We have the most variety, I would think that would lend itself to more segregation. You are going to live with your family who immigrated over most likely, or with or near a friend from the same country/same race. As more and more come over the more segregated it gets because it's just nature to live with people that "look" like you or "are" like you. It's just easier to communicate, know customs, etc.
I think the less differences there are the easier it is to mix.