Surveying identical and fraternal twins allowed the investigators to weigh the genetic and environmental factors that make up the mix of influences related to particular health problems. And because of the sample size, the odds are good that those influences will be true for all Vietnam veterans, not just the twins surveyed for this study.
Less than 9 percent of the Vietnam veterans suffered from hypertension, for example, but for identical twins, the odds were greater than 50 percent that if one twin had high blood pressure, so did his brother. Among fraternal twins, the overall rate of hypertension was 9.5 percent, and if one brother had it, his twin had a 34 chance percent of having it as well. Since the primary difference between pairs of identical and fraternal twins is the extent to which they share genes, this result suggests that inherited factors partially explain the development of hypertension.
Overall, the investigators found that, for the entire group of twins, inherited factors explained about 54 percent of the report of hypertension, while combat exposure explained less than one percent. For joint disorders such as arthritis, genetics accounted for 37 percent while combat exposure explained only 2.6 percent.
In all, combat experiences explained no more than 10 percent of the health problems reported by veterans. By contrast, Eisen and colleagues found that inherited factors and environmental experiences other than combat were much more important in determining whether health problems developed.
The researchers gathered data from the twins either by mail or telephone, using a 30-page questionnaire. When the original survey was conducted in 1987, the mean age of the twins/veterans interviewed was 38, and the average participant had been inducted into the service 19 years earlier.