It's not so much of the "mutual surveillance" during the battle that actually bothers me about Grossman's theory (infact, that makes alot of sense - peer pressure is a big factor) but the events after the battle that seem to countermand the notion that man has a natural aversion to killing.
For most of history, after the victorious army has routed the enemy, they've proceeded to butcher retreating forces (usually where most casualties come from), rape and slaughter any undefended populace, and generally loot and pillage their way through any poor society caught in the way. How is it that Grossman's human soldier (that possesses such a resistance to killing) must be pressed into battle and psychologically pressured into killing his fellow man, and then, when victorious, suddenly becomes capable of executing captured enemies, sticking their heads on pikes and putting civilian populations to the sword.
History is chock full of these events. The Assyrian war machine, victorious Greeks (Read Euripides The Trojan Woman, the original anti-war play), Tamerlane's mountains of skulls, the marauding armies of the Thirty Years War, savage butchery on the American plains between the US Army and Native groups, and the march of the Soviet Army into Germany in 1945.
Why do we have things like the Laws of Land Warfare and the Geneva Conventions, when Grossman's human would willingly resort to such measures due to an innate resistance to killing? Dr Jonathan Shay identifies rage as a key component in the undoing of character, an essential characteristic of all the Vietnam vets with PTSD that he treats. I can't see rage and the combat berserker (See Achilles in Vietnam: Combat Trauma and the Undoing of Character) fitting into Grossman's model of 98% of humans possessing innate resistance to killing and 2% cool-headed, cold-blooded killers - rather, a "Freudian" conflict between the instinctive, animal-like lower brain and the rational, civilized upper brain seem to be competing when the psychological and physical effects of combat crash into the human fighter.
This is why I have never been fully supportive of Grossman's theory and eagerly took on to some of the ideas that Ghiglieri provides. I still believe that On Killing is very important for identifying factors of motivation - things like the Shalit and Milgram factors. As well, I agree with Grossman when he states that there exists some sort of aversion to killing; SLA Marshall's work, although flawed, points to conclusions that are too profound to ignore. However, I think Grossman has it backwards - man does not have a natural aversion to killing which is overcome through social factors (training); rather man is instinctively a very proficient killing machine and society and culture have been formed to act as social, artificial barriers which lead to the non-firer. In cultures where social cohesion is strong and violence is fundamentally frowned upon, the barrier is much more stronger - hence why the non-firer becomes more obvious in our societies.
However, history seems too quick to prove that once these key social barriers are broken down, the Dark Side of Man is all too eager to take over - turning man into one of the most ruthless killing machines that nature has yet to turn out.
Infanteer