The population pyramid's shape effectively communicates significant information about a population's age-sex composition. A broad base with sharply tapering sides (a true pyramid shape) indicates high fertility and high mortality rates in younger age groups. A flat or slightly rising bottom line shows that birthrates are higher than death rates, which is typical of older populations. A peak at one end or both ends indicates that mortality rates are high for children under five years old or for elderly people.

A population pyramid can be used to estimate **the average age** of a group. If we know the proportion of young adults, middle-aged people, and seniors then we can calculate what average age they are likely to be. For example, if a population pyramid looks like this:

Young adults make up 35% of the population. Middle-aged people 45%, and seniors 20%. The average age of **this group** is 36.0. If it was known that their ages were distributed as follows:

Young adults: 30% aged 15-24, 70% aged 35-44

Middle-aged people: 50% aged 25-34, 50% aged 45-54

Seniors: 10% aged 65-74, 90% aged 75+

- How do you describe the shape of a population pyramid?
- What shape of pyramid best reflects the structure of a rapidly growing country?
- What does the shape of the US population pyramid tell you about the country’s population structure?
- What does the rectangle shape represent in a population pyramid?
- How does the shape of the age pyramid reflect the growth status of the population?

The form of the population pyramid also reflects population fertility and death. A large base with sharply tapering sides (a real pyramid form) indicates high fertility and high death rates in younger age groups. A wide base with gently sloping sides (a nearly inverted pyramid form) shows low fertility and mortality throughout the whole population.

A country like China, which has one of **the most rapidly growing populations** in the world, would therefore have **a rapidly increasing number** of people at the top and bottom of the pyramid and a gradually decreasing number in the middle. The shape of **the Chinese population pyramid** has recently started to tilt away from its peak, indicating that birth and death rates are becoming more equal. But it will take several decades before this happens completely.

In conclusion, a country like China which has a largely agrarian economy and where most people live in rural areas will have a pyramid-shaped population profile.

A population pyramid depicts **the age and gender makeup** of a population visually. As we discussed in **our post** "What is a Population Pyramid?" the more rectangular the graph is structured, the slower the population grows; the more a graph resembles a pyramid, the quicker the population grows.

The US population pyramid has been changing over time due to changes in birth and death rates. If we look at the pyramid from 1970 to 2008, we can see that it has become less rectangular and more pyramidal. This means that birth rates are higher than death rates, which will lead to future growth. However, during this same period, immigration has increased the size of the base of the pyramid, so it has not changed the overall shape of the pyramid.

Because immigration has increased the size of the base of the pyramid, it has not changed **the overall shape** of the pyramid. The more a graph resembles a pyramid, the quicker the population grows.

In addition, women live longer today than they did in 1970. This leads to fewer children being born, since parents want to ensure their families do not die out. Immigration may play a role here as well, since fewer births mean **less competition** for jobs.

These factors all work together to create a growing population base. Since mortality rates are still high, however, the actual rate of growth is lower than it would be if mortality were lower.

A population that is growing at a steady rate will tend to be more circular than rectilinear.

In a circle, the area increases as the square of the radius, whereas in a rectangle, the area increases as the length times the width. Thus, a population that is increasing in size will tend to be more rectangular than circular.

This is why populations tend to become more and more skewed toward **older people** as time goes on. There are simply more old people than young people. This is not always the case, but it tends to happen as things get more balanced between the ages of 0 and 19 then 20 or 29 or 39 or 49 or 59. The only real way to stop this from happening is through death or migration, etc.

So, skew toward **older people** means that there are more very old people than young people. And since people with names like "Alexander" and "Emily" do not usually live past 100 (unless they're famous), we can assume that most of the people in **these populations** are actually going to be over 99 years old.

The form of pyramids indicates the population's growth rate. The population with **a high proportion** of youthful members increases swiftly, whereas the population with a high proportion of post-reproductive members tends to fall. This sort of age pyramid indicates that the population is increasing. Conversely, a pyramid shaped like a point or a linear line suggests that the population is stable or declining.

Around the world, most populations are aging and becoming less fertile. As a result, their age-pyramid shapes are changing. In some countries, such as Japan, Germany, and Sweden, the proportion of young people in the population is high, while in other countries, such as Russia and Brazil, it is not.

In general, countries with a large number of children have their populations growing rapidly, while countries where most people are already past **childbearing age** have their populations changing more slowly. However, there are exceptions to this rule. For example, India has **a relatively small population** compared with many other countries but has a significantly younger age structure due to its long life expectancy. Similarly, China had a low birth rate but an elevated death rate during its rapid economic development, which resulted in a decrease of its overall age structure over time.

Age structures of populations can change for **many reasons**.