Population growth slows dramatically because women who start their families later almost always have fewer children. At the same time, the generation time (the interval between birth and reproduction) increases.
We can illustrate this using a simple online tool:
Let’s imagine two Earths, each of which have 1 billion people. On Unstable Earth, girls begin their families at age 16 and have 6 children each. After one century, how many people would that Earth have?
About 73 billion — an incredible increase.
But on Stable Earth, girls begin their families later, at age 23, and have just 3 children each. After one century, how many people would it have?
Half a billion. About 150 times less.
We can make this ultra-simple example more realistic by including the fact that, in smaller families, a larger fraction of the children will survive — as they’ll be better-off financially, with better food and health care.
If we assume child survival increases from 70 percent in large families to 90 percent in smaller families, then Unstable Earth will have 8.6 billion people after a century, whereas Stable Earth will have one-third of a billion people — still about 25 times less. Amazing.
The point is to demonstrate the incredibly powerful effect of allowing girls and young women to get an education. It’s a complete world-changer. Doing so slams the brakes on runaway population growth. And it works fast — we start seeing the benefits almost immediately.