Multi - culturalism

Continuing with my theme borrowed from John Kenneth Press's blog, here is a quote about the effects of multi-culturalism.
Putnam's recent study on diversity involved nearly 30,000 people in 41 communities. He found that the more diverse neighborhoods are the less social capital they create. People in diverse communities volunteer less, give to charity less, vote less and work on less community projects. The simple fact is that people do not trust people they share little in common with.

Now, this research seems to fly in the face of conventional wisdom (as JKGalbraith would phrase it). We all accept that multi-culturalism adds diversity and flavor to a community, doesn't it? So a more diverse community (diverse culturally) results in a more fragmented and insular community rather than a stronger community. Interesting proposition isn't it?