How does group diversity lead to better decisions?

... [W]hite jurors were actually responsible for a large proportion of the group differences, as they behaved differently in a racially mixed jury than in one all-white. White jurors in diverse groups mentioned more facts, made fewer factual errors, corrected more mistakes and raised the possibility of racism more often than did white jurors in homogeneous groups. Even before the deliberations began, white participants who expected to deliberate with black jurors privately espoused less harsh views of the (black) defendant than did white participants who expected to deliberate in an all-white group. Both the anticipation and the experience of serving on a diverse jury seemed to sharpen the white jurors' sensitivity not just to race but to accuracy and due process.

One explanation for this pattern is that white jurors felt more motivation to avoid prejudice in the presence of black jurors. The motivation to avoid prejudice could lead not only to more careful consideration of racism itself, but also to more systematic and thorough information processing of all relevant facts about the defendant.

From this article in Scientific American's Mind Matters feature by Rebecca Saxe who describes a fascinating piece of research by Samuel Sommers of Tufts.

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On a related note, both Henry Farrell (at Crooked Timber) and Cosma Shalizi have reviewed Scott Page's The Difference: How the Power of Diversity Creates Better Groups, Firms, Schools and Societies.