Is the UN underestimating the global fall in fertility?

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Hello and thanks for downloading the More or Less podcast.

We're the program that looks at the numbers in the news and in life and in graphs from the United Nations.

I'm Tom Coles.

Graphs are a particular challenge in our line of work.

The line goes up a bit.

The line goes down a bit.

It's not exactly radio gold.

But sometimes there's a graph that does something so odd, even radio can do it justice.

The graph we're going to talk about today involves a line that plummets downwards for five years heading towards zero.

And then seemingly miraculously, at exactly the point at which it turns from data that's been collected to a projection of what's going to happen in the future, it stops falling and immediately the trend is flat.

like a roller coaster that speeds downhill before abruptly flattening out.

Which is fun for a fairground ride, but a bit odd for a trend on a graph.

And why does this graph matter?

Well, it's the UN's graph of the projected fertility rate in Argentina.

It's an extreme example of something you see in the UN fertility projections for some other countries too.

And the strangeness of those graphs suggests that maybe something strange is going on in how the UN forecasts what the world's fertility rate will be like in the future.

So, what is going on?

And could the UN's predictions be downplaying a global fertility crisis?

Let's start with some context.

Every two years, the United Nations releases the World Population Prospects.

This document contains projections for the population of just about every country on the planet.

And to do that projection, amongst other things, they use fertility rates.

That is, how many children on average each woman will have in her lifetime.

The trend here is pretty straightforward.

I'm Patrick Jarlon.

I work for the United Nations in the population division.

Since the 1950s, since the United Nations exists, the fertility in the world has declined from several children on average per woman to close to two children.

And as we move forward in the rest of the century, the fertility is expected to get lower.

When the fertility rate drops below two, that means there aren't enough babies to replace the baby's parents.

It takes, well, a lifetime for these trends to come through.

But as a result of the global fertility trends, the UN predicts that the world's population will peak at around 10.3 billion in the mid-2080s and then start to decline.

The concern is that if, for instance, the Argentina graph is anything to go by, the UN might be underestimating the fall in fertility.

Currently, remember, the fertility rate in Argentina is dropping fast, but the UN predicts it's going to suddenly level off.

Why do they do that?

The first thing you need to know is that the UN don't base their projections on a complex analysis of all the factors that might influence a country's birth rate.

Patrick says if you try to predict the future of the world's economy, society and culture, it just multiplies the uncertainty.

And many countries don't collect the kind of data you'd need to do that anyway.

If you think you can predict with more accuracy all of these other indicators and eventually do it for the next 50 years or 100 years, good.

But the approach we are using is basically purely and only based on the fertility itself.

The model is based just on historical trends in fertility.

A story that starts with the so-called demographic transition that Patrick described earlier, with fertility rates dropping as economies grow and the role of women changes.

This happened first in Europe and then countries around the world followed suit.

Basically, if a country's fertility rate is falling, the UN will predict that it'll continue to fall.

Until that is, the fertility rate falls below the replacement level of two children per woman.

A country like Argentina has been experiencing below replacement fertility.

For countries that are already at that advanced stage of their demographic transition, we need to look at the experience of other countries that have already experienced this kind of situation in the past.

The historic fertility data from other countries suggests that once the fertility rate goes down below replacement level, at some point the decline will stop and the fertility rate will start to fluctuate up and down instead.

After all, at some point the decline has to stop.

They cannot keep going down forever when it comes to human population and human society.

All human society have a minimum sense of self-preservation.

There's no case in history where a country ceased to exist because they just stopped having children, so it's highly unlikely the fertility rate is going to go to zero.

As for the precise point at which the decline will stop, Patrick is pretty clear about the weakness of the UN method.

Once the countries reach this ultra-low fertility, basically it's the end of that transition, this fertility transition.

It's basically a kind of terra incognita.

There is not much theoretical evidences

to be able to know with perfect accuracy what to anticipate next.

Either it's the bottom or it's not yet the bottom, but we don't know exactly where the bottom for sure will be.

And in this case, we are using a more naive, neutral model that is called an autoregressive model.

What this simply means is simply that you take the last point and you, by default, assume a static woe.

This autoregressive model is why the Argentina graph looks so odd.

Based on the general experience of other countries, when fertility rates are as low as they are in Argentina, roughly speaking, the model just assumes it's hit bottom and projects a fluctuation around that point.

But that's just the average of the UN's future predictions.

If you look more carefully at the actual graph, it predicts a wide range of possible fluctuations into the future, faint lines branching off in all directions, including cases where the rate continues to fall.

And as strange as the main line on that graph looks, there are countries where this kind of trend has taken place.

You should take a look at Cuba.

You can think of it like a canary in the coal mine in the region.

Cuba has reached this kind of situation basically since the 1980s or something like that.

It has been going up and going down and going up and going down around 1.5.

At the same time, though, there are also countries where, in recent times, fertility has continued to fall below the rate in Argentina or Cuba.

Why aren't those trends part of the UN method?

If you look at a country like Singapore or

in East Asia, Korea, you will see eventually fertility that has declined already below one child on average.

And the question becomes, will every country ever reach this ultra-low?

and for how long will it stay at this ultra-low level?

And the answer is no one has an answer to these kind of questions.

So by default, the current assumptions we are making are somewhat more neutral by assuming basically a stagnation rather than a further decline because there is really no way to anticipate to where can those decline happen

and for how long.

The UN method is pretty blunt and its assumptions are basically a little conservative.

As Patrick says, when the birth rate is low, it predicts stagnation instead of continued decline.

You can see this methodological optimism in their predictions over time.

In recent years, the UN's predictions for the peak world population are lower and lower every time they release them, though not by huge amounts given the timescales involved.

It's probably true that the fertility declines that we have anticipated based on the experience of the past have in some cases been too conservative.

Because the UN predictions are based on historic trends in fertility and not more complicated modelling, they are bound to struggle if we're entering a brand new phase of ultra-low global fertility.

But it's not yet clear that we are doing that.

Not yet clear if Argentina is doing a Cuba or going much lower.

Like their near-neighbour Chile, never mind the world's lowest birth rate in Korea.

Whichever one transpires, the UN's population division will be following along and updating their graphs every two years.

Thanks to Patrick Jalin from the UN.

And that's it for more or less this week.

If you'd like to get in touch, please email moreless at bbc.co.uk.

Until next week, goodbye.

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