In the past children improved well-being of the average family, because more labour for their fields was almost always good, and children started working when being 5-6 years old. After industrialization children become detrimental, because they can't really help the family, while their upbringing and education became much more relatively expensive.

modern women have relative financial, social, and medical independence compared to what they had in the past and quite reasonably don't want to go through pregnancy and raising children as early in their lives or as often as women in the past did

GDR had higher birthrates than FRG, and socialist countries in general had significantly higher birth rates than the same countries after 1991.

At this stage in development, it's less about income level than it is the enfolding of the entire domestic sphere into the commodity system and the dissolution of social systems beyond the nuclear family. Children in advanced capitalist countries are an enormous financial burden because every single part of their rearing has been capitalized and commodified. The reproductive sphere has been almost eliminated, and the declining rate of profit means the costs of all that capitalized reproduction are climbing far higher than wages. It's a loss of female agency of a different kind than women in underdeveloped countries have. Instead of being compelled by patriarchy to have children, they are compelled by neoliberalism not to.

In a preindustrial society, your children are your social safety net. Having a bunch of kids insures that there is someone who can be with you as you age. Also, some of those children are likely to die, so if you want to be taken care of, better have an extra child or three. And since you are likely farming the land anyway, it's fine to have another pair of hands to do that around, means more work gets done and so more food and actually more wealth as well, once that kid can help out. And if you are not farming, congrats, you are one of the few people in preindustrial society that are rich and you don't have to worry at all about feeding and housing your children.

In a modern society, there is supposedly social safety nets that make having many children less necessary as a survival tool. Now these are being eroded and living conditions get worse, but people aren't simply going to forget they are supposed to be there. It is known how rich you have to be now to afford children, because every child is a real burden on your wealth, because the kid will not be able to do any useful work and will need costly help in the form of education to be ready to even leave the house in 20+ years. And it is bad parenting to not provide for your child. Most people can do the math if they can afford that cost on a napkin and for many, the answer is no. And yea, of course you could just not provide your child with what it needs, but that's not exactly a recipe for getting the child to help you out later.

So no, getting even poorer is not going to raise the birth rate. Maybe in the long term if capitalism collapses and gives way to feudalism 2, electric boogaloo, where a few trillionaires own everything and everyone else doesn't kill them and instead works for them for some weird reason.

Yeah that's the debate that always comes up. I haven't really heard a great answer yet.

On an individual level, people make decisions shaped by cost, hope for the future, social support, all that stuff. On a population level, the poorest countries have the most kids per capita; as far as I'm aware it's a strong correlation. If there are any non-fascist demographics nerds I'd like to hear some expert opinions on this contradiction.

But personally my opinion is that we should do the things that make people's lives better and give them more options, regardless of its impact on birth rate. The population level, and it's moderate increase or decrease, is only a problem because we live in a system that can't do any planning and is terrible at allocating resources. A hypothetical communist world system could work with basically any realistic population level. In the current system, the population shrinking (or growing, or changing demographics) at any realistic rate is not even a top-5 problem so why worry about it? We have war and genocide and fascism and climate change and tons of other urgent problems, I don't see why anyone would care so much about demographics unless they're huge racists or a certain sub-type of neoliberal nerd that thinks this will threaten markets over the long term (and somehow doesn't see any larger financial risks in the next several decades).