To resolve the apparent paradox of the US's continued population growth in the face of the average woman doing no more than replacement level fertility, we need to explore how population growth rates are affected by age structures.
We'll introduce the concepts of lags and momentum.
The figure below represents the age structure of the population in Mexico as of a few years ago, which is representative of that in many lesser developed, rapidly growing nations. (In fairness to Mexico, we must recognize that it has made significant progress in slowing population growth over the past decades. TFR in Mexico is now 2.3 children per woman -- almost every time I revise these notes, that number gets smaller!).
The age structure diagram breaks the population into males (on the left) and females (on the right). The percentage of the population falling into each 5-year age class is indicated, beginning with individuals between 0 and 4 years old, and ending with individuals 85+ years old. Dark shading represents the ages that are considered prereproductive and reproductive, while lighter shading indicates post-reproductive individuals.
(Information on population age structures can also be found in the Population Fact Sheet given in class. The column is headed "%<15 / %65 +", which means the percentage of the population that is less than 15 years old and the percentage that is over 65 years old. For Mexico in mid-2012, these percentages were 29 and 6%, respectively.
By studying the age structure diagram, you can see that there are so many young people that even if all of them did no more than RLF (replacing themselves and their mates), the population would still grow for a couple of generations. Imagine the young moving up in the diagram and replacing themselves. The diagram would fill in at a width corresponding to the percentages in the young age classes, and THEN the population would stabilize at that new, higher level. Essentially, because of past reproduction at greater than RLF (which you can see because there are more young people than there were older people who gave birth to them), there are many pre-reproductive and reproductive-aged people relative to a small number in older age classes, who will be doing most of the dying
Age structure confers MOMENTUM on population growth, and it takes a couple of generations after a population reaches RLF to move past themomentum that is conferred by prior growth. Population momentum measures the effect of current age structure on future population growth. Note that, while for many lesser developed nations, which have triangular age structures like that pictured above, momentum can also be negative, as it is for many nations in Europe at present.
This concept of population momentum is discussed further in the article by Bongaarts, "Population policy options in the developing world" in your assigned readings on the BlackBoard site.
These concepts of lags and momentum are very important globally, but can be tricky to catch onto at first. Maybe the following examples will help to clarify.
Past high rates of reproduction give momentum to population growth because they resulted in a large number of women who are now of reproductive age. Once RLF is achieved, it takes a couple of generations to get rid of that momentum that is, there is a lag. Note that age structures such as Mexico's -- triangular, with a wide base, are said to confer "positive" momentum on population growth. This use of "positive" does not mean "good," necessarily -- simply means causes the population to continue to grow.
Another way to express this is to say that a population's birth rate falls slower than fertility at the level of the individual woman, because of this momentum.
There are way more people moving INTO reproductive age classes than moving OUT - that is, this is basically an ACCOUNTING issue!
Another way to put it, as of 2012, on any given day, more than twice as many people BEGIN their lives as END their lives .
(1) If RLF had been reached globally in 1990 and longevity remained the same, world population would have increased to 9.4 Bill by about 2050 and then would have continued to climb until stabilizing at about 11 billion
(2) In Bangladesh, about 42% of the pop was < 15 yrs old in 1998. If starting then, each couple had only 2 children, its population would have increased by 80 million people by 2050.
(4) Over the 25 yrs starting in 1998, about 3 billion people (= Earth's pop in 1960) will enter reproductive years but only 1.8 billion will leave this phase of life. That is, lags in death rates matter a lot - it is basically an accounting problem! It takes several generations for death rates to catch up with birth rates at the pop level once RLF has been reached.
(5) If global fertility had hit RLF in 2000 and stayed there (which it did not), we'd still have grown by about 43% before stabilizing in about 60 years.
