Pretrauma Cognitive Ability and Risk for Posttraumatic Stress Disorder: a Twin Study

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.