Is Trauma a Causal Agent of Psychopathologic Symptoms in Posttraumatic Stress Disorder? Findings From Identical Twins Discordant for Combat Exposure

Objective: The diagnosis of posttraumatic stress disorder (PTSD) is unique in that its criteria are embedded with a presumed causal agent, viz, a traumatic event. This assumption has come under scrutiny as a number of recent studies have suggested that many symptoms of PTSD may not necessarily be the result of trauma and may merely represent general psychiatric symptoms that would have existed even in the absence of a trauma event but are subsequently misattributed to it. The current study tests this hypothesis.

Method: A case-control twin study conducted between 1996–2001 examined psychopathologic symptoms in a national convenience sample of 104 identical twin pairs discordant for combat exposure in Vietnam, with (n = 50) or without (n = 54) combat-related PTSD (DSM-IV–diagnosed) in the exposed twin. Psychometric measures used were the Symptom Checklist-90-Revised, the Clinician-Administered PTSD Scale, and the Mississippi Scale for Combat-Related PTSD. If a psychopathologic feature represents a factor that would have existed even without traumatic exposure, then there is a high chance that it would also be found at elevated rates in the non–trauma-exposed, identical cotwins of trauma-exposed twins with PTSD. In contrast, if a psychopathologic feature is acquired as a result of an environmental factor unique to the exposed twin, eg, the traumatic event, their cotwins should not have an increased incidence of the feature.

Results: Combat veterans with PTSD demonstrated significantly higher scores (P < .0001) on the Symptom Checklist-90-Revised and other psychometric measures of psychopathology than their own combat-unexposed cotwins (and than combat veterans without PTSD and their cotwins).

Conclusions: These results support the conclusion that the majority of psychiatric symptoms reported by combat veterans with PTSD would not have been present were it not for their exposure to traumatic events.

J Clin Psychiatry

Submitted: March 17, 2010; accepted July 8, 2010.

Online ahead of print: September 7, 2010 (doi:10.4088/JCP.10m06121blu).

Corresponding author: Mark W. Gilbertson, PhD, Department of Research Service, Manchester VA Medical Center, 718 Smyth Rd, Manchester, NH 03104 (

J Clin Psychiatry 2010;71(10):1324-1330