It may be, though, that far from being a contradiction, these effects are two sides of the same coin. A new study published in the Personality and Social Psychology Bulletin argues that we see improvements in institutional performance through the addition "Socially Distinct Newcomers" in tandem with increased discomfort by the "old-timers" at their presence. From the abstract:
The impact of diversity on group functioning is multifaceted. Exploring the impact of having a newcomer join a group, the authors conducted a 2 (social similarity of newcomer to oldtimers; in-group or out-group) x 3 (opinion agreement: newcomer has no opinion ally, one opinion ally, or two opinion allies) interacting group experiment with four-person groups. Groups with out-group newcomers (i.e., diverse groups) reported less confidence in their performance and perceived their interactions as less effective, yet they performed better than groups with in-group newcomers (i.e., homogeneous groups). Moreover, performance gains were not due to newcomers bringing new ideas to the group discussion. Instead, the results demonstrate that the mere presence of socially distinct newcomers and the social concerns their presence stimulates among oldtimers motivates behavior that can convert affective pains into cognitive gains.
This is definitely an interesting study -- the finding that the increased performance gains were not due to the unique ideas of the out-group. That's in itself counter-intuitive and deserving of further study. But the overall observation of the paper -- that the discomfort in-groups feel about diversity (that Putnam observes) actually translates into concrete performance gains (Sommers) has many important implications for policy-making in the context of meritocratic structures (i.e., ones where "performance" matters).
Katherine W. Phillips, Katie A. Liljenquist, Margaret A. Neale, "Is the Pain Worth the Gain? The Advantages and Liabilities of Agreeing With Socially Distinct Newcomers," Personality and Social Psychology Bulletin, Vol. 35, No. 3, 336-350 (2009) (via)