The book is based on research that was flat made up by S.L.A. Marshall. There is no evidence he ever collected the data he used to spread that myth.
On Killing: The Psychological Cost of Learning to Kill in War and Society, Dave Grossman
Quote From Wikipedia
The book is based on SLA Marshall's studies from World War II, which proposed that contrary to popular perception,[1] the majority of soldiers in war do not ever fire their weapons and that this is due to an innate resistance to killing. Based on Marshall's studies the military instituted training measures to break down this resistance and successfully raised soldiers' firing rates to over ninety percent during the war in Vietnam.[2]
Below quote from
http://www.warchronicle.com/us/combat_historians_wwii/marsha...
Emphasis not in original
This calculation assumes, however, that of all the questions Marshall might ask the soldiers of a rifle company during his interviews, he would unfailingly want to know who had fired his weapon and who had not. Such a question, posed interview after interview, would have signalled that Marshall was on a particular line of inquiry, and that regardless of the other information Marshall might discover, he was devoted to investigating this facet of combat performance. John Westover, usually in attendance during Marshall's sessions with the troops, does not recall Marshall's ever asking this question. Nor does Westover recall Marshall ever talking about ratios of weapons usage in their many private conversations. Marshall's own personal correspondence leaves no hint that he was ever collecting statistics. His surviving field notebooks show no signs of statistical compilations that would have been necessary to deduce a ratio as precise as Marshall reported later in Men Against Fire. The "systematic collection of data" that made Marshall's ratio of fire so authoritative appears to have been an invention.
