It's contraceptives. It started dropping as soon as they became widely available, starting in the West, and never stopped. (The norms around family rearing certainly changed as well, in response)

People really like sex. Kids get a more mixed reception.

[–] borari@lemmy.dbzer0.com 3 points 1 day ago (1 children)

Contraceptives and easy access to them are definitely a big part of it. The largest factor in birth rates is access to education for women. Higher levels of schooling lead to delayed marriage, later childbearing, and fewer total children.

https://pmc.ncbi.nlm.nih.gov/articles/PMC3449224/

https://wol.iza.org/articles/female-education-and-its-impact-on-fertility/long

[–] CanadaPlus@lemmy.sdf.org 2 points 22 hours ago* (last edited 22 hours ago)

Yep, and that's probably through better access to and knowledge of contraception and other kinds of family planning. In really patriarchal societies men don't have to worry as much about the difficulty of raising a high number children. Or giving birth to them.

You can find anecdotes supporting that causation, although there's no hard data that I'm aware of.