[somenameforme]

The problem with that correlation is that it fails all over the place. In the past, people of high and low income alike were having healthy, large families. And even in the present places like Thailand, with a nominal GDP/capita of $9,300, has a catastrophically low fertility rate, lower than even the US and most of Europe.

IMO there's a really simple explanation for what it's "really" observing - consumerism. Poor individuals don't have enough money to fall into consumerism, the ultra wealthy have so much that there's no carrot to be dangled in front of them that they couldn't grab on a whim. The correlation captures the remaining middle class that has just enough money to always have a carrot just slightly out of reach.

And so that drives different motivations for this group of people. They'd rather chase the carrot, rather than go through the sacrifice involved in raising a family. This not only explains the past, when consumerism was much less of a thing, but also the present when even poor countries can be disproportionately driven by consumerism. This is probably why religion is correlated strongly with fertility. It's simply anti-consumerist by nature, so religious individuals become somewhat less likely to fall into the carrot loop.

[paulryanrogers]

People in the past didn't have birth control yet did have high infant and mother mortality

[somenameforme]

Birth control dates back at least 3800 years, and probably much longer, as that 3800 year old reference comes from documentation accurately referring to various substances with spermicidal characteristics. [1] The same is true of abortion and ways to induce it.

And anybody who's had a child can tell you that getting pregnant is not as easy as you might think. In general women are only fertile for a window of several days per month, which they are capable of also determining due to various physiological changes that happen during that window (and also the fact that the window occurs during the, more or less, exact same time each month following their period). And even if you nail that window, the chances of a successful pregnancy are relatively low - only about 20% per month for young couples, and then rapidly decreasing for women beyond the age of 30.

Notably, in the century prior to the Roman Empire's collapse, fertility rates collapsed for reasons that are still unclear. Anyhow, this is all a very long-winded way of saying that the higher birth rates of the past weren't simply because of unplanned pregnancies. If they wanted to lower their fertility rates, they would have been fully capable of doing so.

[1] - https://en.wikipedia.org/wiki/History_of_birth_control