I suspect close family ties and living with parents was the default throughout human prehistory. Our Hunter-gatherer ancestors were probably not leaving their tribe behind and moving away. It's only since the industrial revolution that people have been leaving their birth family behind en mass.

I think in any low trust environment where social mobility outside the group is poor, sticking together based on blood lines probably was the default for sure.

However I think there's plenty of evidence that migration and intermingling between tribes occurred frequently. If only as social practices to prevent inbreeding. Probably a bunch of bride kidnapping sadly but also young males leaving to seek opportunity isn't an exclusively modern phenomenon.

> also young males leaving to seek opportunity isn't an exclusively modern phenomenon.

True but as far as we can tell it was usually (if not entirely exclusively) done through the same social networks that existed locally [unless you were leaving to murder/rape/rob people]. Unless they really, really had to you only moved to another city/location because you had a cousin, uncle etc. or someone else there you had some ties with. Outside of organizations like the church or the army (and even then) a complete outsider was at an extreme disadvantage (relative to today).

From what I can tell (though this varied by culture!) free first born males stayed with the parents. Females were often traded to other tribes (both as war spoils and more peaceful ways). Free second and later sons often discovered the family land couldn't support them and their older brothers families and left looking for anyplace to live (often resulting in war which in turned eased population pressure, though sometimes a city job existed though they were worse than farming until the industrial revolution). Slaves of course had no control of where the children went. The sexism above was real, though how is manifested varied from culture to culture with some worse than others (sometimes it was the oldest female who stayed home).

Genetic diversity requires someone leave their family and join a different one. How that happened varied but nearly every culture recognized siblings having children together resulted in deformed kids and thus developed a culture to prevent that. Every culture includes other animals.

>Our Hunter-gatherer ancestors were probably not leaving their tribe behind and moving away.

It is like literally what they did - more than ever after and before.

As far as I understand the recent decline in the numbers is that people stopped lying about which church they don't go to. Because church-goer numbers are stable.

What happened during the last century is ... complicated. For one thing religion was never really that organized in the US. (Maybe except in Utah. But that's also relatively new.)

I think simply WWII, and the post-WWII economic boom (plus the GI bill), plus then the heating up Cold War slowly but surely transformed society. For the new generations the various Christian belief systems offered by churches were simply not a real option.

People got an appetite for different answers whether be that science or pseudoscience based. Cults and other ideology-based groups filled some of the vacuum. (And of course the counterculture eventually and then after Vietnam and the race riots came the backlash. The Southern Strategy, which platformed evangelicals, but as a political group, not as organized religion. Basically emptying out the spiritual part, etc. And of course it still works.)

It’s a trend that’s a few centuries old at least. The French and Indian War was partly caused by Americans who didn’t have enough land to farm to support a family moving west, and the British trying to stop that was one cause of the Revolution, and then the whole of 19th century American history is a combination of farmers moving west because the east was full and immigration from Europe largely driven by the farmland of Europe being full. Before that almost everywhere was stuck in a Malthusian equilibrium and if you couldn’t support a family in your home, moving away would just make it harder.