LOS ANGELES - A study of 40 men who saw combat in Vietnam and their twins who did not suggests the size of the brain region involved in storing memory can predict vulnerability to post-traumatic stress disorder.

Earlier studies have found the region, called the hippocampus, is smaller than normal in veterans who have the disorder. The assumption has been that stress caused the region to shrink in volume.

But the study involving 40 sets of identical twins found the smaller volume is likely inherited and not a consequence of combat trauma.

"That would probably be the most likely explanation of the results," said Mark Gilbertson, a psychologist with the Veterans Administration Medical Center in Manchester, N.H., and co-author of the study in Tuesday's electronic edition of the journal Nature Neuroscience. The study was sponsored by the Veterans Administration.

Post-traumatic stress disorder has afflicted nearly 31 percent of Vietnam combat veterans at some time, according to government estimates.

In the new study, about half the 40 combat veterans suffered from chronic, unremitting post-traumatic stress disorder. The other half had never been affected, nor had any of the 40 stay-at-home twins.

In veterans who were affected, hippocampal volume was 10 percent smaller on average than in veterans who never suffered from the syndrome but had seen combat. Twins of the combat veterans who reported problems also had smaller hippocampi, even though they had seen no combat.

Since identical twins have similar brain structures, the finding suggests those veterans suffering from the disorder had smaller hippocampi before they entered combat.

Dr. J. Douglas Bremner of Emory University said it is still possible that environmental stresses caused the smaller brain region volumes, ruling out a purely hereditary effect. Early environmental stresses were presumably shared by the twins, he said, explaining the similarity in their hippocampus volumes.

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Nature journal: http://www.nature.com/neuro