Ismist

Tribalism is core to our social nature, because the first thing we want to know about anyone else is whether they are friend or foe, a threat or harmless. This judgment happens all the time, instinctively.

The complexity of such identification is almost fractally infinite. If we know someone it is easy (if we know them well enough to predict what they will do.) Otherwise it is a set of snap decisions based on a series of subconscious signals _ height, age, sex, appearance, situation and on and on. 

A racist element attaches to things that cannot be changed, such as skin color, shape of eyes or nose. A sexist element to male or female. An ageist response to child or ancient elder. And so on. Things a person cannot easily disguise. 

Then we move to cultural markers. What are they wearing, what tattoos do they have, how do they speak. All of those can be more or less easily obscured, so we tend to be suspicious.  And we worry about anyone conspicuously trying to not be part of our accepted tribal norms.

Back to Socratic logic, I guess. I am human. All humans are tribal. I am tribal. And, beyond that, what tribe are you? 

As a civilized person, I can (and should) try to expand my inner definition of my tribe. But the ancient instincts remain.