Fertility Dynamics

The graph below shows the age distribution for the population of a typical developing country. There are many children and very few elderly people. The height of each bar shows the size of each age group, as a percent of the total population.

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The “slider” at the bottom of the panel is a control that you can adjust by dragging the pointer left or right. Try it! You will see that the age distribution reacts as you move the slider.

If you reduce the fertility to just below 3, then the age distribution will stabilize in a shape that is more characteristic of countries in the developed world.

When China's population reached 950 million, in 1978, its government finally became alarmed. Their population was growing by 13 million each year, and food was becoming scarce. In 1979 the government instituted the “one-child” rule, requiring all urban Han Chinese to limit their children to one per family. To see how such a policy would affect the age distribution, move the slider to the extreme left-hand side of its range. It takes about 100 years for the oscillations to die down, by when it does finally stabilize the result is clear: a distribution in which each generation has half as many children as the previous.

You can create a "baby boom" by increasing the fertility sharply for several years, and then return it to about 2.5. You will see a bulge that ripples outwards towards the higher age levels. There is also an "echo boom", that occurs when the people in the bulge reach their peak child-bearing ages. This replicates the experience of the United States, whose baby boom began in 1946, right after World War II, and lasted until 1964. The dates of the Canadian baby boom were slightly later: 1947 to 1966. The echo boom in both countries was smaller and broader, beginning about 1975 and lasting until 1990. Both have had an enormous impact on North American culture and politics

Of course, fertility is not the only parameter of importance in population dynamics. The age at which women start having children has an effect also, but it is different from the effect of changes in fertility. Several mortality parameters are important as well: infant mortality, and the rate at which adult mortality increases with advancing age. Most important of all, the bar chart shown above does not show whether the growth rate of the population. To move on to the next step in this analysis, click the “Next Model” link below.

Written: 9 Jan 2005
Revised: 13 Feb 2005
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Copyright © 2005 by Loren Cobb