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Russia, with the largest territory in the world,

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has roughly the same total population as Nigeria,

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a country one-sixteenth its size.

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But this similarity won't last long.

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One of the populations is rapidly growing,

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while the other is slowly declining.

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What can this tell us about the two countries?

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Population statistics are some of the most important data

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social scientists and policy experts have to work with.

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But understanding a country's situation

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and making accurate predictions

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requires knowing not just the total size of the population,

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but its internal characteristics, such as age and gender distribution.

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So how can we keep track of all that data

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in a way that makes it easy to comprehend?

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Complex data is more easily interpreted through visualization,

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and one of the ways that demographers represent

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the internal distribution of a population is the population pyramid.

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Here, the data is divided by gender,

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with females on one side and males on the other.

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The population numbers are shown for each five-year age interval,

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starting from 0 to 4 and continuing up to 100 and up.

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These intervals are grouped together into pre-reproductive, 0 to 14,

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reproductive, 15 to 44,

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and post-reproductive years, 45 and up.

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Such a population pyramid can be a powerful predictor

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of future population trends.

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For example, Rwanda's population pyramid shows it to be a fast-growing country,

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With most of the population being in the youngest age groups

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at the bottom of the pyramid,

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the number will grow rapidly in the coming years.

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As today's children reach their reproductive years

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and have children of their own,

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the total population is almost certain to double

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within the next few decades.

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For our second example, let's look at Canada,

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where most of the population is clustered around the middle of the graph.

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Because there are less people in the pre-reproductive age groups

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than there are in the reproductive ones,

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the population will grow more slowly

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as the number of people reaching their reproductive years decreases.

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Finally, let's look at Japan.

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Because the majority of its population is in its post-reproductive years,

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and the number of people is smaller at each younger interval,

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this means that at current rates of reproduction,

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the population will begin to decline

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as fewer and fewer people reach reproductive age.

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Comparing these three population pyramids side by side

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shows us three different stages in the demographic transition

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as a country moves from a pre-industrial society

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to one with an industrial or post-industrial economy.

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Countries that have only recently begun the process of industrialization

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typically see an increase in life expectancy

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and a fall in child mortality rates

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as a result of improvements in medicine, sanitation, and food supply,

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while birth rates remain constant, leading to a population boom.

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Developing countries that are farther along in the industrialization process

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begin to see a fall in birth rates

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due to factors such as increased education and opportunities for women

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outside of child-rearing and a move from rural to urban living

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that makes having large families less economically advantageous.

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Finally, countries in advanced stages of industrialization

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reach a point where both birth and death rates are low,

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and the population remains stable or even begins to decline.

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Now let's take a look at the projected population pyramids

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for the same three countries in 2050.

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What do these tell us about the expected changes in each country's population?

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And what kinds of factors can alter the shape of these future pyramids?

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A population pyramid can be useful not only as a predictor of a country's future,

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but as a record of its past.

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Russia's population pyramid still bears the scars of World War II,

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which explains both the fewer numbers of elderly men compared to elderly women

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and the relatively sudden population increase

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as soldiers returned from the war and normal life resumed.

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China's population pyramid reflects the establishment of the one-child policy

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35 years before,

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which prevented a population boom such as that of Rwanda,

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but also led to sex-selective abortions,

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resulting in more male children than female children.

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Finally, the pyramid for the United States shows the baby boom

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that followed World War II.

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As you can see, population pyramids tell us far more about a country

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than just a set of numbers

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by showing both where it's been and where it's headed within a single image.

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And in today's increasingly interconnected world,

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facing issues such as food shortages, ecological threats,

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and economic disparities,

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it is increasingly important for both scientists and policymakers

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to have a rich and complex understanding of populations

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and the factors affecting them.