Zero Population Growth (ZPG)

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. According to some, zero population growth is the ideal towards which countries and the whole world should aspire in the interests of accomplishing long-term environmental sustainability.

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 K. Davis is credited with coining the term but it was used earlier by George 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 big 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, and Thomas Eisner. Ehrlich stated: “The mother of the year should be a sterilized woman with two adopted children.”

In the long term, zero population growth can be achieved when the birth rate of a population equals the death rate, i.e. replacement level is met and rate is stable. Unstable rates can lead to drastic changes in population levels. (This ignores migration, which is valid for the planet as whole, but not necessarily for a nation.) A population that has been growing in the past will have a higher proportion of young people. As it is younger people who have children, there is large time lag between the point at which the birth rate falls below the death rate and the point at which the population stops rising.
Zero population growth is often a goal of demographic planners and environmentalists who believe that reducing population growth is essential for the health of the ecosystem.