I try to understand it. Is there a biological basis for rejecting people who appear to be different? A sense of security, keeping the 'stranger' out? Or marshaling resources for the group?

We rise above so many other biological impulses, one would think this would be easy.

I experienced this schism (based on ethnicity) in my own family. My mother's parents immigrated from Sicily in 1906. My father's family was WASP--British, German, Scottish, Dutch. When my parents married in 1939 there was strong resistance to my mother because of her Italian and Catholic background. I don't know if that prejudice ever went away.