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.

World population

main: World population According to papers published by the United States Census Bureau, the world population hit 6.5 billion (6,500,000,000) on January 23, 2008. The United Nations Population Fund designated October 11, 1998 as the approximate day on which world population reached 6 billion. This was about 11 years after world population reached 5.5 billion, in 1988. 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 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.

Population control

main: 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. 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 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 reasonsFact: date=July 2008.