Darwinian Reactionary has written a three-parter on why diversity destroys social capital. You might want to save this one for the weekend.
It’s dense and has lots of references.
Here’s the conclusion, to spoil you:
In conclusion, since social capital is possible within a culture to the extent that the coordination of stabilizing functions proceed Normally, and cultural diversity is possible in the sense of a diversity of cultures each maintained through the cultural homeostasis that is produced by bonding social capital, they are compatible as long as cultural homeostasis is allowed to persist and not disrupted by too frequent abnormal conditions. The promotion of diversity should not become mere neikophilia—love of breaking the bonds that bring a people together–for it is by these bonds that cultures can exist and persist, and that individuals can enjoy the benefits of cooperation that social capital bestows.
There you go. Make some time and check it out.