This study investigates the relationship between smaller hippocampal volume and the development of stress-related psychopathology, particularly posttraumatic stress disorder (PTSD). Using a case-control design with monozygotic twin pairs, where one twin was a Vietnam combat veteran (exposed to trauma) and the other was not, the researchers found that smaller hippocampal volume was a pre-existing vulnerability factor, rather than a neurotoxic effect of trauma. Specifically, the severity of PTSD symptoms in exposed twins was negatively correlated with the hippocampal volume of both the exposed and unexposed co-twins. The study also controlled for other factors such as combat severity, alcohol abuse history, and lifetime non-combat trauma, finding that these did not significantly influence the results. The findings suggest that smaller hippocampal volume may predispose individuals to stronger and more persistent emotional and hormonal stress responses when exposed to trauma.This study investigates the relationship between smaller hippocampal volume and the development of stress-related psychopathology, particularly posttraumatic stress disorder (PTSD). Using a case-control design with monozygotic twin pairs, where one twin was a Vietnam combat veteran (exposed to trauma) and the other was not, the researchers found that smaller hippocampal volume was a pre-existing vulnerability factor, rather than a neurotoxic effect of trauma. Specifically, the severity of PTSD symptoms in exposed twins was negatively correlated with the hippocampal volume of both the exposed and unexposed co-twins. The study also controlled for other factors such as combat severity, alcohol abuse history, and lifetime non-combat trauma, finding that these did not significantly influence the results. The findings suggest that smaller hippocampal volume may predispose individuals to stronger and more persistent emotional and hormonal stress responses when exposed to trauma.