SO, IN THE US, CONTINUED GROWTH IS NOT PRIMARILY RESULT OF WOMEN HAVING MORE BABIES, BUT OF MORE WOMEN HAVING BABIES
These concepts may be clarified by looking at the age structure for a nation whose population has stabilized; Sweden (mid-2012 r% = 0.2%); it is a ZPG nation). The TFR in Sweden is right at RLF (mid-2011 TFR = 1.9). The age structure for Sweden is given below; the diagram is constructed as was the one above for Mexico, with five-year age classes indicated.
You can see that there is no bulge in the younger age classes. The bulge is characteristic of an expanding population, which Sweden is not. Rather, the age structure is quite columnar, a characteristic of stable populations. You can see that for a couple of generations, people have been doing no more than replacing themselves, and if the young people continue to do that, the age structure will remain columnar and the population will not grow.
The age structure diagram for the US is presented for three times periods below: 1960, 1970 and 1990.
In the figure for 1960, you can see the "baby boom" graphically. Post-World War II, TFR in the US reached its high point (an average of 3.8 children per woman in 1957).
In the diagram for 1970, you can see the "baby boomers" moving up as a bulge through the age classes, and you can also see that, by 1970, TFR decreased (babies represent a smaller percentage of the population than in 1960).
Despite TFR's that have been at RLF since the early 1970's, the US population has continued to grow. We have experienced the "baby boom echo" or "baby boom aftershock," in which the people born during the baby boom reproduced. Even though we (I'm a baby boomer...) didn't have more than an average of two children per woman, there were simply so many of us that we created another bulge of young people as we replaced ourselves. (This is clearest in the age structure diagram for 1990. I will show you diagram that brings us into the 21st century in class.)
The continued growth of the US population is then not the result of women having more babies than RLF, but of more women having babies as a result of past high birth rates during the "baby boom." Of course, immigration has some influence here too; immigration brings more women into the US who then go on to have babies.
Again, the population birth rate falls more slowly than do birth rates at the level of the individual woman because of momentum and lags.
If TFR in the US stays at or below RLF, we will reach ZPG (as defined based on (births - deaths) -- that is, not including added people through immigration) in a couple of generations, as you can visualize from the age structure diagram. However, for the U.S. immigration may cause us to continue to experience a net gain in people after we have reached ZPG in terms of births and deaths.
Check yourself #3: Can you make reliable inferences about a population's current growth rate if you know only what TFR in that population is? (Click on Answers to check yourself.)
Some people believe that it will take a conscious effort to keep TFR low in the US, and that as a step towards insuring that it stays low, the US should have an explicit policy on population growth. (We are the only one of the of the 25 large nations that produce >80% of the growth in the world population that does not have an explicit population policy.)
If you have comments on whether the US should or should not have an explicit population policy, and what that should contain, feel free to send them to the Blackboard Discussion Group for BI 301.
There are some projections that suggest that TFR in the U.S., which is actually higher than TFR's in most developed countries of the world right now, may actually increase slightly over the next couple of decades. See the section of notes that discusses patterns of change in TFR for more details on this.
As of 2012, the population of the US is predicted to be almost 350 million by 2025 and 423 million by 2050. Given that we're about 314 million now, that would be about a 35% increase in our population by 2050 .hard to imagine? Just think about extremely resource-poor nations who are looking at doubling populations in less time that that!?!
V. Trends in TFR
What is the global picture in terms of TFR? We'll look at this for the developing world in general, focusing on steps that can be taken to encourage stabilization (meeting unmet demand for contraception , decreasing demand for large families , and decreasing population momentum ) and then will explore some patterns of change in TFR for particular nations or regions in more detail.
I can't overemphasize how important patterns in TFR are. For example, the UN's widest ranging projections for the size at which the human population will stabiize range between 3.6 and 27 billion people on Earth. The difference between these two extremes involves only a difference of one child per woman!
(To move to the next section in these notes ( on population growth patterns in the developing world overall), click the box at the bottom of the page labeled ">>." To return to the previous section on total fertility rates, click the box labeled "<<" and to return to the master directory for the BI301 home web site, click the box labeled "CONTENTS.")
This page is maintained by Patricia Muir at Oregon State University. Page last updated Oct. 26, 2012.