In carpentry, it is common knowledge that planing wood against the grain will result in a jagged and undesirable finish. A true craftsman respects the grain. To do otherwise would be reckless and damaging. In other fields, such as psychology, it is understood that going against the grain of human nature and morality is similarly problematic. For example, research has suggested that people, by and large, are naturally averse to taking the life of another. We are not a species that is hard-wired for killing. Such behavior goes against the grain. Obviously, a disturbing number of people manage to override this natural aversion, but, even so, the reality is that the vast majority of individuals do their best to resist such urges. Our survival as a species is, in part, a testament to our respect for the grain of our being.
But what of warfare? Despite our natural inclinations, we continue to perpetrate murder on a massive scale in the name of nation, clan, faith, and what-not. Even in the developed world, as such, where one might presume we know better, we continue to wage war and take lives by the thousands. How is this possible? Here in the United States, how do those who serve in the military, the brave men and women with boots on the ground in Iraq and Afghanistan, manage to override their nature and morality and kill their fellow human beings? How do they go so deeply against the grain, and what does that do to their psyches?
The following excerpt from an article by Lt. Col. Dave Grossman in Greater Good addresses some of these questions and finds that U.S. soldiers today, in a very calculated and perhaps immoral way, are psychologically conditioned to kill and, as a result, are suffering the jagged consequences of going against the grain:
During World War II, U.S. Army Brigadier General S.L.A. Marshall asked average soldiers how they conducted themselves in battle. Before that, it had always been assumed that the average soldier would kill in combat simply because his country and his leaders had told him to do so, and because it might be essential to defend his own life and the lives of his friends. Marshall’s singularly unexpected discovery was that, of every hundred men along the line of fire during the combat period, an average of only 15 to 20 “would take any part with their weapons.� This was consistently true, “whether the action was spread over a day, or two days, or three.�
Marshall was a U.S. Army historian in the Pacific theater during World War II and later became the official U.S. historian of the European theater of operations. He had a team of historians working for him, and they based their findings on individual and mass interviews with thousands of soldiers in more than 400 infantry companies immediately after they had been in close combat with German or Japanese troops. The results were consistently the same: Only 15 to 20 percent of the American riflemen in combat during World War II would fire at the enemy. Those who would not fire did not run or hide—in many cases they were willing to risk greater danger to rescue comrades, get ammunition, or run messages. They simply would not fire their weapons at the enemy, even when faced with repeated waves of banzai charges.
Why did these men fail to fire? As a historian, psychologist, and soldier, I examined this question and studied the process of killing in combat. I have realized that there was one major factor missing from the common understanding of this process, a factor that answers this question and more: the simple and demonstrable fact that there is, within most men and women, an intense resistance to killing other people. A resistance so strong that, in many circumstances, soldiers on the battlefield will die before they can overcome it.
Indeed, the study of killing by military scientists, historians, and psychologists gives us good reason to feel optimistic about human nature, for it reveals that almost all of us are overwhelmingly reluctant to kill a member of our own species, under just about any circumstance. Yet this understanding has also propelled armies to develop sophisticated methods for overcoming our innate aversion to killing, and, as a result, we have seen a sharp increase in the magnitude and frequency of post-traumatic response among combat veterans. Because human beings are astonishingly resilient, most soldiers who return from war will be fine. But some will need help coping with memories of violence. When those soldiers return from war—especially an unpopular one like Iraq—society faces formidable moral and mental health challenges in caring for and re-integrating its veterans. [full text]
(Thanks to Kevin for bringing this article, which is well worth reading in its entirety, to my attention.)
Kmareka.com 
The only problem in using SLA Marshall (SLAM) is that for years SLAM’s theories have been considered worthless by both historians and veterans. His methodology is highly questionable, if he even had one. Various authors and historians (such as Roger Spiller) have discredited SLAM’s work on the subject, “Men Against Fire”, as well as some of his other works, including his unfounded allegations that the men of the IX Troop Carrier Command were cowards and incompetent, allegations which Stephen Ambrose repeated in his works, neither author doing any serious research into the allegations nor even interviewing the veterans of that command.
Mr. Merriam’s comments are on target (forgive the pun–the use of overstatement to gain attention is more comfortable in the hands of the tabloid writer than the historian. Sadly, the reputation of th late Mr. Ambrose has been badly tarnished by reproted evidence of plagerism and other issues in his reporting. The life and death issues that our soldiers confront every day and every moment they serve are too significant to be treated in an off-hand manner and with less than rigorous analysis. Those who have meet the challenges of combat in any form do so with a rigor and level of skill never seen in any armed force at any time in history.