Ingroup/outgroup dynamics are actually very well studied area of sociology/evolutionary psychology. It's extremely fascinating.

https://en.wikipedia.org/wiki/Ingroups_and_outgroups

There's this thing called independent thinking. Not everything must come in pre-digested chunks of peer-reviewed journal meta-studies...

(That said, there's tons of research in group dynamics, including the ideas the parent mentioned).

The Robber's Cave Experiment for one https://www.lesswrong.com/posts/MBpj3QKfPg9xKNeXW/the-robber...