Demographic transition

Demographic transition occurs in societies that transition from high birth rates and high death rates to low birth rates and low death rates as part of the economic development of a country from a pre-industrial to an industrialized economy. Usually it is described through the "Demographic Transition Model" (DTM) that describes the population changes over time. It is based on an interpretation begun in 1929 by the American demographer Warren Thompson of prior observed changes, or transitions, in birth and death rates in industrialized societies over the past two hundred years. Most developed countries are already in stage four of the model, the majority of developing countries are in stage 2 or stage 3, and no country is currently still in stage 1. The model has explained human population evolution relatively well in Europe and other highly developed countries. Many developing countries have moved into stage 3. The major exceptions are poor countries, mainly in sub-Saharan Africa and several Middle Eastern countries, which are poor or affected by government policy or civil strife, notably Pakistan, Palestine, Yemen and Afghanistan.

Origins Of DTM
The idea of DTM was first advanced by Warren Thompson in 1929. He divided the world into three major groups: Frank W. Notestein developed this theory in 1945 and suggested that there was a relationship between population change and industrial development. He suggested that with time, countries go through a linear evolution from traditional, non-industrial society to a modern, industrial and urban one.
 * Countries with rapidly declining birth and death rates, with fertility declining more rapidly than mortality, resulting in a declining growth rate (North and Western Europe, North America and Australasia)
 * Countries with declining birth and death rates in certain socio-economic strata, with the rate of decline of the death rate higher than the decline in the birth rate (Central and Southern Europe)
 * Countries with high birth rates but declining death rates (rest of the world)

Summary of the theory
The transition involves four stages, or possibly five.
 * In stage one, pre-industrial society, death rates and birth rates are high and roughly in balance.
 * In stage two, that of a developing country, the death rates drop rapidly due to improvements in food supply and sanitation, which increase life spans and reduce disease. These changes usually come about due to improvements in farming techniques, access to technology, basic healthcare, and education. Without a corresponding fall in birth rates this produces an imbalance, and the countries in this stage experience a large increase in population.
 * In stage three, birth rates fall due to access to contraception, increases in wages, urbanization, a reduction in subsistence agriculture, an increase in the status and education of women, a reduction in the value of children's work, an increase in parental investment in the education of children and other social changes. Population growth begins to level off.
 * During stage four there are both low birth rates and low death rates. Birth rates may drop to well below replacement level as has happened in countries like Italy, Spain and Japan, leading to a shrinking population, a threat to many industries that rely on population growth. The large group born during stage two ages creates an economic burden on the shrinking working population. Death rates may remain consistently low or increase slightly due to increases in lifestyle diseases due to low exercise levels and high obesity and an ageing population in developed countries.

As with all models, this is an idealized picture of population change in these countries. The model is a generalization that applies to these countries as a group and may not accurately describe all individual cases. The extent to which it applies to less-developed societies today remains to be seen. Many countries such as China, Brazil and Thailand have passed through the DTM very quickly due to fast social and economic change. Some countries, particularly African countries, appear to be stalled in the second stage due to stagnant development and the effect of AIDS.

Stage One
In pre-industrial society death rates and birth rates were both high and fluctuated rapidly according to natural events, such as drought and disease, to produce a relatively constant and young population. Children contributed to the economy of the household from an early age by carrying water, firewood, and messages, caring for younger siblings, sweeping, washing dishes, preparing food, and doing some work in the fields. Raising a child cost little more than feeding him: there were no education or entertainment expenses, and in equatorial Africa, there were no clothing expenses either. Thus, the total cost of raising children barely exceeded their contribution to the household. In addition, as they became adults they became a major input into the family business, mainly farming, and were the primary form of insurance in old age. In India an adult son was all that prevented a widow from falling into destitution. While death rates remained high there was no question as to the need for children, even if the means to prevent them had existed.

