 Approximately 30% of the variance in PTSD is caused from genetics alone.For twin pairs exposed to combat in Vietnam, having a monozygotic (identical) twin with PTSD was associated with an increased risk of the co-twin's having PTSD compared to twins that were dizygotic (non-identical twins).There is evidence that those with a genetically smaller hippocampus are more likely to develop PTSD following a traumatic event.