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.