—that is, parents on average are not having enough children to replace those parents, pointing to an eventual decline in population if the low rate continues.
The fertility rate needs to be about 2.1 children per family for the population to remain level over a long period—the extra .1 allows for mortality. The Birth Registration Agency announced Saturday that the fertility rate in 19 of Iran’s 31 provinces is below replacement level. The lowest rate for any province is in the largest province, Tehran, where the rate is merely 1.4 children per family, a very low figure.
Oddly, the agency did not give the national fertility rate. But the implication of having two-thirds of the provinces below the replacement level and the largest province far below the replacement level was that the national fertility rate was below the replacement level.
It is important to note that this does not mean Iran’s population has now topped out at 75 million and will now decline.
It may take a few generations for a change in the total fertility rate to cause the population to level off and then fall because the age distribution must reach equilibrium. For example, a country where the fertility rate has just recently dropped below replacement level, as in Iran, will continue to grow because the previous high fertility produced large numbers of young couples who are now in their childbearing years. This phenomenon carries forward for a few generations and is called population momentum.
Ali-Akbar Mahzoon of the Birth Registry Agency said the highest fertility rates were in such provinces as Hormuzgan, Sistan va Baluchestan and Khuzestan, which have high populations of non-Farsi speaking minorities. Since Iran has only a bare majority of citizens who speak Farsi as their first language, the declining fertility rate in Farsi-speaking areas could mean that native Farsi speakers will be in the minority at some point.
The Birth Registry calculations were make for the first half of the current Persian year, March-September 2011.
Worldwide, the fertility rate is about 2.3 children per family. In Europe, the rate is 1.6 children, with many countries seeing a population decline. In the United States, the fertility rate has been below replacement level for decades, but immigration continues to build up the population.