Japanese culture is quite stifling, I mean that's what's with all the tentacles and men being crushed by giant shoes and whatever. But I wonder if it's actually more closely related to the social disconnect involved in high density living.
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Japan's fertility rate is one of the lowest in the world. They have 1.3 births per woman, that's way below replacement rate (2, one for the man and one for the woman).
Think about how many old people they have in homes versus young people working to pay for them. Ageing populations are a huge economic problem, we have them in most Western nations as well. And it shows no sign of turning around, this problem will continue to get worse.