Stage Two
This stage leads to a fall in death rates and an increase in population. The changes leading to this stage in Europe were initiated in the Agricultural Revolution of the 18th century and were initially quite slow. In the 20th century, the falls in death rates in developing countries tended to be substantially faster. Countries in this stage include Yemen, Afghanistan, Palestine, Bhutan and Laos and much of Sub-Saharan Africa (but do not include South Africa, Zimbabwe, Botswana, Swaziland, Lesotho, Namibia, Kenya and Ghana, which have begun to move into stage 3). The decline in the death rate is due initially to two factors:
 * First, improvements in the food supply brought about by higher yields in agricultural practices and better transportation prevent death due to starvation. Agricultural improvements included crop rotation, selective breeding, and seed drill technology.
 * Second, significant improvements in public health reduce mortality, particularly in childhood. These are not so much medical breakthroughs (Europe passed through stage two before the advances of the mid-20th century, although there was significant medical progress in the 19th century, such as the development of vaccination) as they are improvements in water supply, sewerage, food handling, and general personal hygiene following from growing scientific knowledge of the causes of disease and the improved education and social status of mothers.

A consequence of the decline in mortality in Stage Two is an increasingly rapid rise in population growth (a "population explosion") as the gap between deaths and births grows wider. Note that this growth is not due to an increase in fertility (or birth rates) but to a decline in deaths. This change in population occurred in northwestern Europe during the 19th century due to the Industrial Revolution. During the second half of the 20th century less-developed countries entered Stage Two, creating the worldwide population explosion that has demographers concerned today.

Another characteristic of Stage Two of the demographic transition is a change in the age structure of the population. In Stage One, the majority of deaths are concentrated in the first 5–10 years of life. Therefore, more than anything else, the decline in death rates in Stage Two entails the increasing survival of children and a growing population. Hence, the age structure of the population becomes increasingly youthful and more of these children enter the reproductive cycle of their lives while maintaining the high fertility rates of their parents. The bottom of the "age pyramid" widens first, accelerating population growth. The age structure of such a population is illustrated by using an example from the Third World today.

Stage Three
Stage Three moves the population towards stability through a decline in the birth rate. There are several factors contributing to this eventual decline, although some of them remain speculative:
 * In rural areas continued decline in childhood death means that at some point parents realize they need not require so many children to be born to ensure a comfortable old age. As childhood death continues to fall and incomes increase parents can become increasingly confident that fewer children will suffice to help in family business and care for them in old age.
 * Increasing urbanization changes the traditional values placed upon fertility and the value of children in rural society. Urban living also raises the cost of dependent children to a family.
 * In both rural and urban areas, the cost of children to parents is exacerbated by the introduction of compulsory education acts and the increased need to educate children so they can take up a respected position in society. Children are increasingly prohibited under law from working outside the household and make an increasingly limited contribution to the household, as school children are increasingly exempted from the expectation of making a significant contribution to domestic work.  Even in equatorial Africa, children now need to be clothed, and may even require school uniforms.  Parents begin to consider it a duty to buy children books and toys.  Partly due to education and access to family planning, people begin to reassess their need for children and their ability to raise them.
 * Increasing female literacy and employment lower the uncritical acceptance of childbearing and motherhood as measures of the status of women. Working women have less time to raise children; this is particularly an issue where fathers traditionally make little or no contribution to child-raising, such as southern Europe or Japan. Valuation of women beyond childbearing and motherhood becomes important.
 * Improvements in contraceptive technology are now a major factor. Fertility decline is caused as much by changes in values about children and sex as by the availability of contraceptives and knowledge of how to use them.

The resulting changes in the age structure of the population include a reduction in the youth dependency ratio and eventually population aging. The population structure becomes less triangular and more like an elongated balloon. During the period between the decline in youth dependency and rise in old age dependency there is a demographic window of opportunity that can potentially produce economic growth through an increase in the ratio of working age to dependent population; the demographic dividend.

However, unless factors such as those listed above are allowed to work, a society's birth rates may not drop to a low level in due time, which means that the society cannot proceed to Stage Four and is locked in what is called a demographic trap.

Countries that have experienced a fertility decline of over 40% from their pre-transition levels include: Costa Rica, El Salvador, Panama, Jamaica, Mexico, Colombia, Ecuador, Guyana, Surinam, Philippines, Indonesia, Malaysia, Sri Lanka, Turkey, Azerbaijan, Turkmenistan, Uzbekistan, Egypt, Tunisia, Algeria, Morocco, Lebanon, South Africa and many Pacific islands.

