Zero population growth, sometimes abbreviated ZPG (also called the replacement level of fertility), is a condition of demographic balance where the number of people in a specified population neither grows nor declines, considered as a social aim by some.[by whom? This is related to the optimum population theory, where a certain population is the ideal towards which countries and the whole world should aspire in the interests of accomplishing long-term environmental sustainability. What it means by 'the number of people neither grows nor declines' is that births plus in-migrants equal deaths plus out-migrants.
History
A loosely defined goal of ZPG is to match the replacement fertility rate, which is the average number of children per woman which would hold the population constant. This replacement fertility will depend on mortality rates and the sex ratio at birth, and varies from around 2.1 in developed countries to over 3.0 in some developing countries.
The American sociologist and demographer Kingsley Davis is credited with coining the term but it was used earlier by George J. Stolnitz, who stated that the concept of a stationary population dated back to 1693. A mathematical description was given by James Mirrlees.
In the late 1960s ZPG became a prominent political movement in the U.S. and parts of Europe, with strong links to environmentalism and feminism. Yale University was a stronghold of the ZPG activists who believed "that a constantly increasing population is responsible for many of our problems: pollution, violence, loss of values and of individual privacy." Founding fathers of the movement were Paul Ehrlich, author of The Population Bomb, Richard Bowers, a Connecticut lawyer, and Professor Charles Lee Remington. Ehrlich stated: "The mother of the year should be a sterilized woman with two adopted children."
