# The Population Pyramid

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• 0:01 Population Pyramid
• 1:41 Evolving Pyramid
• 3:13 Pyramid Shapes
• 4:28 Constrictive & Baby Boomers
• 5:34 Lesson Summary

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Lesson Transcript
Instructor: Devin Kowalczyk

Devin has taught psychology and has a master's degree in clinical forensic psychology. He is working on his PhD.

In this lesson, we look at what a population pyramid is, how to read it, some interesting features about them, and what are some prototypical examples of them.

## Population Pyramid

Numbers are really good for describing things in a very accurate way. If I say I have three apples and two oranges, you know exactly what I am talking about without needing to reference charts or tables. But, there are some limitations to using numbers. Sometimes, looking at relationships between different things can be difficult, especially if you're dealing with large numbers or a large number of numbers. Think about populations and how large those numbers are: in the millions usually, maybe even the billions! If we then started splitting up the population and trying to look at smaller groups, like 20-year-olds and 30-year-olds, it would get very confusing when we deal with these numbers.

What we need is visual representations of the populations. I think that would do quite nicely. Oh wait! I have the perfect solution! A population pyramid is a visual representation of any population split up by age groups. This can be about a city, a country, or even the world. The pyramid consists of gender and age categories that usually go by decade, so 0 to 10, 11 to 20, 21 to 30, and so on. With each new generation, the old generation gets bumped up. And for the very sad reason that not everyone lives to be very old, each time a generation moves up the pyramid it gets a little smaller. In the rest of this lesson, we will look at what causes changes to the population pyramid as well as some of the information it can depict.

## Evolving Pyramid

The population pyramid represents people by age and gender. Each new bottom layer represents fertility, which, in terms of a population pyramid, is the number of babies born per decade. Let's use a visual description of this. Let's say generation 1 is made up of 100 babies: 50 males and 50 females. We already know it is an unrealistic example because in real life more females are born than males, but I digress. There are 100 babies, 50 males, 50 females. They grow up, and because life is cruel there are about five of each gender that die. This is reflected in the mortality rate, which is the number of deaths per so many people.

Now we have generation 2, and let's say there was a bit of a bump, now we have 150 babies, 75 male and 75 female. Generation 1, sitting on top of generation 2, is now 45 males and 45 females. Again, fickle fate was cruel and takes 10 from each generation. When generation 3 comes around, let's say there are only 80 children born, 40 of each gender. This is the general structure and shape of the pyramid.

We could even change it and say that instead of measuring a country we are measuring the world. Or we could have two pyramids next to each other and compare what one country's pyramid looks like to another.

## Pyramid Shapes

When you have a visual representation of something, people start looking at it and seeing shapes in them. Three prototypical shapes have been identified by researchers who use population pyramids.

Expansive pyramids have a high rate of childbirth but a rapid depletion. This is found in many developing countries that have a high birthrate, but hard and short lives. What this looks like is a pyramid that is very wide at the bottom and quickly begins to narrow, sort of like an upside down spinning top.

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