Countries that have experienced a fertility decline of 25-40% include: Honduras, Guatemala, Nicaragua, Paraguay, Bolivia, Vietnam, Myanmar, India, Bangladesh, Tajikistan, Iran, Jordan, Qatar, United Arab Emirates, Zimbabwe and Botswana.

Countries that have experienced a fertility decline of 10-25% include: Haiti, Papua New Guinea, Nepal, Pakistan, Syria, Iraq, Saudi Arabia, Libya, Sudan, Botswana, Kenya, Ghana and Senegal.

Stage Four
Traditionally many demographers have assumed that the demographic transition would be complete when populations reached similarly low birth and death rates so that populations would become essentially stable, although no convincing social mechanism has been put forward for this view.

Countries that are at this stage (Total Fertility Rate of less than 2.5 in 1997) include: United States, Canada, Australia, New Zealand, most of Europe, Bahamas, Puerto Rico, Trinidad and Tobago, Brazil, Sri Lanka, South Korea, Singapore, China, North Korea, Thailand and Mauritius.

Stage Five
The original Demographic Transition model has just four stages, but it is now widely accepted that a fifth stage is needed to represent countries that have undergone the economic transition from manufacturing based industries into service and information based industries called deindustrialization. Countries such as Germany, Italy, Spain, Portugal, Greece and most notably Japan, whose populations are now reproducing well below their replacement levels, that is they are not producing enough children to replace their parent's generation. China, South Korea, Hong Kong, Singapore, Thailand and Cuba are also below replacement, but this is not producing a fall in population yet in these countries, because their populations are relatively young due to strong growth in the recent past. The population of southern Europe is already falling and Japan and some of western Europe will soon begin to fall without significant immigration. However, many countries that now have sub-replacement fertility did not reach this stage gradually but rather suddenly as a result of economic crisis brought on by the post-communist transition in the late 1980's and the 1990's. Examples include Russia, Ukraine, and the Baltic States. The population of these countries is falling due to fertility decline, emigration and, particularly in Russia, increased male mortality.

Non-Applicability to Less Developed Countries
One of the principal criticisms of the DTM is the questionable applicability to less developed countries, where the prerequisites for wealth and information access are limited. Applicability of the DTM to less developed countries has been questioned on several grounds. For example, the DTM has been validated primarily in Europe, Japan and North America where demographic data exists over centuries, whereas high quality demographic data for most LDCs did not become widely available until the mid 20th century. Secondly, the DTM does not account for recent phenomena such as AIDS; fully 94 percent of all HIV cases are found in underdeveloped countries, and thus the mortality decline of most of Sub-Saharan Africa has been arrested starting in the mid-1990s. This trend is so marked that two thirds of children in many sub-Saharan African countries are projected to have HIV infection by the time they have reached age 50 (or die from HIV before). In these areas HIV has become the leading source of mortality. Some trends in waterborne bacterial infant mortality are also disturbing in countries like Malawi, Sudan and Nigeria; for example, progress in the DTM model clearly arrested and reversed between 1975 and 2005. The above data lead to the questioning of the applicability to many lesser developed countries. DTM is used widely to minimize concerns regarding overpopulation and paint an overly optimistic version of the future.

Neo-Malthusian objections
The DTM is used to classify countries into general groups. Hence, it is unable to take into account evolutionary changes in the process of population growth and classify diverse population into distinct but coherent stages of the development process. Neo-Malthusians argue that the long-term fertility of a population depends on the most rapidly-breeding subgroups within the population. If a rapidly-breeding subgroup sustains its high fertility, it will eventually expand its numbers and restore the whole population to higher fertility.

All over the world groups with lower income, lower educational levels and more traditional social beliefs have higher fertility. In most countries poor parents decide to have many children and invest little in education, which implies that more weight gets placed on such families. Consequently, an increase in inequality lowers average education and, therefore, economic growth. Until now, DTM provides no explanation for this demographic-economic paradox and its possible dysgenic consequences, which may cause a cyclic behavior of human population dynamics.

