Physiologic Responses to Sudden, Loud Tones in Monozygotic Twins Discordant for Combat Exposure<subtitle>Association With Posttraumatic Stress Disorder</subtitle>

Larger heart rate responses to sudden, loud (startling) tones represent one of the best-replicated psychophysiologic markers for posttraumatic stress disorder (PTSD). This abnormality may be a pretrauma vulnerability factor, ie, it may have been present prior to the event's occurrence and increased the individual's likelihood of developing PTSD on traumatic exposure. Alternately, it may be an acquired PTSD sign, ie, it may have developed after the traumatic exposure, along with the PTSD. Studying identical twins discordant for traumatic exposure offers an opportunity to resolve these competing origins.