Hippocampal Volumes of War Veterans and Their Identical Twins Average hippocampal volumes for four groups of participants: (1) combat-exposed veterans who developed PTSD; (2) their combat-unexposed twins with no PTSD themselves; (3) combat-exposed veterans who never developed PTSD; and (4) their unexposed twins, also with no PTSD. Smaller hippocampal volumes were found both for the combat-exposed veterans with PTSD (Group 1) and their twins who had not been exposed to combat (Group 2) in comparison to veterans without PTSD (Group 3) and their twins (Group 4). This pattern of findings suggests that an inherited smaller hippocampus may make some people sensitive to conditions that cause PTSD (Gilbertson et al., 2002).