Effects
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.
Read more about this topic: Zero Population Growth
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