It's actually pretty common sense. Pulling a trigger is not easy when it's aimed at another creature, and I've seen people refuse to shoot deer. When training infantry costs a million dollars, you have to economize by distinguishing who will be a state-sponsored killer, and who will shoot in the air out of compunction.
http://en.wikipedia.org/wiki/Military_psychology
It's actually pretty common sense. Pulling a trigger is not easy when it's aimed at another creature, and I've seen people refuse to shoot deer. When training infantry costs a million dollars, you have to economize by distinguishing who will be a state-sponsored killer, and who will shoot in the air out of compunction.
http://www.amazon.com/Men-Against-Fire-Problem-Command/dp/08...
I maybe wouldn't believe it, but I've seen it.