[grzm]

References from the Wikipedia article on tribalism would likely be a good starting point.

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

Group selection would be another area that might provide some more information on related arguments:

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

[didibus]

I don't see any empirical data pointing that Humans are tribal by nature in neither of those. The first link just explains the concept, the second is a highly criticised hypothesis.

The article base all its premise on the axiom that we're tribal by nature, and I think while we might all think that makes sense, I've never seen data to indicate we are innately tribal.