[shaded2]

That definately happens but is an underlying cause. I'm not sure what it is. I've lived in other countries where this natural segregation does not happen.

[semi-extrinsic]

There's a fascinating example in computational studies that partly explains this: if you have people of different groups moving around randomly on a lattice, even a very slight preference (51%/49%) for being next to people in the same group as oneself ends up leading to a self-segregated pattern.

EDIT: see e.g http://ccl.northwestern.edu/netlogo/models/Segregation

[learc83]

I'd be interested in hearing about what countries you're talking about and the percentages of total population that these integrated groups make up.