"Human beings are by nature tribalistic." I'd love too see the empirical evidence for this.
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
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.