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.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.
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.
Conversely, a large elderly generation can be the result of an aging “baby boom”, but if that generation had failed to replace its population during its fertile years, the result is a subsequent “population bust”, or decrease in population, as that 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 adjust for changes in 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 ecosphere. 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.
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 since the beginning of this century. It is because China's family planning policy, which was formulated in the early 1970s, encourages late marriages and late childbearing, and limits most urban couples to one child and most rural couples to two children. Without the policy, the country's population would be 400 million more than the current 1.3 billion people. According to the government projection, the work-age population will then drop to 870 million. The Chinese government is hoping to see the zero population growth in the future.