Bryan Caplan  

Fun Facts About Population Projections; or, Are People Like CDs?

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Tyler already blogged the best sentences from this excellent piece on population decline. So I've decided to supply a complement: A brief critique of U.N. population projections.

Ben Wattenberg explains that the U.N.'s World Population Prospects gives four basic projections: High, Medium, and Low, plus the Constant-Fertility Variant.

The Constant-Fertility Variant mechanically assumes that fertility rates will remain right where they are. If you're at 1.1 kids per woman, it assumes that you'll always have 1.1 kids per woman.

The other approaches do something quite different: With a few provisos, they assume that countries' Total Fertility Rates (TFRs) will asymptote to a specific rate. In the Medium scenario, the rate is 1.85 children per woman (it used to be the "replacement rate" of 2.1). If this sounds weird, it is. As Wattenberg explains:

Until the publication of the 2002 data, the UNPD "Medium" scenario took every modern nation with a TFR below 2.1 children per woman and by statistical fiat sloped it upward toward 2.1 by 2050. Nations above the replacement rate were sloped downward to 2.1.

Now the UN projects that TFR will fall to 1.85. Recall that every single developed nation - all of Europe, Japan, the United States, Canada, Australia - is currently in subreplacement status, and almost all of them have been there for decades. Yet for the most modern countries the new UN projections will raise the TFR for the next fifty year cycle.

In other words:

[F]or nations with low TFRs, this does not mean a decrease in fertility, but rather a large increase. Thus the current German TFR is 1.35. Using the new level of 1.85 means going up by half a child per woman, which is quite substantial. As we shall see, there is no evidence that such an increase is occuring.
The High and Low projections basically use the same methodology as the Medium, but asymptote TFRs to different numbers: the High assumes TFR will go to 2.35 children per woman; the Low assumes it will go to 1.35 children per woman. Thus, for many countries like Russia, Japan, China, and Italy, even the Low projection actually predicts a reversal of a decades-long fertility decline.

To get some perspective, suppose that you're trying to predict CD sales in the developed world in 2050. You could assume that sales per person will continue at their current rate. That's like the Constant-Fertility Variant. You could assume that sales per person will asymptote to a level somewhat above the current rate. That's like the Medium Fertility Variant. You could assume that sales per person will asymptote to a level far above the current rate. That's like the High Fertility Variant. And you could assume that sales per person will asymptote to a level somewhat below the current rate - even though large segments of the market are already well below that rate. That's like the Low Fertility Variant.

Now notice: The most obvious projection - one in which the decline in CD purchases continues - is conspicuously absent! And at least for CDs, isn't that the most reasonable projection? CD sales per person will fall to near-zero, and the world stock of CDs will slowly do the same.

Are people like CDs? At least for the near-term, there's every reason to think so. If fertility has been declining for decades, it's strange to assume that fertility coincidentally bottomed out yesterday. So Constant-Fertility Projections are actually probably on the high side.

In the longer-run, though, evolution will almost surely save us. If the average woman has one child, population size shrinks by 50% per generation. But if 10% of women have three kids, and if family size (like virtually every trait) is partly heritable, the proportion of the population that wants 3 kids will exceed 50% in a few generations. In a century or two the desolate villages of Italy will be reclaimed by the descendants of those of us who think that life is a chain letter worth continuing.


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COMMENTS (9 to date)
ErikR writes:
In the longer-run, though, evolution will almost surely save us.

In the long run, evolution will not be a problem as we will control our DNA and/or digital copies...

Bob Knaus writes:

My family is Old German Baptist Brethren, one of the "plain sects" most broadly characterized by the Amish. I'm the only one of my immediate family who has not followed the old path.

According to Wikipedia, there are about 200,000 Amish in the US. The have an average family size of 6.8 children. This is a product of their culture, which seems to be strongly passed on to succeeding generations.

Assume that each family loses one or two children to "the world" as mine did. That still leaves 5 children per family.

