Expansive access to contraceptives, later marriages, fewer forced marriages, education and professional opportunities for women, formal social safety nets for seniors, plunging childhood mortality. All leads to a rational choice for high investment, high intensity parenting of fewer kids as opposed to low-investment production of lots of kids. Holds true across cultures.
Whats interesting is that things like parental leave and subsidies for kids probably further incentivize “invest more in fewer kids” as opposed to “have more kids.”
There should be no beating about the bush: this is going to cause some short and medium term pain even if over the long term it will create more stability and be more sustainable.
More interesting than declining birth-rates is to track what this model of parenting (high-investment, fewer kids) does to sex ratios. The picture is not pretty for big swathes of the world where they have adopted this model but retain a bias in favor of men. Fewer young people and more of them males with literally not enough women to pair off with.