Zero population growth , sometimes abbreviated ZPG (also called Expired the replacement level of fertility ),  is a requirement of demographic balance Where the number of people in a specified population Neither grows nor Declines , regarded as a social aim by some.  According to some, zero population growth, perhaps after stabilizing at some optimum population , is the ideal towards which countries and the world should aspire in the interests of achieving long-term environmental sustainability . What it means by ‚the number of people nor grows nor declines‘ is that births more in-migrants equal deaths more out-migrants. 
A loosely defined goal of ZPG is to match the replacement fertility rate , which is the average number of children per woman who 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 developing 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 by George Stolnitz, who stated that the concept of a stationary population was back to 1693.  A mathematical description was given by James Mirrlees . 
In the late 1960s ZPG became a prominent political movement in the US and parts of Europe, with strong links to environmentalism and feminism . Yale University was a stronghold of the ZPG activists who believed that it is responsible for many of our problems: pollution, violence, loss of values, and 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 child with two children.“
In the long term, zero population growth can be achieved when the birth rate of a population equals the death rate , ie, fertility is at rest and is 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 whole, but not Necessarily for a country or area as it ignored migration. A population that has been growing in the past will have a higher proportion of young people. As it is young people who have children, there is a lot of difference in the fertility rate (ie, the number of children in the childbearing years) birth and death rates for a population at equilibrium and the point at which the population stops rising. The reason for this is the fact that the population is fertile, but it is a population that continues to live for some time. 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 of the population’s overall mortality rate, and also vary 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 recover 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-beens That Way for at least Several decades (to Stabilize icts age distribution), Then That country’s population Could still experience coincide 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 human population levels or rates to fall too low. Achieving ZPG is difficult because it is often determined by economic factors, incidence of poverty, natural disasters, disease, etc.
However, even if there is zero population growth, there may be some differences in economic importance , such as changes in age distribution .
How ZPG can be achieved
Number of demographic experts have suggested a few ways to reach zero population growth.
Biologist Alan D. Thornhill and Author Daniel Quinn argues that human population growth is a function of the human food supply  and that human population growth can only be achieved by an expanded food supply to support the growing population. In 1998, they produced a video entitled “ Food Production and Human Population Growth “ where they explain the theory and answer audience questions.
Albert Bartlett , an emeritus professor of physics at the University of Colorado at Boulder in his lifetime, suggests that
- Voluntarily limited births and immigration to achieve zero population growth;
- Continue on the path of the population is so large that draconian measures become necessary to stop the growth of population;
- Do nothing and let nature stop the growth through disease, starvation, war and pestilence. If humans do not solve the problem, nature will. 
Similarly, Jason Brent, another demographic expert, argues that there are three ways to achieve zero population growth. His argument is as follows:
- By war, with or without weapons of mass destruction , starvation , disease , rape , murder , ethnic cleansing , concentration camps , and other horrors beyond the imagination, when humanity has exceeded the carrying capacity of the Earth.
- By the voluntary action of the world of the earth. If any group or even a single-family failed to control its population would have failed.
- By coercive population control prior to the human population the carrying capacity of the Earth. 
China is the largest country by population in the world, being home to 1.3 billion people. China is expected to witness a zero population growth rate by 2030. China’s population growth has slowed down since the beginning of this century. This was the result of China’s economic growth and increasing living standards. However, many demographers also credit China’s family planning policy, who was formulated in the early 1970s, promotes late marriages, late childbearing, and the use of contraceptives, and since 1980 has 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  goal, in November 2013, a relaxation of the one-child policy was announced unpopularity, reduced labor pool and support for an ageing population. 
- Birth credit
- Demographic transition- Zero population growth is achieved when the birth rate of a population equals a situation where net migration is also zero
- Georgia Guidestones
- List of population concern organizations
- Overpopulation -when organism’s numbers exceed the carrying capacity of its habitat.
- Pledge two or fewer (UK family limitation campaign)
- Steady-state economy
- ZPG -A science fiction movie about the topic of zero population growth.
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