Previous studies on stress suggest that post traumatic stress disorder (PTSD) could be either environmental or genetic. One study found the size of several regions of the brain are inherited and presumably one region in particular, the hippocampus, may increase stress reactivity or impair the capacity for resilience. Another study found that stress can cause brain atrophy and even neural death in some brain regions.

Kiyoto Kasai, affiliated with the Department of Neuropsychiatry, Graduate School of Medicine, University of Tokyo, Japan, and Mark W. Gilbertson, Department of Psychiatry, Harvard Medical School, Boston, Massachusetts, and associates, measured the gray matter density of the brains of combat-exposed Vietnam veterans and their identical twins who were not exposed to combat. Using magnetic resonance imaging (MRI) detailed images provided investigators to compare specific brain regions of the siblings.

Researchers found the gray mater density of the area of the brain involved in emotional functioning (pregenual anterior cingulate cortex) was reduced in veterans with PTSD, but not in their sibling. The scientist feel that this finding supports the idea that psychological stress resulting from a traumatic stressor may damage the brain associated with emotion–contributing to PTSD.