Tribalism explains so much and once one understands it's human context, you see tribalism everywhere and in so many different aspects of life.
You say it's a failing of our mental state, I think that might be a little harsh although tribalism today leads to more negative outcomes than positive. Obviously it had evolutionary advantages which is why I think it's too harsh to label it as mental failing. Fair or not?
Much of the horrors we inflict upon each other are based on this... state of mind we find ourselves in. Failing to overcome it is a serious flaw in our social character. There's not enough effort made to protect people from falling for the "Us VS Them" mentality. In fact, without regard for the human condition, much of our media is fueling the partisanship... the fear... the hatred...ultimately the violence we see today.
I call it a mental health problem, as I find something unique to our present situation in history. More of us recognize that we need to do good for one another, even "others" whom we may not identify with... who may even hold hostilities towards us... but an increasing number of people are falling under the influences of a negative state of mind.
I base my theory on the constant barrage of negativity surrounding "others" in the media. First it was 24/7 cable news... become a junkie and vulnerable folks may think the world is out to get them. Now with social media on your phone it's in your face. We are trapped by a constant stream of negativity when our minds are built to seek comfort. It is in this inversion of our comfort that our brains shut down a constructive meaning and instead take the shortcut to dismissing "others". Dismiss the ideas, the emotions, their appeals. Dismiss their humanity.
I posit that this dehumanizing of other people is increased by the barrage of social media contact giving our brains the signals of a threat. Of stress. We don't put down the phones or turn off the news... instead we turn to fearing the others, to hating what we perceive them to be doing, and that will break us down into acting out on that emotional stress and misguided belief.
I fear we are setting up an increasing number of people to fall victim to a mental state of acting out violence against "others".