Zero population growth

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.
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."
Effects.
In the long term, zero population growth can be achieved when the birth rate of a population equals the death rate, i.e. fertility is at replacement level and birth and death rates are stable, a condition also called demographic equilibrium. Unstable rates can lead to drastic changes in population levels. This analysis is valid for the planet as a whole (assuming that interplanetary travel remains at zero or negligible levels), but not necessarily for a region or country as it ignores migration. 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 fertility rate (mean total number of children each woman has during her childbearing years) falls to the replacement level (the fertility rate which would result in equal birth and death rates for a population at equilibrium) and the point at which the population stops rising. The reason for this is that even though the fertility rate has dropped to replacement level, people already continue to live for some time within a population. Therefore, equilibrium, with a static population, will not be reached until the first "replacement level" birth cohorts reach old age and die. The related calculations are complex because the population's overall death rate can vary over time, and mortality also varies with age (being highest among the old).
Conversely, with fertility below replacement, a large elderly generation eventually results (as in an aging "baby boom"); but since that generation failed to replace itself during its fertile years, a subsequent "population bust", or decrease in population, will occur when the older generation dies off. This effect has been termed birth dearth. In addition, if a country's fertility is at replacement level, and has been that way for at least several decades (to stabilize its age distribution), then that country's population could still experience coincident growth due to continuously increasing life expectancy, even though the population growth is likely to be smaller than it would be from natural population increase.
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. Preserving cultural traditions and ethnic diversity is a factor for not allowing human populations levels or rates to fall too low. Achieving ZPG is difficult because a country's population growth is often determined by economic factors, incidence of poverty, natural disasters, disease, etc.
However, even if there is zero population growth, there may be changes in demographics of great importance to economic factors, such as changes in age distribution.
How ZPG can be achieved.
A number of demographic experts have suggested a few ways to reach zero population growth.
Albert Bartlett, an emeritus professor of physics at University of Colorado at Boulder in his lifetime, suggested that a population has the following choices to achieve ZPG:
Similarly, Jason Brent, another demographic expert, argues that there are three ways to achieve zero population growth. His argument is as follows:
In China.
China is the largest country by population in the world, having some 1.4 billion people. China is expected to have a zero population growth rate by 2030. China's population growth has slowed since the beginning of this century. This was mostly the result of China's economic growth and increasing living standards which led to the decline. However, many demographers also credit China's family planning policy, which was formulated in the early 1970s, encourages late marriages, late childbearing, and the use of contraceptives, and since 1980 has limited most urban couples to one child and most rural couples to two children. According to government projections, the work-age population will then drop to 870 million. It was said, in 2009, that the Chinese government was hoping to see zero population growth in the future but, in November 2013, a relaxation of the one-child policy was announced amid unpopularity, reduced labour pool and support for an ageing population.