This common explanation never made sense. Lower income is correlated with higher fertility within the US, within every country, across the world, and (seemingly) throughout history.

Most of history is subsistence farming economies. More people means more labor. It’s a pretty good ROI. And of course, lack of contraception. Plus with kids dying you need several to reliably end up with a few when you’re old.

In an urban economy kids are largely dead weight and expensive for two decades. The middle class dual income family isn’t typically looking for 3+ kids anymore.

The fertility rate today in the United States is inversely correlated with income. The effects are not minor, either. As far as I'm aware, there's not much subsistence farming going on in the United States.

They are inversely correlated, yes, but the absolute levels of fertility American poor vs subsistence farming poor are much lower.

It’s possible for the poorest people to have more kids than anyone while economic pressures also lower birth rates across the board.

Kids require more investment today in the first world. 400 years ago you just kind of taught your child your craft, your religion, and your culture and that was their education. Extremely few people went to any sort of school. Raising a child was different. Now there's a level of sensitivity and attention that's necessary to ensure emotional and developmental stability that's much greater than in the past. Kids must be educated through to at least college, sometimes even beyond. There's a globalized economy which children will be competing in, and everything that gives them an advantage makes a difference