Is diversity desirable?

The greatest predictor of desertion in the Union army was socioeconomic and demographic diversity.
That's the subtitle of a recent book review of Heroes and Cowards: the social face of war in the Wall Street Journal. A husband and wife team of economists wrote the book and the review itself makes for compelling reading.
What do the numbers say?

The main finding of "Heroes and Cowards" is that companies composed of volunteers of similar age and occupation who were born in the same areas were the least likely to suffer desertion... Factors like age, marital status, pro-Lincoln support back home and whether the army was on a winning streak also made a measurable difference, but the most important predictor of desertion was socioeconomic and demographic diversity.
A good quote used by the reviewer: Ardant du Picq, a 19th- century French colonel and military theorist. "Four brave men who do not know each other will not dare to attack a lion. Four less brave, but knowing each other well, sure of their reliability and consequently of mutual aid, will attack resolutely."

It should be noted though that most deserters in the Union Army did not do so under fire. Considering battles in the Civil War (and other wars) stimulates conflicting thoughts regarding a coherent understanding of rational behavior. Is it cowardice to resist marching across a mile of open field into the teeth of Union Artillery (see Pickett's charge) while some young officer calls out "steady boys" after a shell takes out a couple of your buddies?

Also mortality in POW camps was a function of social cohesion. The interpretation is that POW's were more likely to render aide and support to those who were like them, people with whom they could identify. A sound interpretation, consistent with research that shows that support for social welfare programs increases with the degree of homogeneity in a society - we tend to identify more easily with those who are like us.

Would those who support a stronger social safety net be willing to trade off for less diversity (see Japan and Germany)?
Is it possible to overcome the negatives associated with superficial diversity, race and ethnic differences? The American experience is cause for optimism and hope.