New research on identical twins has found that post-traumatic stress disorder (PTSD) is linked to both genetic factors and exposure to stress. For the study, researchers examined more than 100 combat veterans of the Vietnam War, each of whom have an identical twin who did not serve in combat. Investigators found that both twins had smaller areas of the brain involved in memory, suggesting that this genetic factor may increase a person's risk of developing PTSD. But combat-exposed twins also had...