In biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings. Individuals within a population share a particular characteristic of interest, most often that of living in a given geographic area. In addition, on average, populations often show differences in morphological,[physiological, life history or behavioral characteristics.
Human populations can be defined by any characteristics such as mortality, migration, family, 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.
Human society is controlled by a law which decrees that the population must vary directly in accordance with the land arts and inversely with a given standard of living. Throughout these early ages, even more than at present, the law of supply and demand as concerned men and land determined the estimated value of both. During the times of plentiful land--unoccupied territory--the need for men was great, and therefore the value of human life was much enhanced; hence the loss of life was more horrifying. During periods of land scarcity and associated overpopulation, human life became comparatively cheapened so that war, famine, and pestilence were regarded with less concern. When the land yield is reduced or the population is increased, the inevitable struggle is renewed; the very worst traits of human nature are brought to the surface. The improvement of the land yield, the extension of the mechanical arts, and the reduction of population all tend to foster the development of the better side of human nature.
According to papers published by the United States Census Bureau, the world population hit 6.5 billion (6,500,000,000) on February 25, 2006. 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, and 6 years after world population reached 5.5 billion in 1993. However, the population of some countries, such as Nigeria, is not even known to the nearest million 
In 2007 the United Nations Population Division projected that the world's population will likely surpass 10 billion in 2055. 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.
In the future, world population has been expected to reach a peak of growth, from there it will decline due to economic reasons, health concerns, land exhaustion and environmental hazards. There is around an 85% chance that the world's population will stop growing before the end of the century. There is a 60% probability that the world's population will not exceed 10 billion people before 2100, and around a 15% probability that the world's population at the end of the century will be lower than it is today. For different regions, the date and size of the peak population will vary considerably.
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. Infanticide, including abortion, was encouraged in some Greek city states in order to keep population down.
An important example of mandated population control is People's Republic of China's one-child policy, in which having more than one child is made extremely unattractive. This has led to allegations that practices like forced abortions, forced sterilization, and infanticide are used as a result of the policy. The country's sex ratio at birth of 114 boys to 100 girls may be evidence that the latter is often sex-selective. However, other countries without a one-child policy also have similar sex ratios but for different reasons such as nutrition.
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. In a constant environment, population control is regulated by the availability of food, water, and safety. The maximum number of a species or individuals that can be supported in a certain area is called the carrying capacity. At many times human effects on animal and plant populations are also considered. 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.
India is another example where the government has taken measures to reduce the country’s population. Concerns that the rapidly growing population would adversely affect economic growth and living standards caused India to implement an official family planning program in the late 1950s and early 1960s; it was the first country in the world to do so.
- UNFPA, The United Nations Population Fund
- United Nations Population Division
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- Current World Population
- NECSP HomePage
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- Population in the news homepage
- Optimum Population Trust
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- SIEDS, Italian Society of Economics Demography and Statistics
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- United Nations Economic Commission for Europe - Official Web Site
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- World Population Counter, and separate regions.
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- OECD population data