore, Macau, Taiwan, Hong Kong and South Korea. Only a few countries have had, for the time being, sufficiently sustained sub-replacement fertility (sometimes combined with other population factors like higher emigration than immigration) to have population decline, such as Japan, Germany, Lithuania, and Ukraine. As of 2016, the total fertility rate varied from 0.82 in Singapore to 6.62 in Niger.
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 relationship between higher education and childbearing varies by country: for example, in Switzerland by age 40, childlessness among women who had completed tertiary education is 40%, while in France it is only 15%. In some countries, childlessness has a longer tradition, and was common even before educational levels increased, but in others, such as Southern European ones, it is a recent phenomenon; for instance in Spain the childlessness rate for women aged 40–44 in 2011 was 21.60%, but historically throughout the 20th century it was around 10%. Not all countries show a relationship between low fertility and education: in Czech Republic, of women born in 1961-1965, low educated women were more likely to be childless than high educated women.
In Eastern European countries, the fall of communism was followed by an economic collapse in many of these countries in the 1990s. Some countries, such as those that experienced violent conflicts in the 1990s, were badly affected. Large numbers of people lost their jobs, and massive unemployment, lack of jobs outside the big cities, and economic uncertainty discourages people from having children. For instance, in Bosnia-Herzegovina, the total fertility rate in 2016 was only 1.28 children born/woman.
Some consider the increase of urbanization around the world 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 their 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.
Although fertility rates are often discussed in terms of state policies (e.g. financial benefits, combining work with family etc.), the deeply entranced social views on what constitutes an "ideal" family may play a crucial role: if parents do not envision large families in a positive way, it is difficult to "convince" them to have many children. In this regard, there are major differences between European countries: while 50.23% of women aged 15–39 state that the "ideal" family has 3 or more children in Estonia, and 46.43% say this in Finland; only 11.3% say this in Czech Republic, and 11.39% in Bulgaria.
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; however, for instance in a European context where its prevalence has always been very high in the modern era, the fertility rates do not seem to be influenced significantly by availability of contraception.
The availability of assisted reproductive technology (ART) may foster delay of childbearing, because many couples think that it can solve any future fertility problems. Its effect on total fertility rate is extremely small but government support for it is beneficial for families.
The Human Development Index (HDI) is a composite statistic of life expectancy, education, and per capita income indicators, which are used to rank countries into four tiers of human development. A country scores higher HDI when the lifespan is higher, the education level is higher, and the GDP per capita is higher. There is a strong inverse correlation between the HDI and the fertility rate of the population: the higher the HDI, the lower the fertility rate. As of 2016, the countries with the highest fertility rate are Burundi, Mali, Somalia, Uganda, Burkina Faso, Zambia, Malawi, Angola, and Afghanistan; while most Western countries have sub-replacement fertility rates. This is part of the fertility-income paradox, as these high fertility countries are very poor, and it may seem counter-intuitive for families there to have so many children. The inverse relationship between income and fertility has been termed a demographic-economic "paradox" by the notion that greater means would enable the production of more offspring as suggested by the influential Thomas Malthus.
Some governments have launched programmes to reduce fertility rates and curb population growth. Notably, China implemented a one-child policy for 35 years (from 1979 to 2015); this was relaxed to a two-child policy in 2016.
Although today Singapore has a very low fertility rate, and the government encourages parents to have more children because birth rates have fallen below the replacement rate, in the 1970s the situation was the opposite: the government wished to slow and reverse the boom in births that started after World War II (see the poster).
The total fertility rate is also influenced by the ability to choose what type of family to have, if and when to have children, and the number of children to have - free from coercion, pressure, or interference from the community, extended family, state or church. This includes prohibition on practices such as child marriage, forced marriage or bride price. In some cultures for instance, the payment of the bride price creates an obligation on the wife to have children, and failure to do so often results in threats and violence. Western countries have substantially lower fertility rates, and increased childlessness, because people who remain childless or who have small families are less likely to be stigmatized. In many cultures childless women suffer discrimination, stigma, ostracism, and social isolation.
It has been shown, both historically and in the present day, that societies engaged in a prolonged state of war experience a substantial lag in fertility rate. The most notable examples of this phenomenon are widely accredited to the First and Second World Wars. Modeled by these examples, "total war" subjects individuals to intense social upheavals and a heavy psychological impact that forcefully prioritizes survival and economic stability over the need to reproduce for the duration of the conflict. Events like these subsequently pave way  for an active effort to repopulate, such as the "baby boom" after the Second World War.
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Total fertility rate (TFR) is affected by a phenomenon called the tempo effect, which ultimately results in "distortions due to changes in the timing of births."  This allows for misleading measures of overall fertility.
A classic example of the tempo effect is seen when considering the increasing age of childbearing. As the total number of births over a life cycle remains the same yet the age of childbearing increases, the measured TFR will appear low. This is misleading because the same number of births are occurring, they are just occurring over a longer period of time.
For illustration, if women in the past always had 1 child at the age of 20 (TFR of 1), and all women born in 1980 or later postponed having children until age 30 in the year 2000, there would be no births for 10 years (TFR of 0). After 10 years (assuming flat population structure, no deaths, etc.), in 2010, it would suddenly jump back up (TFR of 1), even though the life cycle TFR was always 1.
Additionally, if the age of childbearing were to decrease, the TFR would be inflated. It would appear as though more births are occurring, but in reality, the same number of births are occurring, just over a shorter period of time.
