Louis J. Sheehan, Esquire.  In 1861 my ancestor Richard Courtwright heeded Abraham Lincoln's call for troops and joined the 32nd Ohio Volunteer Infantry. He and his comrades slept on frozen ground in winter and marched through man-high horse weeds in summer. The mosquitoes, wrote his company historian, "took sides with the Confederates."

Dora Costa and Matthew Kahn, husband-and-wife economics professors, want to know why men like Richard Courtwright stuck it out while 200,000 others, roughly 10% of the Union army, chose to desert. To answer this question they analyzed 41,000 digitized life histories of Union troops (35,000 white and 6,000 black) collected under the leadership of the economic historian Robert Fogel. Southern armies had their share of deserters, particularly late in the war, but no comparable Confederate records are available.

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. (I checked and, sure enough, most of the volunteers in Richard Courtwright's company were farm boys from the same Ohio county.) 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. Ms. Costa and Mr. Kahn approvingly quote 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."  http://57100louis0j0sheehan0esquire.wordpress.com

"Heroes and Cowards" is interesting to read but hard to label. It is a work of military sociology written with one eye on the debate about the social costs of diversity. Except for a chapter on prison camps, it is only secondarily a book about the Civil War.  http://57100louis0j0sheehan0esquire.wordpress.com  In a way, it is only secondarily about heroism and cowardice. Few of the cases that Ms. Costa and Mr. Kahn discuss involve soldiers who fled their comrades under fire. Desertion wasn't necessarily an act of individual physical cowardice or panic; sometimes it was just a walking away from war and the hardships of army life. Men often deserted in groups, after sober consideration of their circumstances. http://57100louis0j0sheehan0esquire.wordpress.com

A paradox lurks in the authors' findings. Social cohesion was good for morale, and good morale kept men fighting. But soldiering on in this particular war -- fought before aseptic surgery but after the advent of rifled musketry -- could have unusually deadly consequences.

When the 1st Minnesota Volunteers made a headlong attack on the second day of Gettysburg, 215 out of 262 men were killed or wounded. The next day 17 more fell while repulsing Confederate Gen. George Pickett's equally desperate attack on Cemetery Ridge. The war left whole towns reeling. Winchendon, Mass., lost 21% of its men who served. Assuming that loyalty to socially similar comrades was hard-wired -- that soldiers in the Union army manifested the trait because it had been selected for in the distant past -- could it be that such solidarity had become maladaptive in the killing fields of Gettysburg and Cold Harbor? Ms. Costa and Mr. Kahn's statistics suggest that Civil War desertion was a shrewd survival move. Six in 10 deserters made good their escape. Of the 80,000 who did not, only 147 were executed. Many deserters faced ostracism after the war, but that problem lent itself to the solution of a changed address and name.

Northern states did not usually replenish depleted regiments, preferring to raise new ones instead. Gen. William T. Sherman thought that the policy of allowing old regiments to dwindle away into "mere skeleton organizations" was the single greatest mistake of the war. The Confederates replenished their regiments, which inevitably increased their diversity. Why the steady influx of strangers failed to undermine Southern veterans' morale the authors do not explain. One possibility: It might have been easier for soldiers to keep their morale from flagging if they felt that they were playing for higher stakes, defending their homes and families from Yankee invaders.

If social cohesion increased the likelihood that a Union soldier would stay in the field with his company, of course, the chances of his being killed or wounded rose. But we also learn in "Heroes and Cowards" that such social cohesion improved his odds of survival in Confederate prison camps. Controlling for crowding -- the most important predictor of camp mortality -- and age, height, occupation and rank (commissioned officers got better treatment), the more loyal comrades a prisoner had, the more likely he was to survive. At Andersonville you needed someone to watch your back. "If one was captured alone, put with strangers and became sick," a memoirist wrote, "it was ten chances to one he would die unattended by any human being." Ms. Costa and Mr. Kahn, who have a knack for comparisons, note that life in Nazi and Soviet camps operated in a similar fashion.

Were there any advantages to diversity in the ranks? When former slaves were mixed into companies with large numbers of free blacks, they were slightly more likely to learn how to read and write. In the main, though, Ms. Costa and Mr. Kahn emphasize the advantages of trust and mutual sacrifice that come from social similarity. They understand full well the contemporary implications of their historical study. When we contemplate helping others, whether through volunteer organizations or welfare-state transfers, we are less likely to provide for -- and more likely to abandon -- those who are unlike ourselves.  Louis J. Sheehan, Esquire