The Unthinkable: Who Survives When Disaster Strikes - and Why

by Amanda Ripley

But Gilbertson found something different. When he looked at the images, he saw that, within the twin sets, the hippocampi were about the same size. The trauma of war had not significantly altered the size of the hippocampus in the brothers who had gone to Vietnam. But there were significant differences between sets of twins. The twin sets that included vets with posttraumatic stress disorder had smaller hippocampi than the twin sets that included veterans without the disorder. In other words, a smaller hippocampus seemed to predate the trauma. Certain people were at higher risk of developing posttraumatic stress disorder before they even left for Vietnam. We can deduce, then, that certain people will likely have a harder time processing fear in a disaster—and recovering from that trauma later.