I'm Jonathan Tomkin, University of Illinois.
So far this week, we’ve talked about how you can use the current snapshot of demography
to estimate what the current, the future population will be.
We saw how young ages in Yemen predicted high fertility rates and high population growth
and old ages in places like Greece and other places in Europe
indicate that population growth will be low or maybe even negative.
We can actually get more information than just that if we look at the population in the form of a so-called population pyramid
which shows the age groups, um, for different slices of the population group.
Population pyramids are histograms, and they show the distribution of ages in a population
usually by country, and they are called pyramids because they tend have, or at least historically have, a pyramid-type shape. They’re wider at the base and they steadily get thinner and thinner as you get to the top.
So let’s just look at this example from the S-Sudan.
We can see the x-axis here is the number of people in millions in each 5-year cohort
and then the y-axis is the different 5-year cohorts.
So we have people aged 0-4 , 5-9, 10-14 and so on
and as you get to older and older cohorts there are less and less people
and on the face of it this should make perfect sense, because as people die in each age group they get removed, and so they don’t progress to the next age group.
So if you die when you’re, when you’re 42, you will show up in the 40-44 age group, but you will never be seen in any of the higher ones.
So on the face of it, pyramid is the perfect shape; there’s no other possible shape. You can’t have an additional birth; someone’s not born in age 45.
This is what’s known as a youthful age pyramid, and this is what most countries, or most regions around the world, would have looked like for most of human history.
Because of the demographic transition, age pyramids look less like triangles today in many countries and more like blocks, and we’ll look at that in some detail.
Let's start with an example.
This is the population pyramid of Japan in 1920.
in this case, the space for each single year, so they’re much thinner, and so there’s a little bit more variation.
We do expect some noise; we don’t expect a perfect pyramid in any population
because some groups might go through an unexpected death due to, for example, disease or a conflict
and sometimes there might be a mini baby boom, so there’ll be a few more people born in a particular year than they years on either side
but nevertheless the general trend is clear.
Now this is a youthful population group, just as we saw for Sudan: lots of young people and progressively less and less old people as we go up, um, the age pyramid.
Now, over the 20th century, average life spans in Japan soared
so we’re going to see, that big i-impact from that demographic transition from high death rates to low death rates.
Indeed, over the century the Japanese average length of, uh, life doubled, so this has a big impact on the size of this population.
By 1940 the shape is still youthful.
So remember the demographic transition takes some time to work, so even though death rates are decreased - there are more older people - people are still having large families, and so the fertility rate is still rather high.
And here we can see an example of what can happen when there is an outside force to a population pyramid.
there was of course the World War II was a very big impact on Japan, and so it, there were some people who were, uh, being killed in action, and of course there are people who are overseas, and so a lot of the males are actually missing from this sample of what the population in Japan at the time was like
and you can also see that there’s an impact, that there is a sort of a drop in the number of births around this period as well, as the, young, marriageable-age men are away.
Family life was disrupted, and that has an impact on the population pyramid
and this happened all around the world in this period, of course, and we’ll, and you probably know this from your own country.
By 1960, though, the war had been over for 15 years, and so notice this post-war baby boom.
There’s this giant increase in the number of people born, about the time, um, 10 to 15 years ago, when the, the war had finished an-and servicemen returned home and family life resumed.
Again, this is something that will be very familiar to lots of people, uh, if you’re from, example, United States or Europe or Australia.
They all experienced the exact same baby boom, and so you see this big increase, and so now we no longer have a balanced pyramid.
Because of that baby boom, there are more people who are in their early teens than there are in their newborn areas of 0-5 or 5-10.
So what we also see is that the number of people in their 20s is about the same as the number of people who are in those earlier age groups.
This is the indication that Japan is progressing through that demographic transition.
Fertility rates are declining, and as a consequence there are just as many people in the 0-10 age group as there is in the 20-30 age group
the pyramid is becoming square.
By 1980 this is a little bit clearer. We still see those booms and busts in the baby boom.
The baby boom is still very clear, and we can also see the impact of the lack of births during World War II
but there’s a generally a flat structure, uh, in this case from people aged 50 down, so we have this much more flatter structure.
This is very different from that early pyramid that we saw.
This is the same structure of we see in a lot of other countries such as the United States
and not only do we have the, the original baby boom, then we have a little boomlet, and here it’s for people who are almost 10
and that’s what we call Generation X in the United States, and so this happened in lots of other places around the world as well
So the, the pyramid structure is not perfect in describing these features.
Cultural impacts, conflicts, all of these things can change the structure
but the fact that we have this flat growth between 50 and 0
there’s the same number of people aged 0-5 as there is age 45-50 - suggests that there’s been a profound change in the way this population reproduces itself.
At this point, Japan is deeply into the demographic transition, and so this leads to a further prediction.
By 2010, which is the most recent, um, piece of data I’m going to show you, the birth rate is now the lowest in more than 80 years.
If you look at this graph here, you can see that there must have been more people born every year in the last 80 years than there is today
because there are more people alive in each of those age groups.
So even though some people die before they reach the age of 80, or 70, or 60, or 50, or even 1, there were more people alive in those age groups than at 0.
So the population has experienced a profound shift. Japan is now well into that final stage of the demographic transition
and that most families are much smaller. In fact, in Japan, the average family size is well below the replacement rate
and so we can see here that this is reflected in the structure.
We have moved from a pyramid, to a more flat structure, to what you might call a cup-shaped structure where we have a wedge
where there is more people in the older age groups than the younger age groups.
This sort of structure is not sustainable. As the older age groups die off
that wedge shape will become thinner and thinner, and a sustainable structure looks more like a square
so there is a more stable age between the young and the old.
This indicates that Japan would expect to see a large decrease in population over the next few decades as those older age groups and the people in their 40s, 50s, 60s and 70s, progress and then die, they’re not going to be replaced by the same number of people.
So we can use this pattern, then, to predict the future population growth of a country.
We can see this pyramid shape; this is the youthful shape where we have a growing, um, population.
And then in the 40s and 60s we saw a more transitional shape as it was moving from this youthful pyramid to a more mature, block-type shape.
And it was stable in the 1980s; it had this more mature age shape.
And you might even call this, the shape we see today, this wedge-shape or cup-shape is sort of super-mature.
It’s gone past that point of sustainability where the population’s actually slowly decreasing.
And I’d make the point that this is not like a J-shaped population growth curve collapse
because we’re not seeing people die out because of some disaster.
We’re merely seeing a, a slow slide down to a new lower population point
and it’s actually not clear what that will be for Japan
because as we can see here, the fertility rates has been decreasing for a number of years
and of course if you have a fertility rate which is below the replacement rate, below 2 per personper woman rather, then we don’t expect to see
the population to, uh, stay stable; it should continue to decrease.
I also mentioned in the earlier, uh, discussion, that the people of Yemen were much younger than the people of Greece, and so we could use that to predict the population future.
In this case we can see there are far more people in modern Japan
who are in their 40s, 50s and 60s than in their 20s and 30s.
People who are older don’t have kids, and so this would indicate, again, that we have these, uh, few people in that reproducing age between 15 and 45
and so again we can predict that Japan will face, uh, a greying future with more old people and less young people.
Here’s an animation of that, and you can see, you can follow population growth through time.
Strictly speaking, these population pyramids are influenced by things such as immigration and emigration
so if people leave or enter the country they can change. So it’s not strictly related to birth and death rates
but for the world as a whole we’re not experiencing any, uh, net migration from other planets
so we can use this model to describe global trends.
Produced by OCE-ATLAS Digital Media at the University of Illinois Urbana-Champaign