In a little more than a century, assuming other sectors of US society remain at replacement level, that puts the Amish at 1/3 of the US population.

What's all this high-tech future everyone is talking about???

Stewart writes:

Williams makes the argument that people are substituting from low quality children to high quality children. Specifically parents are investing more of their resources into fewer children. This is a wealth effect, but as our wealth continues to grow, high quality children are still a normal good and one might suspect that birth rates would increase.

John Thacker writes:

The assumptions can work in cases where, e.g., the birth rate is declining because of a trend towards having children later. If everyone still intends on having 2.1 children, but there's an ongoing transition towards having children at 30 or 35 instead of 20 or 25, then it will show up in the data as a fertility decline temporarily, and will "snap back" at some point as the adjustment finishes. There are other hypotheses that could lead to that result, too; the article notes that the countries with the lowest fertility are ones where unmarried and childless women have gained a lot of freedom, but married women with children are still heavily expected to engage in traditional roles. (This describes Italy as much as Japan and South Korea.) The same sort of argument applies-- once those countries finish adjusting to modernity, things will change and people will have their ideal number of children. Built into all these arguments is that there is an ideal number of children in an equilibrium that all (modern) countries are moving towards.

Of course, it's far from clear that that's what's going on, and you're right that it seems like the projections are randomly picking a few scenarios out of a hat.

j writes:

evolution will almost surely save us

Us...?

Arnold may be saved. His descendants will be davening with mine.

But you? Who knows...

Kurbla writes:

Go, Amish go! They are (perhaps) the evidence that greed and consumerism can be avioided without state violence.

Bob Knaus, could you answer some private questions? How do you explain that so many people grown in Amish families succeed to abandon consumer goods other people find so attractive? Religion or something else? Because, religion itself is generally not enough, other Christians usually find the way to be both greedy and religious!

What kind of relation do you have with your family now? Do they make strong pressure on you or you can maintain warm relations in despite of the world thing? Could you describe growing in the Amish family as "brainwashing" or adults influence children mostly by their own example - and children follow. Or something in between. Your brothers and sisters - did they stay because of family, secular or religious values?

I hope you'll not see such question as unpleasant or insult - if you do, please, just ignore them - it was not intentional, it is intercultural difference. Good luck to you. :)

Bob Knaus writes:

Kurbla - I don't mind answering questions!

You are correct that religion alone is not enough to ensure rejection of consumerism. It requires a strong cultural matrix. When I was a teenager, a woman came to my family's produce stand and told my uncle she had to interview someone from a different culture for a class, could she interview him? My uncle was offended. "We are a religion, not a culture! Go interview a Miccosukee if you want someone from a different culture!" He was wrong, of course, my family has a culture every bit as different from the mainstream as their religion.

In my opinion, some behaviors of plain people substitute for consumerism. In my family's church, men are allowed to have one of three haircuts, broadfall rather than zipper pants, and no collars on their coats. Cars are OK, but not two-tone paint on them. And those are the written rules... there is a whole complicated layer of unwritten ones underneath them. True, they do not care what the latest clothing fashions are, but they have plenty of other things to soak up bandwidth when it comes to personal attire and behavior.

My family relationships are reasonably good. Unlike some of the Amish, they do not practice "shunnning." Parents do not use brainwashing techniques on their children, at least not in the modern sense. It is a strongly patriarchal society. It takes a big effort, or a big accident, to get out. My sisters have remained because nothing happened to push them away. I am one of the minority who left due to my own intellect and force of character. The more common exit route is to fall in with bad company during the late teens.

Dan Weber writes:
In a little more than a century, assuming other sectors of US society remain at replacement level, that puts the Amish at 1/3 of the US population.
They'll have to deal with the descendants of the super-families of the rest of the US. The Duggars have 17 kids.

I'm not sure why people are like CD's, though. CD's will definitely be replaced by something better in the next forty years. People might want fewer children, but I don't see how this necessarily leads to people wanting no children.

Kurbla writes:

Thanks, Bob! Beware of sharktopuses!

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