I can easily relate to solidarity in the face of physical attack. I'm not half as sure about ideological solidarity. Maybe it needs to be fleshed out as to what the benefits and the disadvantages are. It's weird how it's a bit like a new civil war in America these days.

Where is the research?

Interesting, you may find this a good read too.
[pnas.org]

The only thing that the two do not address, which I think comes into the picture, though I have nothing more than hunch, is the effects that weapon technology has on aggression. Making inter group aggression both more dangerous to the aggressor and more likely to result in the deaths of near relatives with common genes.

We are indeed tribal animals. But not all human groups practice coalitionary proactive aggression. Whether a group or tribe adpots such behavior depends entirely on the experiences and dominant norms, mores, and values of the group. Groups with excessively exclusionary values adopt such behavior both to prove and protect their falsely adopted "special" status.

If we are "domesticated", then my definition of that term needs modification.

Surely you mean us apes, all of us, not just Homo sapien.

It's really simple- murder everyone who doesn't think or act like you and you have more resources for those more like you. Then you have to be nice to everyone who is left, unless they deviate from the social norm. Then you need to get rid of them.

Among all the studies...I wonder! No society has all mentally healthy people, even if the most are non-violent! It seems the tipping point is only a short stretch away!

I am not sure but it seems to me that at least in most major Western cultures only the remnants of tribal culture remain, the positions of power and authority are now business related, and for the most part these relationships are outside the family considered as a tribe. Position and to an extent personal success in such cultures is not guaranteed.

There are other cultures which are more strictly tribal, where social positions are given in birth, where a lowly individual may have very little chance of moving beyond the position of their birth. Life is easier on tribal members in the sense that they don't have struggle to find their way in life, certain possibilities have been predetermined for them from birth

"Me against my brother. My brother and I against my cousin. My brother and my cousin and I against the world." Said to be an old Arab aphorism. It would seem that "tribal" depends upon the scope.

Would you explain "coalitionary proactive aggression"?