From a recent Hacker News comment by me that was both popular and controversial, in response to the declining fertility levels, declining interest in family, and projected downturn in Asia -
I have a theory that I haven't found expounded before. It came from a combination of travel through 60+ countries, living and working and interacting with local people on a pretty intimate level sometimes, and study of lots of history.
It's going to be controversial and maybe even shocking, so brace yourself for a moment before reacting please.
I think peaceful societies self-destruct.
With a few notable exceptions that require a geography suitable to isolationism, long term peace has historically been achieved through your country or one of your ally's having military supremacy over the rest of your neighbors.
Obviously, diplomacy can keep the peace for long periods of time, even human lifetimes, but eventually incidents happen when there's a hothead in one government, and then that's when the military supremacy determines whether you get attacked or not.
Anyways, I've found the more a country renounces war and gets further away from it, the more birth rates go down. You get an explosion of commerce and art for ~30 to ~70 years, and then the society self-destructs.
No longer forced to confront mortality and with no externally unifying cause, people start living for luxury, pleasure, and consumption. They stop having children. Birth rates fall off.
Eventually, this destroys a country's economy, the military supremacy fades, and one of their neighbors comes in and cleans house, and the cycle begins anew.
This has happened many times through history. It's happening in Japan right now. If I became an advisor to anyone in the Japanese government, I'd advocate two things as chief priorities - (1) exceedingly good relations with China, and (2) re-militarize.
Then join the next war they can on America's or China's side. Combined with some standard messages of nationalism/strength/growth/unity, birth rates would almost certainly increase.