I think that we should be able to test this in a simulation.

I think the crux of the idea is that if a group of people stop fighting for a bit amongst themselves and instead wipe out another group, then the other group will no longer reproduce.

Any mechanism to allows this to happen more effectively will reproduce itself. Tribal thinking is supposedly one such mechanism. So genes/culture that will promote tribalism are selected for to some extent.

Human history seems to be the natural history of this.