> why must we pick ethnicity as the dividing feature?

Human behavior is largely heritable. Ethnic groups represent groupings of heredity.

(Look: I know the first sentence is one of those things you just can't say. Nobody wants to admit to believing it. But it's true. Instead of raking me over the coals for saying something that is true and unpleasant, how about we find a way of grappling in a fair, humane way with reality?)