China's birth rates took a nosedive in the 2015-2016 period. I don't think even China's own experts have figured out what precipated that crash. To add irony to injury, this happened literally right after China reversed its One-Child Policy. Nevertheless, it can be said unequivocally that policy is not the cause of nor the panacea to the population crisis.
Going by opinions expressed by Chinese people on the Internet and on the street, I reckon that the greatest contributors to the birth decline are the following:
1. Prosperity & Personal Development
This would explain why the TFR is so low in urban areas and higher in rural regions. With access to modern technology, entertainment, and a 996 job to boot, modern Chinese citydwellers have little incentive or time to raise kids. This isn't just confined to China; take any of the more developed countries and you'd see that it is becoming increasingly easier to support oneself. People will begin questioning what benefits kids would bring to their lives and if such benefits outweigh the effort, time, and resources the parents would ultimately need to devote. Gone are the days when more children = more helping hands in the fields.
2. Economic & Time Restraints
I've read that rearing children is more expensive in China than just about anywhere else in the world. That, coupled with high youth unemployment and intense work cultures/expectations, would make having children an undesirable prospect for even the most pro-natalist couples. Again, this is an issue exacerbated by urbanization.
3. Historical Expectations
It is difficult to change perceptions of what a "normal nuclear family" looks like when people have been forced to cap their offspring at 1 child for the past several decades. This has become the "default" for Chinese families and it seems that the reversal of the One-Child Policy has yet to extend its cultural influence over Chinese people. You also don't get the peer pressure of having large families from friends and neighbors because they too have been conditioned to look favorably at the single-child family structure. This is one of the few times wherein I directly attribute the recent population decline to the One Child Policy. It's almost like trying to undo generations of propaganda/misinformation. I expect this to die out within the next generations but that change may be too slow for the Chinese planners to accept.
4. Generational Attitudes
This is more of a gobal issue rather than a China-specific one. Younger generations (Z, alpha, and to a certain extent millennials) are more cavalier towards the idea of having children and family. On a global scale, the rate at which young people are having relationships and marriages is plummeting. They no longer hold their parents' or grandparents' emphasis on building a family and having a strong urge for posterity. This is what I suspect to be the reason behind the collapse of marriage rates in China.
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A lot of the aforementioned factors are ultimately ingrained in China's collective mindset and difficult to reverse with policy; only time and eventual retrospection could shift the paradigm. Nevertheless, the Chinese leadership could still tackle the modifiable factors driving the population decline, starting with ameliorating the economic cost of childbearing/childrearing for couples that do want multiple children.