So if the ancients did want to avoid having kids…maybe they couldn’t avoid having them altogether, but they could have leaned very strongly towards having fewer and later kids, and we’d see historical evidence of that preference.

There is some evidence for such a preference in some ancient cultures during some times and places (these were rare for obvious reasons), e.g., Italians during the late Roman Empire.

Conservatives need to realize that the cultural ground has mysteriously shifted under their feet and they’re up against a huge, evolutionary novel change in social attitudes and that a handful of measly tax breaks is a ridiculously underpowered tool to prevent it. Liberals need to realize that something really powerful and strange is going on and that in the long run, there's no reason to believe that these forces are necessarily friendly to the liberal – or the human – project.

This reminds me of Calhoun's mouse universes.