I think that’s more of a confounding correlation. America had birth rates above 3 when it landed a man in the moon. And Europe was “highly developed” before birth rates started collapsing in the 1970s. Several highly successful American subgroups, such as Muslims and Mormons, still have birth rates above replacement.
And Asia has its own distinct issue, which is that development coincided with population control evangelism from the west.
As far as I can tell, it's one of the more robust sociological correlations that has been established worldwide. Fertility rates are dropping on every continent, and the drops are strongly correlated with economic development. See links below, they seem pretty damning.
And yes, it begun in the second half of the 20th century, but I don't think that makes it spurious.
I wouldn't be surprised if western cultural influence played a meaningful role, but I can't imagine that explaining the entire phenomenon.
As you might know, Malthus was quite concerned about the issues of overpopulation. His solution was to educate women. This seems to be working surprisingly well !
I seem to remember that he practiced what he preached (not only as the first ever professor of economics, but also as a pastor), and has founded at least one school for women, but annoyingly I can't seem to find the source any more ??
Of note also : by the end of the 19th century the (neo-)Malthusian feminists had already embraced ideas that he disagreed with like contraception and for some even abortion.
You’ve mentioned Mormonism twice now as an example of successful monogamous societies but it should be noted that the growth rate of the church has become negative. I’d be amazed if the LDS church is still around in any meaningful way 30 years from now. It’s hardly meaningful now.
And Asia has its own distinct issue, which is that development coincided with population control evangelism from the west.