Tribalism is hardly a shocking revelation. However, tribalism made systemic in the Legal System is a bit further away from saying Boston Bruin fans are dumb.
Racism is about willful ignorance. You have to work at being a racist.
If it is about ignorance, then it doesn't have to be willful. Even if there were no differences between the races except looks, nothing else biological, not even any cultural differences, then the evidence seems to suggest that racism would still be a natural psychological outcome. The difficulty is exacerbated by the objective systemic cultural differences (differences in styles of speaking, music, politics, religion, income, educational achievement, crime and so on). It really does take a lot of work to correct the natural human inclinations. Anti-racism is among the strongest social forces in the white world for that reason, I believe. Any perceived racist is a commonly excessive target of social correction or silencing.
What you say about tribalism is probably true, but you have left out one layer of the learning process.
Babies include as "us" people, and types of people, who they see their parents and trusted adults, tribe members if you will, acting comfortable around.
They don't have an inherent distrust of anyone, and get over their distrust of bearded men as soon as Mummy assures them that it's OK. But they quickly assign people to the "out" group if they see those people, or groups of people creating tension in their "in" group.
Babies will react with fear and long term aversion to those they see their parents in conflict with and pick up on very subtle behavioural cues. In the photo above, that child will need to have very few extra contacts with black people before they conclude, without ever consciously thinking about it, that "there is nothing openly wrong with black people, they seem friendly enough, but Mum isn't comfortable around them and gets tense when I play with them. There must be a problem." That kid looks about 8 months old.
The fear of travelling salesman is fairly well established and that doesn't have to arise from overt violence, just Mother answering the door apprehensive of potential for harm.
Two other examples of this are that parents who are afraid of dogs or water, and who are aware of their phobia and work hard to hide it and to avoid passing it on, are still very likely to raise children who are nervous around the particular aversive stimulus. Kids react to the truth of bodily tension in preference to the contradictory verbal assurance that everything is all right. "If everything is all right, how come you are all tense and worried and trying to cover it up?" they reason unconsciously.
Hence, "some of my best friends are ..." racists, if they are uncomfortable in mixed race situations, will still pass on the tension and the out group assignment.
This is the origin of the saying "Took it in with his mother's milk." It is all the stuff you learned from your family without ever knowing you were learning it, or they that they were teaching.
Here's an article from Time magazine addressing this issue:
You always suspected babies were no good, didn’t you? They’re loud, narcissistic, spoiled, volatile and not exactly possessed of good table manners. Now it turns out that they’re racists too.
The latest evidence for that decidedly unlovely trait comes from research out of the University of Washington that actually sought to explore one of babies’ more admirable characteristics: their basic sense of fairness. In the study, 15-month-old toddlers watched an experimenter with a collection of four small toys share them either evenly or unevenly with two other adult volunteers. When allowed to choose which experimenters the babies wanted to play with later, 70% of them preferred the ones who had divided the toys evenly.
Nice, but there was an exception: when the two adults who were receiving the evenly or unevenly divided toys were of different races and the race of the one who got more toys matched the babies’ own, the 70% preference for the fair distributor dropped and the share of babies wanting to play with the unfair one rose. The implication: unfairness is bad, unless someone from your clan is getting the extra goodies.
http://time.com/67092/baby-racists-survival-strategy/
From what I've read, we are born racist, or at least xenophobic. Babies show more thrust towards people that look and sound like their parents.
See above. At 15 months kids have had plenty of time to define their "tribe" according to their family's definition. I would like to see an extension of that research to investigate the reactions of kids that the researchers knew to have come from genuinely colour blind backgrounds.