I recently came upon an interesting paper by D. Benjamin Barros called “Group Size, Heterogeneity, and Prosocial Behavior: Designing Legal Structures to Facilitate Cooperation in a Diverse Society”. He analyzes how group size and group heterogeneity affects the ability of the group to cooperate, self-police or be civicly engaged and suggests how laws and legal structures might encourage this collaboration in the face of greater diversity. Draft of paper available here.
Also, a Human Resources Journal had an interesting article on this diversity research called Diverse and Disengaged? by Scott Flanders (Human Resouce Executive Online, 9/24/07)