>>2183087Poor societal decisions about what to prioritize (work >>> everything else) during the 80s and they never reversed course until it was too late.
Their population pyramid is inverted now and their working adults will be too busy propping up their giant retiree population to make their own babies. The real pain hits in 15-20 years when their Gen X retires.
The US and Europe are in similar situations with TFR but they're making up for it with immigration. Japan can either try to tough it out for 40 years until their retirees die and try to rebalance their demographics or open their country up.
