In biology a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings. A population shares a particular characteristic of interest, most often that of living in a given geographic area. In taxonomy population is a low-level taxonomic rank.
Human populations can be defined by any characteristics such as mortality, migration, 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.
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.
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 112 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.
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.