In my lifetime, three astonishing events have happened - ones that I never would have predicted as a know-it-all teen: 1. The fall of the Soviet Union and communism. 2. The end of public smoking in the U.S. 3. The sharp decline in the world’s birthrate.

The last is the most important, of course, and perhaps the most surprising of all. The New York Times Magazine has a long, interesting piece examining its possible causes.

It comes to this conclusion: Women who are comfortable being able to work and make money are more likely to have children. This means, it argues, that to keep a level population, a developed economy should either have lots of subsidized day care and forced gender equality (the Scandinavian model) or have an extremely open economy where women can enter and leave at will (the American model). In a nutshell, societies have to be either generous or flexible.

Places that are neither (e.g., southern and eastern Europe, Japan, South Korea and Singapore) have seen fertility rates collapse within a single generation, falling far below replacement level.

The situation is not entirely economic, however. Social factors like immigration, religious mores and tradition have an enormous effect, and the argument doesn’t really apply to developing countries. Still, the article makes a compelling case.