If I remember correctly it was a study during the Vietnam war. Something like this: In a firefight, 1 in 10 soldiers aims and fires his weapon at the enemy, 2 in 10 fires their weapon in the general direction of the enemy, 5 in 10 hugs the ground and wont fire unless specifically told to, and the remaining 2 in 10 will hug the ground and not fire their weapon at all.
I might be off on the percentages but I think it was something like that.