Well, these are ultimately economic arguments.
In my opinion, the root cause of birth rate decline is the change in social norms. I don't want to echo right-wing punditry by calling it a "breakdown" of traditional social values, but it is certainly a change. Specifically, the formation of family units, i.e. marriage. Here is a that's helpful.
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In my opinion, and I want to underline that this is my opinion, not backed by more than a cursory interest in the subject, is that the decline in birth rates is caused by increasingly higher burden of society upon young people. To be a "good" husband, you have to be educated, make a decent salary, be of sufficiently "good character", and a host of other requirements.
All this, while the pre-requisites to accomplish all of those things are rising higher and higher. Now, you have to finish a 4 year college, possibly finish even more education if you want to "guarantee" that high salary through specializations like an M.D, J.D., Ph.D... To say nothing of "life experience" which is hard to accomplish when you are buried neck-deep in studies.
And for the women? Well, it's possibly even worse. Women are expected to choose "good" partners which necessitates being picky. Moreover, women are expected to be attractive, educated, and capable of being to contribute both professionally and domestically through being able to cook, host, and raise children. While educational and professional requirements are most likely lesser than those demanded of potential husbands, the extra requirements outside of those are possibly even greater! And they have to learn and accomplish all of those things while still being relatively young. Unlike men, women have an "expiration date", and I'm sorry if it offends someone, but that's the biological and social reality.
Now sure, plenty of women do get all of those things done, but I hardly fault women who want to live a little and either do not get married or simply do not accomplish everything they "need to"
And that's in United States, I don't know how much worse or better it is in China, but considering the hyper-competitive economic environment there, I can't imagine that the social demands are any lower.
Agreed on those points, but I would argue that the costs of earning degrees that can access good jobs and having a great educated background all add to the costs of raising a child since having and raising a successful child requires two people to be in a relationship to begin with. The good character part is more or less a personal thing. Such burdens can be alleciated via active government programs that educate and help the populace while suppressing the nasty materalism that is an unfortunate side effect of a powerful economy. The Chinese are currently working on that and the labor of such programs will take a while to bear fruit.All of this puts off children for years. And very little of it has to do with the direct costs of raising children, in my opinion of course.
I mentioned that one should include tax raises for those who don't have kids by a certain age. I haven't seen a single country do that since they most likely fear the backlash from their populace (particularly European ones). Simultaneously, generous pro-natalism policies must be offered in order to alleviate the financial pressure.I don't think you will be able to force people to have kids just off economic incentives. If European countries are having the same issue despite generous pro-natalism policies and a generally high standard of living for the majority of the population, pulling the same levers in China will have the same limited effect.