Diversity was a killer

The more diverse a unit's enlistees were, the higher the casualty rate, the higher the desertion rate, the more likely the unit was to fail in battle.

That's the quick and dirty summary by a journalist encountering Heroes and Cowards: The Social Face of War, a new book that applies social science methodology to 41,000 Union ACW records.
"Union soldiers, whether in prison camps or in the field, were the most loyal to men who looked like themselves — of the same ethnicity and occupation, from the same state or hometown, or of the same age or related by blood," said Dora Costa, a UCLA economics professor and co-author of the book Heroes and Cowards: The Social Face of War.
I am reading this volume and will report back soon. Meanwhile, the publisher's pitch says:
When are people willing to sacrifice for the common good? What are the benefits of friendship? How do communities deal with betrayal? And what are the costs and benefits of being in a diverse community? Using the life histories of more than forty thousand Civil War soldiers, Dora Costa and Matthew Kahn answer these questions and uncover the vivid stories, social influences, and crucial networks that influenced soldiers' lives both during and after the war.
You can read a sample chapter at the link.