Suppose you had 100 women. Each has one daughter in her lifetime on average, and the daughter has 1 in hers. You would have 100 mothers, 100 daughters, and 100 granddaughters. The population (of active mothers) would stay the same for 3 generations, and the fertility rate would be 1.0 female babies per lifetime.
Now, let’s factor in the males. Suppose the 100 women had 100 husbands. To keep the population stable, each mother would have to produce 2.13 babies, including the male ones, because there are more male babies than females. The 2.13 number is an estimate, the exact figure depends on other factors.
If you have 100 couples, and they each have only 1 child, the birth rate per couple would be 1.0, and they would produce only 50 couples in that generation. If the fertility rate of 1.0 held again, then there would only be 25 couples after two generations. Inevitably, the population would decline.
Population size is complex, and you would think that if the birth rate exceeds the death rate, It would grow. This is not always true. Other factors, such as the number of immigrants to a country and the number of people who leave, play a role. But for most countries, a population, has to maintain an average fertility rate of about 2.13 births per mother (or better) to keep from declining. Here’s more: Human birth rate replacement level
Why do I bring this up? Because, many countries, including the U.S., have fertility rates below 2.13; others are expanding rapidly. The CIA World Factbook is an excellent reference, and you can find fertility rate estimates for every country in the world, so you can learn which countries have declining populations. CIA World Factbook. To locate the fertility rate, select a country (in my example, Belarus), and expand the section on People and Society (click on those words.)
Going down through People and Society, you come to population growth rate in Belarus, which is (today) –0.362%. It’s negative! The birth rate is given as 973 per 1000 people per year, or in my terms above, .973%! The death rate per 1000 is 13.73 deaths per thousand per year, or 1.373%. But, the birth rate is not the number we’re looking for. The total fertility rate per mother is given farther down, as 1.27 children per mother. A country comparison to the world is given, and you see that Belarus is 213th. Notice a line under the number 213, which means it’s a link to a list of all the countries and their fertility rates. Click on it, and you see that Niger is the country with the highest fertility rate in the world, with 7.52 children per mother. In last place is Singapore, with only .78 children per mother. Not every country is rated, but Canada is 177th at 1.59, and the United States is 121st at a comfortable 2.06, which is a very modest decline, so why worry?
Many developed countries suffer from declining population. Forty countries, including the Vatican (for obvious reasons) are at zero to negative annual population growth rate. Russia, for example, is at –.48% per year. You can get that from the CIA World Factbook. If we wait long enough, Russia will be empty, and there should be real estate opportunities there. A writeup on Russia’s fertility problem.
Take a look at The U.S. Census of 2010. Check the growth since the 2000 census by race: white alone, 5.7%, all others, much higher. It sounds as if our nation’s white women aren’t doing their job.
Should we worry? Probably not, we have enough other stuff to worry about. But, the world is always changing. The trends are there, and we’ll have to wait and see what comes of them.