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A Twin Study

William S. Kremen, PhD; Karestan C. Koenen, PhD; Corwin Boake, PhD; Shaun Purcell, PhD; Seth A. Eisen, MD; Carol E. Franz, PhD; Ming T. Tsuang, MD, PhD; Michael J. Lyons, PhD

Arch Gen Psychiatry. 2007;64(3):361-368.

Context  Cognitive deficits are associated with posttraumatic stress disorder (PTSD), but whether such deficits reflect sequelae or risk factors is not fully resolved.

Objective  To determine, in a representative sample, whether preexposure cognitive ability is associated with risk for PTSD, and whether that risk is genetically mediated.

Design, Setting, and Participants  The co-twin–control study involved 2386 male Vietnam-era twin veterans with a mean (SD) age of 41.9 (2.7) years, a population-based sample of men who were in military service during this era. Cognitive ability scores were obtained just before military induction at a mean (SD) age of 19.7 (1.5) years. Participants included only individuals who were exposed to potentially traumatic events and underwent preexposure cognitive testing.

Main Outcome Measures  Armed Forces Qualification Test (of cognitive ability) percentile scores and PTSD diagnosed by means of structured interviews.

Results  We found a significant dose-response relationship between preexposure cognitive ability and risk for PTSD. After controlling for confounders, the highest cognitive ability quartile had a 48% lower risk than the lowest ability quartile (P<.001). Non–PTSD-concordant pairs had the highest scores; PTSD-concordant pairs had the lowest scores; and PTSD-discordant pairs had intermediate scores. Differences in Armed Forces Qualification Test scores within twin pairs were significant only in PTSD-discordant pairs (P=.04) and were accounted for specifically by the discordant dizygotic pairs (P=.002). Genetic influences on preexposure cognitive ability explained 5% of the variation in PTSD, but 100% of that relationship was explained by common genes.

Conclusions  Preexposure cognitive ability is a risk or a protective factor for PTSD. The variance in PTSD explained by preexposure cognitive ability is accounted for entirely by common genetic factors. Lower cognitive ability may be a marker of less adaptive coping against adverse mental health consequences of exposure to potentially traumatic events. Further study of the potential mechanisms through which cognitive ability confers risk is needed.

Author Affiliations: Department of Psychiatry (Drs Kremen, Franz, and Tsuang) and Center for Behavioral Genomics (Drs Kremen and Tsuang), University of California, San Diego, La Jolla; Department of Society, Human Development, and Health, Harvard School of Public Health (Dr Koenen), Psychiatric and Neurodevelopmental Genetics Unit, Massachusetts General Hospital (Dr Purcell), Department of Psychology, Boston University (Dr Lyons), and Harvard Institute of Psychiatric Epidemiology and Genetics, Departments of Epidemiology and Psychiatry, Harvard University (Dr Tsuang), Boston, Mass; Institute for Rehabilitation and Research, Houston, Tex (Dr Boake); and Research and Medical Services, St Louis Veterans Affairs Medical Center (Dr Eisen) and Departments of Internal Medicine and Psychiatry, Washington University School of Medicine (Dr Eisen), St Louis, Mo.