I'm a bit reluctant to write this, but it made me worried, so here we go :
I had a chat with a taxi driver in Stockholm the other day, and we talked about the situation in Somalia and that he planned to move back there as it's getting better there.
He had 8 kids, but wanted another wife and 10 more kids when he became my age - 40.
I mentioned that I have been married before so I have 3 kids with 2 mothers and was quite happy with that number...
He actually laughed and looked at me and said - "man, you Swedish guys will disappear."
I don't know how representative this was but looking at some societies in Sweden, I'm guessing it's not unique. It would be nice with some research.
Anyway.I'm hoping that the Somalian women will get some say in the matter, because if they don't, looking at the demography and the socio-economic mechanisms, both Sweden and eastern Africa might end up in a spot of bother.
The horror, if you would adapt to this hellish circumstances, by speeding up the cycle, creating "specialists" for each cycle stage and survived through the hard times by reducing all that makes up human society. No arts, no compassion, no creation, no school, all of these are calories wasted, instead go full zombie and walk the earth till the cycle of strife ends.
PS: Before somebody yells racism. This behavior could be seen everywhere during world war 2. So one could claim that white racists are actually the pro-zombie equivalent found elsewhere.
This will no longer be the case, so there would be no rush for either gender to have kids. You can focus on your career for the first hundred years, and then have kids. At worst, there would be a minor uptick in population while people get accustomed to the new reality.
One child per couple means that the total population caps out at double the current population (a population of 32 would have 16 children, who'd have 8, who'd have 4, who'd 2, then 1 person who's SOL - 32+16+8+4+2+1=63). A single order of magnitude is an acceptable loss, and everyone gets a child of their own.
Note: Not as in "literally force people to have only one child", just make it frowned upon like smoking generally is - most people won't care enough to defy it, and in the long term it should be enough. Although IMO families like the Duggars[1] shouldn't be provided immortality unless they start using birth control.
Accidents and suicide could be handled too. Former by outlawing machines of murder (cars) and spare or artificial organs, latter by improving our understanding of neuroscience and psychology.
This already happens. The more affluent a country becomes, with its better healthcare, better education, and greater career opportunity, the lower its birthrate. The big crisis in 50 years will be population decline.