OK, I'll bite. How do you "know" this?
just been my observation in life. of course it is a generalization, and there are exceptions.
i guess it dawned on me as a teen. my parents were looking to buy a house. most of the neighborhoods were upper middle class...where the 'successful' live. the real estate agent went out of her way...many times to emphasize (always in a whisper) about the lack of blacks in the neighborhoods. obviously, that is a big selling point.
so we can laugh at the racist peckerwoods in mississippi...but unlike those upper income liberals, the guy in mississippi isnt likely to yank his kids out of a school if too many blacks attend. he isnt likely to bitch about his real estate value going down the drain if a black family moves in. and he probably wont call you a nasty name if you disagree with him politically.