As for lifecycle TFR statistics, they are lagging. This is because life cycle TFR measures require women to be completely done having children before they can be "counted" as data. As women continue to delay the onset of their childbearing years, it is difficult to determine when women will conclude childbearing. Because of this difficulty in collecting data, adjusted measures of TFR are suggested to give more accurate measures of life cycle fertility. It is additionally suggested that greater value should be placed upon cohort fertility measures, which instead estimate the average number of children that women have over their lifetime. Cohort fertility measures, therefore, account for all children born to a woman throughout her lifetime as opposed to total fertility rates, which only account for children born to women of "child-bearing" ages. 
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, during which there was a steady rise in childbearing age. Hence, the period TFR dropped sharply, overstating the decline in life cycle fertility.
If the age of childbearing is increasing and life cycle fertility is decreasing, period TFR measures will initially overstate the decline, and may then show a spurious increase even if life cycle fertility is actually declining. For instance, this is computed to be the case in Spain in the period 1980–2002.
A study of the United States and multiple countries in Europe came to the result that women who continue to cohabit rather than get married after birth have significantly lower probability of having a second child than married women in all countries except those in Eastern Europe. Another study, on the contrary, came to the result that cohabiting couples in France have equal fertility as married ones.
A large survey in the United States came to the result that married women had an average of 1.9 children, compared to 1.3 among those cohabiting. The corresponding numbers for men were 1.7 and 1.1, respectively. The difference of 0.6 children for both sexes was expected to decrease to between 0.2 and 0.3 over the lifetime when correcting for the confounder that married people have their children earlier in life. In the United States, those who cohabit without marrying had increased fertility when the male earns considerably more than the female.
Another explanation for falling fertility could be a reduction in the frequency of sex. For example, according to the Japan Family Planning Association 2016 survey, 47.3% of the men and 47.1% of the women had not had sex with their spouse in the previous month.
In recent years, the marriage rate in many countries (especially in the West) has decreased. As more young people reject traditional lifestyles that include marriage, the issue arises whether the state accepts non-traditional families such as those based on non-marital cohabitation, or whether it actively discourages non-marital childbearing. In Western countries, the former position of accepting cohabiting couples and supporting them as a legitimate form of family has been connected with a higher fertility rate (e.g. Scandinavian countries, France, where most births occur outside of marriage) while the latter (such as East Asia - Japan, Hong Kong, Taiwan, Singapore) has been connected with a very low fertility rate.
People are more likely in modern society to invest strongly in the needs of their children, such as offering them the best education, shelter (a room only for the child), travel, cultural activities etc. In the past, when child mortality was high, people had more children, but invested less in them. Today, parents usually experience much less doubt about whether the child will live to adulthood, and so are more likely to strongly invest in that child. But strongly investing in each child makes it more difficult to have large numbers of children—a "quantity vs. quality trade-off" effect.
Social norms both within the family and in society at large determine fertility levels. The quality of couple relations in terms of support given to the woman matters, with studies on fertility in the West showing a U-shaped relationship between gender equity within the couple and fertility: in countries with very low fertility rates, the probability of a woman to have the second child occurs at the extremes - either very low gender equality or very high gender equality. This is also reflected at a social level: countries that are neither sufficiently patriarchal to coerce women into having large families, nor sufficiently egalitarian to incentivize women to have more children through strong support (such as subsidized childcare and good support of working mothers), have very low fertility rates, especially among educated women. Where women are expected to 'choose' between their professional and public life, or having children, the more educated the woman is, the more likely she is to choose the former. The strong emphases on the domestic role of women in Germany (unlike Scandinavia and France) was described as the cause of the very low fertility in that country.
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|“||"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 our 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.[dubious ] 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.
One of the most notorious policies of forceful attempts to increase the TFR occurred in communist Romania between 1967 and 1990. Communist leader Nicolae Ceaușescu adopted a very aggressive natalist policy that included outlawing abortion and contraception, routine pregnancy tests for women, taxes on childlessness, and legal discrimination against childless people. This period was depicted in movies and documentaries (such as 4 Months, 3 Weeks and 2 Days, Children of the Decree). These policies increased birth rates for a few years, but this was followed by a decline due to increased illegal abortion. Ceaușescu's policy resulted in over 9,000 women who died due to illegal abortions, large numbers of children put into orphanages by parents who couldn't cope with raising them, street children in the 1990s (when many orphanages were closed and the children ended on the streets), and overcrowding in homes and schools. In addition, Ceaușescu's demographic policies are feared of having very serious effects in the future, because the generations born under Ceaușescu are large (especially the late 1960s and the 1970s), while those born in the 1990s and 2000s are very small. This is believed to cause a very serious demographic shock when the former generations retire, as there will be insufficient young people in the workforce to support the elderly. Apart from Romania, a relatively similar policy of restricted reproductive rights during that period also existed in Communist Albania, under Enver Hoxha (see Abortion in Albania).
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.
Analysing data for 40 countries, Lee et al. show that fertility well above replacement and population growth would typically be most beneficial for government budgets. However, fertility near replacement and population stability would be most beneficial for standards of living when the analysis includes the effects of age structure on families as well as governments. And fertility moderately below replacement and population decline would maximize standards of living when the cost of providing capital for a growing labour force is taken into account.
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Sub-replacement fertility does not automatically translate into a population decline because of increasing life expectancy and 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/or population decline. This is already happening and impacts first 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, though population momentum continues to increase global population for several generations beyond that. The development of the world population is linked with concerns of overpopulation, sustainability and exceeding Earth's carrying capacity.
Some 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. In Italy, for example, natalist policies may have actually discouraged the Italian population from having more children. This "widespread resistance" was the result of the Italian government, at one point,[when?] taxing single persons and criminalizing abortion and even contraception.
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 cannot be used to effectively counter population ageing.
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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 in comparison. 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 rather than lower to the 1.6 level common in Europe. It can also be explained by substantial immigration.
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 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.