Marshall's theory that the majority of soldiers in war do not ever fire their weapons due to an innate resistance to killing.

Based on Marshall's studies, the military instituted training measures to break down this resistance, raising soldiers' firing rates to over 90 percent during the Vietnam War.

This recurring theme in recollections collected from war veterans is the idea that they were not fighting for themselves at the time but more concerned for the people to their left and right.

This ideology is ancient, recorded for example by Sun Tzu in his book The Art of War: "If those who are sent to draw water begin by drinking themselves, the army is suffering from thirst.

Many believe that other factors have been influencing PTSD such as decompression time after conflicts, and the lack of moral sovereignty in engagements.

In contrast, many U.S. soldiers in World War II felt they were stopping an evil empire from conquering the globe.

This shunning was due to the senior members being afraid to befriend someone with a much higher chance of being killed than experienced combatants.

Grossman states in his book that everyone who experiences combat comes back with PTSD, the only question is to what extent their mind and psyche are damaged and how they cope with it.

[1] Professor Roger J. Spiller (deputy director of the Combat Studies Institute, US Army Command and General Staff College) argues in his 1988 article, "S.L.A.

"[9] This revelation has called into question the authenticity of some of Marshall's other books and has lent academic weight to doubts about his integrity that had been raised in military circles even decades earlier.

[10] As a result of Marshall's work, modern military training was modified to attempt to override this instinct, by: By the time of the United States involvement in the Vietnam War, says Grossman, 90% of U.S. soldiers would fire their weapons at other people.

In On Combat (Grossman's sequel to On Killing, based on ten years of additional research and interviews), he addresses the psychology and physiology of human aggression.

Marshall's findings, even after having doubt cast on their methodology, have borne out in further scientific studies and real world experience and, furthermore, have been the cornerstone of military and police training for over a half century.