Most people have heard of China’s infamous one-child policy. it’s exactly what it sounds like: each married couple is only allowed to have one child.

Now the policy has ended. Actually, it ended on Jan. 1st, less than a week ago.

A lot of people don’t know the story behind the concept, and why it was initially implemented. Here are some numbers that made the one-child policy look like a sensible idea at the time:

China’s total population:

1960: 667.1M

1970: 818.3M

China’s population grew 151.2M in 10 years, or at the rate of 15.12M per year. The government was worried that the population would continue growing exponentially at the same rate, with the country eventually becoming unsustainable.

Fertility rate:

1960: 5.76 births/1 woman

1970: 5.47 births/1 woman

The fertility rate stayed stable (and strong) throughout the 1960s.

Crude birth rate:

1960: 20.9

1970: 33.4

This metric shows the “number of of live births occurring during the year, per 1,000 population estimated at midyear.” The number hit a high in 1963 with 43.4, no doubt sending the Chinese government into a full-fledged panic.

With the above stats as historical context, it’s a bit easier to see why the Chinese government implemented the One-Child Policy, and kept it for the 35 years they did.