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|Color||Fertility rate||Long-term impact|
|Red||less than 2||declining population|
|Yellow||about 2||stable population|
|Green||3 to 4||growing population|
|Blue||4 or more||rapidly growing population|
|Gray||data not available|
It was tough to cram all these countries into the map. If a country you are interested in is not marked as a
separate country on the map, check the
we probably have it in there. Our apologies to people from
countries that we didn't manage to fit in. No insult intended: we were trying to make this map all fit on the
screen at one time, even for people with low-resolution screens, and something had to give.
If the average woman has exactly two children in her lifetime, this is just enough to replace herself and one man, and thus maintain the population.
Ultimately, this is the only thing that matters in determining long-term population growth. If the average woman has two children, then the number of people in the next generation will be the same as the number of people in this generation. You often hear people say that improved diet or medicene or other things that cause people to live longer cause population to grow, or that wars or other disasters cause population to shrink. But this is only true indirectly: only if it changes the number of children that the average woman has in her lifetime. That is, if many women die while still young, they may not live long enough to have all the children that they otherwise might have had. But beyond that, how long people live doesn't matter. Everybody still dies sooner or later. If a war wipes out a large number of people, this causes an immediate drop in the population, of course. For the time that the war lasts population growth becomes negative. But once the war is over, if the number of children per woman is the same, the old population growth rate will immediately resume. There is a temporary change in the total numbers, but the rate of growth does not change.
So the way we really calculate fertility is like this: For a given year, we count the percentage of women of any given age who had a child that year, total up these percentages for all ages, and divide by 100. For example, suppose -- just to make things simple for an example -- that women only have children when they are between the ages of 18 and 21, and that there are no twins or other circumstances where a woman has more than one child in a year. Then say that 20% of the 18-year-olds had a baby, 40% of the 19-year-olds, 50% of the 20-year-olds, and 30% of the 21-year-olds. Adding these up gives 140%, so the average woman in this hypothetical society has 1.4 children in her lifetime.
Of course in real life women women are typically having children from ages 17 or 18 up to their 30's, and some have children even younger or older, so there are a lot more than four numbers to add together. But the princple is the same as the example; the calculations are just more tedious.
This sounds like it should give an exact number, but in fact there's one big catch: Some women don't live to reach any given age. When we count what percentage of women of a given age have a baby this year, of course we're only counting the percentage of living women. Going back to our example, suppose that in this hypothetical society, not only do women only have children between the ages of 18 and 21, but also suppose that half the women die the day before their 21st birthday. Then even though 30% of the 21-year-olds are having a baby, this is not 30% of the women who were born 21 years ago, because half of those women are dead. It's really only 15% of the women who are 21, or who would have been 21 if they were still alive. (Assuming none of the women die before they reach 20 for some other reason.) Thus, the actual number of children per woman is more like 1.25.
So the number would be more accurate if instead of counting the percentage of, say, women age 21 who have children this year, we could count the percentage of women who were born 21 years ago who have children this year, including both the living women and the dead women. (Of course the dead women are not having children -- barring some weird horror movie plot -- but including them in the calculation reduces the percentage.) Unfortunately, people just don't keep statistics this way, so we don't have the information available to calculate this.
Another minor adjustment is that slightly more boy babies are born than girl babies, so a woman must have slightly more than two children to make up for this. That is, as fewer girls are born than boys, each girl was to work a little harder at the baby business to make up the difference.
In practice, then a calculated fertility rate of 2.0 doesn't really mean that the population is exactly replacing itself: it's slightly short. For wealthy places with good medical care, like North America and Europe, maintaining the population requires a calculated rate of about 2.1 to 2.2. For poorer countries, where many women don't live out their child-bearing years, and many little girls don't live long enough to have even one child, the calculated rate must be significantly higher.
The calculated fertility rate is a trend for a given period of time. Many other factors are involved.
Some are simple: For example, if a country has many immigrants, even if the natives aren't having many babies, the population could still be growing.
Some are more complex: Suppose that in a given society the average woman is having less than two children. But right now the number of women in their child-bearing years is very large compared to the number of older people. With fewer older people, the death rate will be low, so for a time the relatively small number of children born could still be enough to make up for the older people dying. Such a situation could last for decades. Eventually, this "bulge" of people in child-bearing years will grow old and be replaced by a smaller generation. Now there are a large number of old people dying off, while there is a smaller set of people in their child-bearing years to produce their replacements. Even if they have significantly more than two children per woman, that might not be enough to replace all the old people dying.
In fact, this is exactly what is happening in the United States and some European countries right now. Woman are not having enough children to replace themselves-plus-one-man. But it so happens that there are an unusually large number of women of child-bearing age compared to the number of old people, plus these countries have relatively high rates of immigration, so the population continues to grow despite the low fertility rate.
But this can't last forever. Ultimately fertility rates will have to rise or population will begin to fall.
It's always dangerous to project current trends decades into the future. Life is too complicated for that. But if fertility rates continue, then within a few decades the population of the United States and Europe will begin to fall. This is already happening in Russia and Italy. Some statisticians have estimated that were it not for illegal immigrants, the population of the United States would be falling today.
|Central African Republic||4.9|
|Papua New Guinea||4.6|
|United Arab Emirates||3.4|
For most countries, the data is based on the years 1995-2000. Different periods were used for some countries when data for these years was unavailable.
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Posted 3 March 2003.
Copyright 2003 by Pregnant Pause