True et al,1 in their twin study on genetic and environmental contributions to the liability for posttraumatic stress symptoms in Vietnam veterans, use a bivariate model to postulate significant genetic influences on symptom liability and state that there is no evidence that shared environment contributes to the development of posttraumatic stress symptoms.
This model is erroneous, however, since it is based on the assumption that noncombat environmental influences are not more highly correlated in monozygotic (MZ) twins than in dizygotic (DZ) twins. The authors' own data indicate that the MZ twins have a higher concordance for combat exposure than do DZ twins (0.34 vs 0.17). To assume that their environments previous to this are somehow no more concordant seems obviously false.
In addition, the fact that MZ twins are so much more alike in appearance, tend to play together, and have other childhood similarities, seems to guarantee that their