Fifteen Percent Concentrated Power Of Will

In WWI, some bright spark in the War Office came up with the idea of Pals regiments, where friends from the same town could serve together. The idea was very popular and increased enlistment considerably -- until you had things like 3/2 of a particular group being killed or incapacitated in the first few minutes of the Somme. A town could lose all its young men at once, instead of piecemeal.

Of course, the medical personnel weren't supposed to be on the front lines, so the same concepts didn't necessarily apply.