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No, it’s 100% economics. Why do you think that having “careers, lives and travel” (as if having a family is not having a life?) is more appealing to modern first worlders? Because it doesn’t impact their finances severely. Having more children in impoverished countries is a financial gain because children are free labor and lottery tickets to get the entire family out of poverty. In wealthy countries, children are only a financial loss.
Is this like when they made the kilogram some function of the speed of light instead of the weight of a metal ball in a French museum?