I don't think anyone assumes that the cost of raising children is zero... quite the opposite. I don't think its key, either. Can say about Japan specifically,

I don't think anyone assumes that the cost of raising children is zero... quite the opposite. I don't think its key, either. Can say about Japan specifically, but in the West I think it's down to people wanting to have a life, things, holidays instead of having children. It is a choice that is about the whole commitment and sacrifice of having children no matter what.