Population Patterns in Natural vs. Controlled Fertility Environment

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  • 0:01 Fertility
  • 1:00 Natural Fertility
  • 2:05 Controlled Fertility
  • 3:00 Natural vs. Controlled…
  • 5:10 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Different populations have different fertility rates, and one way to examine this is by studying the difference between natural and controlled fertility environments. Explore the differences in these populations in this lesson.


As any farmer could tell you, the fertility of your soil determines the success of your crops. A more fertile field has more crops, a less fertile field has fewer. Some farmers let their crops grow in natural conditions, others alter the fertility of the soil by controlling the environment. And just so we're clear, farming here is a metaphor for human fertility, or the rates of child birth in a population. Got it? Like, if crops are children, then a country with a higher fertility rate has a higher rate of childbirth? Metaphor!

But, you know, I'm not saying that you should try to sell them at a farmer's market. That's kind of where the metaphor breaks down. The point is human fertility is something that can occur naturally, but is something that we can also control, and each of these has dramatic implications for how our populations grow. See? Metaphor!

Natural Fertility

Okay, so the fertility rate of a population is the average number of births per year. When parents start having children, we call the fact of having children their parity. Some women have a specific parity, or number of children they want, and this can influence their overall fertility, or rates of childbirth. So, whether or not parity is important is a major area of study.

When a population does not attempt to control fertility based on parity, they live in an environment of natural fertility. Basically, natural fertility means the only factors preventing fertility are physiological. Many couples abstain from sex after a woman gives birth for various health or personal reasons, women become less able to conceive while breastfeeding, etc. In these scenarios, fertility rates decrease but not because couples are trying to create a specific size of family. Fertility rates are only based on the biological possibility of having children.

Controlled Fertility

Natural fertility is the rate of childbirth in a population that does not put any effort into controlling family size. However, many populations do attempt to control family size by intentionally altering their behaviors. We call this controlled fertility.

In this population, fertility is influenced by parity, by a desire for a certain number of children. Perhaps couples try to have large families, so they track natural cycles to increase the chances of pregnancy. Perhaps they decide they've had enough children, or want their children to be a few years apart, or want to focus on their careers before having children. As a result, they may intentionally abstain from sex, or might rely on artificial birth control to prevent pregnancies. What they are trying to do is to control their rates of fertility.

Natural vs. Controlled Fertility

In general, populations can be categorized as having either natural or controlled fertility. And, no surprise, each type of fertility has some major impacts on the population. Now, let me preface this by saying that this is not a law of nature, just the general trend, but, generally, populations that embrace natural fertility tend to have higher rates of childbirth.

Since there are no direct efforts to control family size, fertility reflects the natural health of the population and their physical ability to reproduce. This also means that there tends to be a diverse range of family sizes in these populations. On the other hand, areas with very controlled fertility tend to have more consistent family sizes, and fertility rates reflect cultural values and goals, not just physical health.

However, there are some other interesting trends, as well. Look at this graph.


On this graph are three populations with natural fertility and three populations with controlled fertility. Can you tell which are which? It's not actually the fertility rates themselves that matter, it's the shape of the curve. See how the bottom three lines are concave, or bowing in?


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