PTSD, the only question is to what extent their mind and psyche are damaged and how they cope with it. Claims Grossman's theory, based on the World War II research of S.L.A. Marshall, is that most of the population deeply resists killing another human. Some veterans and historians have cast doubt on Marshall's research methodology. 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. Marshall and the Ratio of Fire" (RUSI Journal, Winter 1988, pages 63–71), that Marshall had not actually conducted the research upon which he based his ratio-of-fire theory. "The 'systematic collection of data' appears to have been an invention." 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. As a result of Marshall's work, modern military training was modified to attempt to override this instinct, by: using man-shaped targets instead of bullseye targets in marksmanship practice practicing and drilling how soldiers would actually fight dispersing responsibility for the killing throughout the group displacing responsibility for the killing onto an authority figure, i.e., the commanding officer and the military hierarchy (see the Milgram experiment) 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. He also says the act of killing is psychologically traumatic for the killer, even more so than constant danger or witnessing the death of others. Grossman further argues that violence in television, movies and video games contributes to real-life violence by a similar process of training and desensitization. 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. Reception Robert Engen, in a paper for the Canadian Military Journal critiquing On Killing, both praised and criticized Grossman's works, saying: "On Killing and On Combat form an excellent starting point, there are too many problems with their interpretation for them to be considered the final word on the subject." Grossman's response to Engen, printed in the same journal, addresses the criticisms by showing that S.L.A. Marshall's findings, even after having doubt cast on their methodology, have borne out in further scientific studies and