It's pretty clear that the major driver is education and freedom of career choice for women. The UN has lots of data supporting that. Industrialization and changing attitudes toward children have also played a role.

My paternal grandfather was a farmer. He had 20 children who survived to adulthood. By two wives, the second having been the nursemaid. As I understand it, all of the children worked. Basically as soon as they could walk. Girls focused on childcare and housework, and boys on farm work. If they'd been living in a city, the boys would have had factory jobs.

So anyway, that's how you raise many children. That or be wealthy, of course. But current expectations, social norms, and laws make that impossible. Except perhaps for farm families, where substantial child labor remains legal.

Education comes with higher life standards. You want your kids to have their own rooms instead of whole family living in single-room shack, education for them etc.

"Freedom of career" for women is more like freedom to have to work. It's hard to raise a family on a single salary. Previously, a woman would work at home and husband would work outside. Now both of them have to work outside of the house to pay for appliances that do stuff back at home.

IMO one of the top issues is housing cost. Increasing specialisation, both parents having to work and urbanisation means people are crammed into more and more condensed cities. Space comes at expense. Children need space. Cheaper space out of the city means more transport costs and/or less job opportunities and likely lower wage. All in all, children is damn expensive.

Yes, it's true that (at least in the US) greater career choice for women has come with declining real wages. So that now, both partners need to work. Some argue that effectively doubling the workforce has played a major role. Or that people just expect to have more stuff. But the power of unions to maintain livable wages has eroded over the same time frame. And maybe it's all connected.

What's the resource constraint then if it's not food, energy, or shelter?

People want to buy more stuff? Hardly a requirement of years gone by.

People want more free time? Doubtful, otherwise people would work less and buy less stuff - which we don't see. People keep getting more productive, but they just funnel that into a more expensive lifestyle rather than cutting back on work.

I don't think the reasons are simple (or even the same between people) but I don't think it's dominated by resource constraints.

I know software engineers with no children and school teachers with four. Very different lifestyles, but all middle class. And I don't think resource constraints factored heavily into their decision making.

The resource constraints now are time and location. It's affordable to buy houses in a lot of places, just not so many that have access to good jobs and schools. So people prioritize the latter and sacrifice their time by commuting for work.

Since both parents now work, often with lengthy commutes, there isn't a whole lot of person-hours left to spend with the children. That means people have fewer of them.

When the kids get older and it's time to send them to university, well, that's something which has gone backwards in affordability. The cost of a university degree has shot through the roof. How could anyone afford to send 3 kids to university these days, let alone 8?

I think both parents (or especially the woman) working is probably the big part of it. But I don't think many people factor the cost of university degrees into their decision (whether they should is another matter)

> I think both parents (or especially the woman)

Why do you say especially the woman? Not trying to pick a fight, I'm genuinely curious.

Because traditionally, staying at home taking care of kids has been usually done by women.

Never mind traditionally, this is still true, even in the most egalitarian countries (Scandinavian countries). Women have to choose between family and career, men don't really have to do that. The biggest driver behind the gender pay gap is the difference between women with children and women without. Somehow we haven't solved the issue that it's a cost borne primarily by women.

I think the current answer is quality - people want the best for their kids including a future or they won't have them otherwise. They may feel that their lifestyle consume too much time, that they can't afford what they need, or just that they and their family situation would make poor parent(s) for whatever reason. And this is peripheral to adoption vs reproduction.

Kids have turned from a resource like they were to farm families to an expense and an increasing one at that - and I don't mean in a "kids these days" sort of way - the world changed and not people. Which leaves children to be had by those who want to have them for the sake of having them - some sort of enjoyment or sense of obligation.