Population
In sociology and biology a population is the collection of people or organisms of a particular species living in a given geographic area or mortality, and migration, though the field encompasses many dimensions of population change including the family (marriage and divorce), public health, work and the labor force, and family planning. Various aspects of human behavior in populations are also studied in sociology, economics, and geography. Study of populations is almost always governed by the laws of probability, and the conclusions of the studies may thus not always be applicable to some individuals. This odd factor may be reduced by statistical means, but such a generalization may be too vague to imply anything. Demography is used extensively in marketing, which relates to economic units, such as retailers, to potential customers. For example, a coffee shop that wants to sell to a younger audience looks at the demographics of an area to be able to appeal to this younger audience.

# World population
According to estimates published by the United States Census Bureau, the world population hit 6.5 billion (6,500,000,000) on February 25 2006. It is estimated that by 2012, the Earth will be home to 7 billion. The United Nations Population Fund designated October 12 1999 as the approximate day on which world population reached 6 billion. This was about 12 years after world population reached 5 billion, in 1987. However, the population of some countries, such as Nigeria, is not even known to the nearest million, so there is a considerable margin of error in such estimates.
In 2007 the United Nations Population Division projected that the world's population will likely surpass 9 billion in 2050. The last 50 years have seen a rapid increase in population due to medical advances and substantial increase in agricultural productivity, particularly in the period 1960 to 1995 made by the Green Revolution.

# Population decline
Population decline is a decrease in a region's population. It can be caused by sub-replacement fertility, heavy emigration, or more dramatically disease, famine, or war. In the past, population decline was mostly caused by disease. The Black Death in Europe and the arrival of Old World diseases to the Americas all caused massive population declines.
In biology, population decline of a species is usually described as a result of gradually worsening environmental factors, such as prolonged drought or loss of inhabitable areas for the studied species. These, or other factors, may lead to a small population, in which case genetic factors may become dominant in the survival, or extinction of a population.
Under-population is recognized when there are more resources in an area (for example, food, energy and minerals) than can be used by the people living there. Hence, the maximum human potential of that area is not realized as the resources are not fully exploited. Countries like Canada and Australia can export the surplus of food, energy, and mineral resources, have high incomes, good living conditions and level of technology and immigration.
Some rural areas close to major cities in advanced countries such as the UK are under-populated due to outward migration. In the UK, the Southwest Wales and the highlands of Scotland are less densely populated compared to the rest of the country. This has also happened in older declining industrial areas and the outward movement or migration has been due to lower wages and unemployment. This phenomenon results in a decline in a population. With fewer people, there is a decrease in demands for services. The lower level of services therefore sometimes encourages further outward migration.
However, when making comparisons on a global scale, there does not seem to be any direct correlation between population density and over- or under-population. For example, Brazil is 'over-populated' with two people per square kilometer, whereas portions of California may have further carrying capacity with over 500 people per square kilometer. Therefore, this is related to the amount of available resources. Similarly, population density is not necessarily related to the GDP per capita. The Netherlands and Germany, for example, both have a high GDP per capita and a high population density whereas Canada and Australia have a high GDP per capita and a low population density, while Bangladesh has low GDP per capita and a high population density, etc.
The balance of population and resources within a country may be uneven. For example, a country may have a population, which is too great for one resource such as energy, yet too small to use fully a second such as food supply.
Various attempts to address population decline have been made:
- Improving communication networks and transport facilities makes remote places more accessible. This strategy was used in developing countries like Nigeria and Tanzania where modern railway networks were established, but these attempts were not very successful.
- Establishment of new capital cities, new towns, or development growth points. For example, Brazil has a population imbalance between the coastal parts from east and south and the rest of the country. Brasilia, the new capital was created in the 1960s in the country's geographical center to attract people into the North and Center-West regions, but this had limited effect, as most of these unpopulated areas are occupied by large forests and swamps.
- Regional development programs. In Brazil, the interior improvement of transport networks and development of secondary growth points and rural development have all been enhanced to attract more people and discourage out-migration. The standard of living in such regions is expected to gradually improve due to improved resource utilization.
- Pronatalist policies providing tax incentives, paid maternity leaves, daycare, or other benefits to families to bear more children. Such policies have been tried, with mixed success, in Western Europe in recent years.

# Population control
Population control is the practice of curtailing population increase, usually by reducing the birth rate.  Surviving records from Ancient Greece document the first known examples of population control. These include the colonization movement, which saw Greek outposts being built across the Mediterranean and Black Sea basins to accommodate the excess population of individual states. An important example of mandated population control is China's one-child policy, in which having more than one child is made extremely unattractive. This has led to allegations that practices like infanticide, forced abortions, and forced sterilization are used as a result of the policy.
It is helpful to distinguish between fertility control as individual decision-making and population control as a governmental or state-level policy of regulating population growth. Fertility control may occur when individuals or couples or families take steps to decrease or to regulate the timing of their own child-bearing. In Ansley Coale's oft-cited formulation, three preconditions for a sustained decline in fertility are: (1) acceptance of calculated choice (as opposed to fate or chance or divine will) as a valid element in fertility, (2) perceived advantages from reduced fertility, and (3) knowledge and mastery of effective techniques of control. In contrast to a society with natural fertility, a society that desires to limit fertility and has the means to do so may use those means to delay childbearing, space childbearing, or stop childbearing. Delaying sexual intercourse (or marriage), or the adoption of natural or artificial means of contraception are most often an individual or family decision, not a matter of a state policy or societal-wide sanctions. On the other hand, individuals who assume some sense of control over their own fertility can also accelerate the frequency or success of child-bearing through planning.
At the societal level, declining fertility is almost an inevitable result of growing secular education of women . However, the exercise of moderate to high levels of fertility control does not necessarily imply low fertility rates. Even among societies that exercise substantial fertility control, societies with an equal ability to exercise fertility control (to determine how many children to have and when to bear them) may display widely different levels of fertility (numbers of children borne) associated with individual and cultural preferences for the number of children or size of families.
In contrast to fertility control, which is mainly an individual-level decision, governments may attempt to exercise population control by increasing access to means of contraception or by other population policies and programs. The idea of "population control" as a governmental or societal-level regulation of population growth does not require "fertility control" in the sense that it has been defined above, since a state can affect the growth of a society's population even if that society practices little fertility control. It's also important to embrace policies favoring population increase as an aspect of population control, and not to assume that states want to control population only by limiting its growth. To stimulate population growth, governments may support not only immigration but also pronatalist policies such as tax benefits, financial awards, paid work leaves, and childcare to encourage the bearing of additional children. Such policies have been pursued in recent years in France and Sweden, for example. With the same goal of increasing population growth, on occasion governments have sought to limit the use of abortion or modern means of birth control. An example was Romania's 1966 ban on access to contraception and abortion on demand.
In ecology, population control is on occasions considered to be done solely by predators, diseases, parasites, and environmental factors. At many times human effects on animal and plant populations are also considered. See also . Migrations of animals may be seen as a natural way of population control, for the food on land is more abundant on some seasons. The area of the migrations' start is left to reproduce the food supply for large mass of animals next time around. See also immigration.