In general there's a strong negative correlation between development of a society and birth rate. Also income. It's extremely powerful, and applies literally all over the world. I suspect that in America over the last few decades, development continued. Child-safety laws came along for the ride. [1, 2]
Income is a less compelling argument in America as wages have remained totally constant on an inflation adjusted basis for ages.
Income is a less compelling argument in America as wages have remained totally constant on an inflation adjusted basis for ages.
[1] https://www.researchgate.net/figure/Total-fertility-rate-TFR...
[2] https://www.stlouisfed.org/on-the-economy/2016/december/link...