China is lucky in that they border a country with the opposite problem [1]:

> One feature of the Russian-Chinese relationship seemed especially telling: Cross-border marriages are overwhelmingly between Chinese men and Russian women. Much of this has to do with demographics—Russia has a surplus of women, while China has too many men.

In addition, the demographics issue might be overstated as many of the 'missing women' started to be 'found' once they turned 18 and needed to officially register with the government [2]:

> The researchers believe local government officials informally worked with farmers and acknowledged that they couldn't fully enforce the one-child policy. Instead they made tacit agreements in allowing families to have extra children in exchange for social stability in their communities. The cadres, or local governments, would then under-report “out of plan” births that ultimately influenced the national population statistics.

> To supplement the qualitative data, the researchers then examined Chinese population data by cohort, and they compared the number of children born in 1990 with the number of 20-year-old Chinese men and women in 2010. In that cohort, they discovered 4 million additional people, and of those there were approximately 1 million more women than men.

> "If we go over a course of 25 years, it's possible there are about 25 million women in the statistics that weren't there at birth," Kennedy said.

1. http://www.slate.com/articles/news_and_politics/dispatches/f...

2. https://news.ku.edu/2016/11/22/study-finds-chinas-missing-gi...

One of biggest issues with the 1-child policy is that it affected the middle class much more severely than the ultra-wealth and the rural countryfolk,

> Min was born in 1986, six years after the one-child policy came into effect. She is the second of five children, a reflection of lax enforcement of family planning in the Hebei farming village where she grew up. (Min's father told me that one family in the village had six children; another man, who had fathered seven children, had been the village chief.)

> Min herself, along with everyone she knows, has two children.

> When history's largest social experiment in state-regulated childbearing comes to an end, it will have been borne disproportionately by the Chinese urban middle-class. The elaborate machinery built to enforce these policies barely touched Min at all. She was ignored by the government, living at the margins--in China, that's often the best place to be.

https://www.theatlantic.com/china/archive/2013/03/why-the-on...

Why are girls more likely to be the hidden children, though? Does cultural preference for a son mean "well, he's the favorite anyway, may as well make it official"?

If the firstborn is a girl, the parents might try to have more kids until they have a boy. And if their family registry already says they have a kid, they might run into some bureaucratic issues that might prevent them from having more kids.

If their firstborn is a boy, they will just register him. And if they choose to have more kids after that, be it a girl or a boy, they are forced to leave them off the family registry. In this case, since they already have a son, they might not be too worried about running into issues.

Imagine what will happen in Northern Europe after the large influx of asian single men who will want to marry in a few years.

China - 114 men for 100 women (wikipedia) Sweden - 123 men for 100 women (BBC) https://www.bbc.com/news/magazine-35444173

(I am not sure how much it proves or disproved theory.)

https://constitutioncenter.org/timeline/html/cw08_12159.html

Women as moral citizens

As a “colonial frontier”, New Zealand had a surplus of men, especially in resource towns. Pragmatically, this placed a premium on women for their part as wives, mothers and moral compasses.

There was a fear of a chaotic frontier full of marauding single men. This colonial context saw conservative men who supported family values supporting suffrage. During the 1880s, depression and its accompanying poverty, sexual licence and drunken disorder further enhanced women’s value as settling maternal figures. Women voters promised a stabilising effect on society.

1.) Culturally, there was a broad understanding that males remain within their birth family lineages, whereas women leave and join their husband's family.

2.) Male children were expected to provide for their parents in old age, whereas (again if I recall correctly) it was customary to offer a dowry when female children were married.

3.) As a result wealth tended to transfer from female-heavy lineages to male-heavy lineages.

4.) When new land was available, or technology or new crops could make old land more productive, this effect was manageable, but increased pressure when population butted up against technical limits of agriculture, so that additional new arable land (wealth) could not be easily made available. Particularly when infrastructure and ecological failures strained or reduced the capacity of productive land. In that more zero-sum environment, male children were correspondingly more valuable, and there were upticks in female infanticide.

There's another dynamic in play with regards to the Chinese bureaucratic governmental system, which only men could take advantage of - in the archtypical system, entry into the bureacratic class was at least nominally meritocratic, and through grueling work and education, one could become eligible to take and pass the examinations to gain the bureaucratic degrees, which conferred massive benefits, which were again expected to accrue to not only the newly-minted official, but their family groups. So there are instances of family lineages and even entire villages pooling their resources behind a favored son, investing massively in classical education in hopes of hitting the lottery that way.

I've often wondered to what extent it explains the situation in Afghanistan. If there is a shortage of women anyway due to poor healthcare and other factors, and some men get to have multiple wives, what do the other men do?