The Fill Up

Why Will the Population Stop Increasing?

The chart shows the ages of the 7 billion people alive in 2015: 2 billion were aged 0 to 15, 2 billion aged 15 to 30, and then there were 1 billion each in the 30 to 45, 45 to 60, and 60 to 75 age groups.













In 2030, there will be 2 billion new 0- to 15-year-olds. Everyone else will have grown older. The 0- to 15-year-olds of today will have become 15- to 30-year-olds. The 15- to 30-year-olds of today will have become 2 billion 30- to 45-year-olds. There are only 1 billion 30to 45-year-olds today. So, without any increase in the number of children being born, and without people living for longer, there will be 1 billion more adults.

The 1 billion new adults come not from new children, but from children and young adults who have already been born.

For three generations, this pattern will repeat itself. In 2045, the 2 billion 30- to 45-year-olds will become 45- to 60-year-olds and we will have another 1 billion adults. In 2060, the 2 billion 45- to 60-year-olds will become 60- to 75-year-olds and we will have another 1 billion adults. But look what happens next. From 2060, each generation of 2 billion people will be replaced by another generation of 2 billion people. The fast growth stops.

The large increase in population is going to happen not because there are more children. And not, in the main, because old folks are living longer. In fact the UN experts do predict that by 2100, world life expectancy will have increased by roughly 11 years, adding 1 billion old people to the total and taking it to around 11 billion. The large increase in population will happen mainly because the children who already exist today are going to grow up and “fill up” the diagram with 3 billion more adults. This “fill-up effect” takes three generations, and then it is done.

(This explanation is a brutal simplification. Many die before they reach 75, and many parents have their children after they reach 30. But even including these facts makes no difference to the big picture.)

This phenomenon is usually called the demographic momentum. We prefer to call it the fill up and we usually explain it in animations or with our own hands. 

Download the video above here.






A Powerpoint slideshow where you can see the change year by year. Download the slides here.


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Date Posted: 2018-03-11