Its worth noting that it's actually subjective ethnic diversity that has these effects, rather than actual race. As a really clear example, Kenya and Tanzania both decolonialized with roughly the same mix of tribes in their borders but due to the different paths they've taken post independence most Kenyans think of themselves as primarily of their tribe first and Kenyans second, its the reverse in Tanazia. And Tanzania now has a much higher level of trust than Kenya, just as Putnam's work would suggest.

The second thing worth nothing is that more ethnically diverse societies tend to have faster growing economies than more homogeneous ones, even after taking into account their less efficient governments.