David Allen Grossman (born August 23, 1956) is an American author and trainer who conducts seminars on the psychology of lethal force. He is a retired lieutenant colonel in the United States Army. Early life and military career Grossman was born in Frankfurt, West Germany on August 23, 1956. His career includes service in the U.S. Army. Post-military career Following his retirement from the Army, Grossman founded the Killology Research Group to give seminars about the physiological and the psychological effects of having to use lethal force for law enforcement officers and soldiers. Grossman also speaks at civilian events on ways to reduce violence in society and deal with the aftermath of violent events such as school shootings. As a civilian Grossman has been an expert witness in numerous state and federal court cases and was part of the prosecution team of United States vs. Timothy McVeigh. In 2022, Killology Research Group was renamed and rebranded to Grossman On Truth. Works Grossman's first book, On Killing: The Psychological Cost of Learning to Kill in War and Society is an analysis of the psychological processes involved with killing another human being. In it, he claims that most people have a phobia-level response to violence, and that soldiers have to be specifically trained to kill. He details some of the physical effects that violent stresses produce on humans, ranging from tunnel vision, changes in sonic perception, and post-traumatic stress disorder. Robert Engen, in a paper for the Canadian Military Journal critiquing On Killing, criticized Grossman's works, saying that "although 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, attempted to address the criticisms by arguing that SLA Marshall's findings that man is not by nature a killer, 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.” "On Killing" has been cited over 3300 times, noted by Google Scholar. In Stop Teaching Our Kids to Kill: A Call to Action Against TV, Movie and Video Game Violence, Grossman argues that the techniques used by armies to train soldiers to kill are mirrored in certain types of video games.