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Russia, with the largest territory in the world,
has roughly the same total population as Nigeria,
a country 1/16 its size.
But this similarity won't last long.
One of the populations is rapidly growing,
while the other is slowly declining.
What can this tell us about the two countries?
Population statistics are some of the most important data
social scientists and policy experts have to work with.
But understanding a country's situation
and making accurate predictions
requires knowing not just the total size of the population
but its internal characteristics,
such as age and gender distribution.
So, how can we keep track of all that data
in a way that makes it easy to comprehend?
Complex data is more easily interpreted
through visualization,
and one of the ways that demographers represent
the internal distribution of a population
is the population pyramid.
Here, the data is divided by gender
with females on one side and males on the other.
The population numbers are shown
for each five-year age interval,
starting from 0-4
and continuing up to 100 and up.
These intervals are grouped together
into pre-reproductive (0-14),
reproductive (15-44),
and post-reproductive years (45 and up).
Such a population pyramid can be a powerful predictor
of future population trends.
For example,
Rwanda's population pyramid shows it to be a fast-growing country,
with most of the population
being in the youngest age groups at the bottom of the pyramid.
The number will grow rapidly in the coming years.
As today's children reach their reproductive years
and have children of their own,
the total population is almost certain to double
within the next few decades.
For our second example,
let's look at Canada,
where most of the population is clustered
around the middle of the graph.
Because there are less people
in the pre-reproductive age groups
than there are in the reproductive ones,
the population will grow more slowly,
as the number of people reaching their reproductive years decreases.
Finally, let's look at Japan.
Because the majority of its population
is in its post-reproductive years
and the number of people is smaller
at each younger interval,
this means that at current rates of reproduction
the population will begin to decline
as fewer and fewer people reach reproductive age.
Comparing these three population pyramids
side by side
shows us three different stages
in a demographic transition,
as a country moves from a pre-industrial society
to one with an industrial
or post-industrial economy.
Countries that have only recently begun
the process of industrialization
typically see an increase in life expectancy
and a fall in child mortality rates
as a result of improvements
in medicine, sanitation, and food supply.
While birth rates remain constant,
leading to a population boom.
Developing countries that are farther along
in the industrialization process
begin to see a fall in birth rates,
due to factors such as
increased education and opportunities for women outside of child-rearing
and a move from rural to urban living
that makes having large families
less economically advantageous.
Finally, countries in advanced stages of industrialization
reach a point
where both birth and death rates are low,
and the population remains stable
or even begins to decline.
Now, let's take a look at the projected population pyramids
for the same three countries in 2050.
What do these tell us
about the expected changes
in each country's population,
and what kinds of factors
can alter the shape of these future pyramids?
A population pyramid can be useful
not only as a predictor of a country's future
but as a record of its past.
Russia's population pyramid
still bears the scars of World War II,
which explains both the fewer numbers of elderly men
compared to elderly women
and the relatively sudden population increase
as soldiers returned from the war
and normal life resumed.
China's population pyramid
reflects the establishment of the one child policy
35 years before,
which prevented a population boom
such as that of Rwanda
but also led to sex-selective abortions,
resulting in more male children than female children.
Finally, the pyramid for the United States
shows the baby boom that followed World War II.
As you can see,
population pyramids tell us far more
about a country
than just a set of numbers,
by showing both where it's been
and where it's headed
within a single image.
And in today's increasingly interconnected world,
facing issues such as food shortages,
ecological threats, and economic disparities,
it is increasingly important
for both scientists and policy makers
to have a rich and complex understanding
of populations and the factors affecting them.
コツ：単語をクリックしてすぐ意味を調べられます！

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【TED-Ed】Population pyramids: Powerful predictors of the future - Kim Preshoff

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Precious Annie Liao 2014 年 5 月 12 日 に公開

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