thought I remember learning/reading that urbanization played a big part in bringing down family sizes.
You don’t need as many kids to help out with the farm, and less space. I would think religion would play a role as well. Very few people would consider it a duty to have children for God or anything like that anymore. Although I guess it would be hard to tell how much that played a role in societies past.
We are quickly moving to a generally pagan society which natural follows postmodernity- the idea all beliefs are equally valid leads to forms polytheism. Now since there’s no true metanarrative all we have left is sensation, and thus the intensity over profundity principle. Children get in the way of this since they require nurture and parental responsibility which gets in the way of sensationism. Further historically pagan societies seem to demonstrate a demonic hatred of children which ultimately leads to child sacrifice eg the Canaanites. G K Chesterton makes this observation in the Everlasting Man. For more on the intensity over profundity point see my thread here.