This is similar to the evolution of pesticide resistance in insects, and antibiotic resistance among pathogens: the first applications kill large numbers, but a few surviving resistant individuals may eventually make good on the losses through exponential growth. In other words, a criticism of the DTM is that it is only valid if the fertility-lowering social changes that caused the DTM in present-day industrial nations permanently lower the fertility of every subgroup within each nation (which would imply no tendency of high-fertility individuals to produce high-fertility offspring). Garrett Hardin doubted that purely voluntary birth control could achieve that result; Hardin argued that voluntary birth control merely selects against the people who will use it.

This view appears to be an echo of the eugenics movement of the early 20th century, which was concerned that various minority groups would overwhelm the US or German populations in the long run through higher fertility and immigration. While "overwhelm" may be an excessively alarmist term, the white population in the United States certainly is declining in percentage terms in the 21st century. At the contrary, the critics of the Neo-Malthusian view believe that all human subgroups are subject to social and economic changes and that this is likely to overwhelm any tendency for a subgroup to have higher fertility in the long run. However, Hardin's doubts in the sufficiency of purely voluntary birth control were not based on group behavior but on individual variation. Regardless of what the average fertility is for any group, some people will have more children than the mean for their group, and some will have fewer. According to Hardin, if the daughters of a woman who bore more children than the mean for her group go on to bear more children than the mean for their generation and group, this could be evidence for some sort of heritability of individual fertility, i.e., resistance to the cultural factors that lower the average fertility. If such resistance is heritable (whether culturally or genetically), then over time the percentage of people who are resistant could increase enough to create an entire population with a kind of "pesticide resistance" to the factors that gave rise to the demographic transition in the previously susceptible population, and population growth could resume. However, this effect would require several generations of sustained selection to become visible, if only a small percentage of people were initially carriers of this supposed resistance, and by then culture may have changed in other unpredictable ways.

Generalization from European experience
The DTM is also limited in the sense that it gives a generalized picture of population change over time based on European studies, assuming that all countries would follow suit. In addition, the DTM is rigid in assuming that all countries will go through the stages 1 to 4 in that exact order. There are variables and exceptions such as war and turmoil that may lead to different results. Some countries may even skip stages. Demographic data for lesser developed countries span about five decades compared to 30 or more decades for developed countries, leading to questionable extrapolation of the experiences of the most developed countries.

Subsistence farming and capital formation
The Peruvian economist Hernando de Soto has argued that one obstacle to industrial development is that subsistence farmers can not convert their work into capital which can be used to start new businesses and trigger industrializations. He argues that these obstacles exist often because subsistence farmers do not have clear title to the land which they work and to the crops which they produce.

Insufficiency of wealth effect on fertility
Some versions of the DTM assume that population changes are induced by industrial changes and increased wealth, without taking into account the role of social change in determining birth rates, e.g, the education of women. In fact the developers of the DTM were aware of the importance of social change, but some were content to analyse the statistics of the transition rather than develop a comprehensive explanation for it. In recent decades more work has been done on developing the social mechanisms behind it.

Some have claimed that the DTM assumes that the birth rate is independent of the death rate. For instance, they point to evidence at the level of families that child mortality leads to higher fertility, as parents try to replace their dead children. Nevertheless, demographers maintain that there is no historical evidence for society-wide fertility rates rising significantly after high mortality events. Notably, some historic populations have taken many years to replace lives lost in major mortality events such as the Black Death.

Some have claimed that DTM does not explain the early fertility declines in much of Asia in the second half of the 20th century or the delays in fertility decline in parts of the Middle East. Nevertheless, the demographer John C Caldwell has suggested that the reason for the rapid decline in fertility in some developing countries compared to Western Europe, the United States, Canada, Australia and New Zealand is mainly due to government programs and a massive investment in education both by governments and parents. On the other hand, birth rates remain high in some nations, particularly Saudi Arabia, despite great increases in prosperity, probably partly as a result of government policy and partly as a result of the limited need and opportunity for mothers to work.