Our overall sample consists of 130 male Mz twin pairs, in which one twin served in combat in Vietnam and his cotwin did not. Zygosity was determined by a questionnaire that has been found 97% accurate when validated against blood typing.2
The only exclusion criterion for participation in these noninvasive studies was a serious contraindicating medical (e.g., cardiac) or psychiatric (e.g., psychosis, dementia) condition.
Most the twin pairs were recruited from the Vietnam Era Twin (VET) Registry, which has been described in detail elsewhere.3
VET Registry staff identified a pool of 624 living mz pairs of Vietnam era veterans discordant for combat exposure in Vietnam with known addresses and/or telephone numbers. They succeeded in obtaining preliminary data over the telephone in both twins in 442 (71%) of these pairs. These data included the Mississippi Scale for Combat-Related PTSD in the combat-exposed twin.4
VET Registry staff initially sent letters to each member of twin pairs randomly selected from the subpool of 442 pairs, inviting him to participate in the study. The letter contained a brief description of the study and copies of the consent documents the subject would be required to sign prior to participation. The letters were followed by telephone contact to answer any questions the subject candidate might have and to encourage participation. Unwillingness of one twin disqualified the pair. When recruitment of PTSD twin pairs (i.e., pairs in which the combat-exposed
twin had PTSD) fell behind that of non-PTSD twin pairs, random recruitment was abandoned, and efforts were focused on the combat-exposed subject candidates who had the highest Mississippi scale scores or other suggestive evidence of PTSD from past mail and telephone surveys. Of 210 (48% of the 442) VET Registry twin pairs invited to come to the Manchester, NH VA Research Service Psychophysiology Laboratory, 103 (49%) participated. Most subject candidates who did not participate either declined because they were uninterested, too busy, or could not be scheduled at a time of mutual convenience.
After the VET Registry resource had apparently been exhausted for suitable PTSD pair candidates, to make up the deficit, letters were sent to approximately 80,000 Vietnam veterans receiving compensation for PTSD from the Veterans Benefits Administration. The letter inquired whether the recipient had an identical twin who had not served in combat in Vietnam, and if so, whether he and his brother were interested in participating in a research study. Twenty-seven additional PTSD pairs were recruited in this manner.
Due to ambiguous predictions for veterans with past but not current PTSD twin, some of whom may have substantial residual PTSD abnormalities, and others of whom may not, data from 27 past-PTSD (in the combat-exposed twin) twin pairs were excluded from these studies, leaving 50 PTSD and 53 non (never had)-PTSD twin pairs. All these subjects participated in an initial psychophysiological study.5
Subsequently, a convenience sample of 25 PTSD and 24 non-PTSD pairs from this larger sample returned for neuroimaging. For technical and other reasons described in the respective publications, not all of these subjects’ data were usable in all studies.
All studies were approved by the Manchester, NH VA Medical Center Institutional Review Board. After the procedures had been fully explained, subjects gave written informed consent for participation.
The PTSD status of each combat-exposed subject was determined by the Clinician-Administered PTSD Scale (CAPS) administered with regard to his combat experience. Further methodological details are available in the respective publications cited below.
The general statistical analytic approach to the data entailed a mixed model that treated PTSD Diagnosis (i.e., PTSD versus non-PTSD in the combat-exposed twin
) as a between-pairs fixed effect, combat exposure as a within-pair fixed effect, and twin pairs as a random effect.6
The combat-unexposed cotwins of combat veterans with PTSD are designed “high-risk/vulnerability” because their twin, with whom they share the same genes and developmental environment, developed PTSD. The combat-unexposed cotwins of combat veterans without PTSD are designed “low-risk/vulnerability” for the inverse reason.