If basic income only for adults did drive people not to have kids (where you don't get as much welfare as a child costs now, so it is still a net negative today, yet parents have kids anyway) then you would have to then conflate with cultrual and societal rejection of those who don't procreate.

Even if you got into a situation where you had a sub 2 natural birthrate and were concerned about your long term population sustainability, you are looking post 2050 at the earliest. Why not just use genetic engineering to create human templates and just grow generations in labs, a la Brave New World? Why not raise them in some collectively researched and agreed upon environment to try to grow them into creatively thinking intellectual socially capable human beings? Do you really need to organize your social reproductive structure around random pair bonding that gets funky and creates amalgams of the parents, who said parents then raise, with 13 years of state funded daycare in a factory environment?

It is worth noting, though, that African birth rates are still high for a competitive advantage reason - without state elderly welfare programs, you heavily depend on having children that reach adulthood with enough productivity to care for you when you can no longer work fields. It is really modern where some countries don't have children as their retirement account.

It is also worth noting that female empowerment and education more directly correlate with lower birthrates than any amount of fiscal policy. US birthrates haven't changed much between 80s monthly check per baby welfare and modern unemployment programs, but European birthrates tanked when women were getting state guaranteed higher education.

And, as far as I understand, birth rates of the "people who you want to have kids" - people with education living near a large city - have already plummeted. Which is compensated by people who don't quite fit the society but instead reproduce to get something from the society.

Why will there be a cultural and societal rejection? A lot of people don't have babies already (or defer it until it's too late) and it's okay for everyone. Anyway, when you've got a nuclear family or a single person, it's not easy to reject or influence them - they'll give you a finger and start a new WoW gaming session.

And when you've got a society where childless family is the norm, it won't be easy to reverse until it's too late.

If you're ready to breeding humans in labs like cattle, why would you object to wiping out human populations we don't like? Surely, humans have rights, but only the wild ones. Humans grown in labs are property. Random pair bonding is what gives you a society. Anything else gives you androids that usually malfunct.

And the only reason we're talking about BI is because we think that people have rights and we have to cope with those.

What we're not in Africa is a fact. Female empowerment and education is also a fact. Let's think what are our next steps? Hint: it doesn't involve trying to make a few steps back.

And even if you didn't have a cultural reason, incentivizing reproduction is extremely short sighted in an overpopulated world. Uplift Africa rather than reproduce here. The transition to a sustainable population is another topic, but it is an inveitable problem that must be solved, and we are no where near capable of having 8 billion US (or even the less egregous European nations) level of consumption and pollution without running out of resources, livable space, and breathable air well below that threshold.

Do those parents and grandparents really support their grandchildren or they only want them happen and then do nothing about it?

Anyway, children living near a large city are more capable than ever of giving them the finger, and that's what they going to do. See in the concurrent thread, "If they decide to have children" - meaning your default is to not have children.

World may be overpopulated, but is North America overpopulated? I guess it actually isn't. Most of places outside South Asia aren't overpopulated. Having some population growth and economy growth and no fears that social security would implode is better than having decline and being in panic like Japan does.

There's an abstract sense of overpopulated (sometimes described as turn all of texas into Manhattan and abandon the rest of the planet and texas would still only be half full)

And then there's carrying capacity sense of overpopulated. Both food, and west of the Mississippi, water, is actually the limiting reagent (Los Vegas will be simply out of water in a generation or so...). There's already a cultural impedance bump when westerners talk to easterners WRT water "waste".

Mexico is tremendously overpopulated from a carrying capacity standpoint. Canada is in pretty good shape. USA in between.