Lurie and Geyer question our finding that shared environment did not contribute to susceptibility to posttraumatic stress disorder symptoms in our study of 2224 MZ and 1818 DZ male veteran twins. 1 They base their argument on the validity of the equal environments assumption of twin studies, which is that MZ and DZ twins do not differ in exposure to etiologically relevant environmental influences.Although, as stated in the article, we did not have data in our study to test directly this assumption, the issue has received considerable critical attention in the twin literature, most recently in Kendler' s2 comprehensive review of the role of twin studies of psychiatric illness and in a review of genetic approaches to personality by Eaves et al:3 The issue is that if the phenotypic similarity of MZ twins is actually attributed to shared environments that are more uniform, then genetic contributions will
True WR, Rice J, Heath AC. Genetic and Environmental Influences of Twins in Posttraumatic Stress-Reply. Arch Gen Psychiatry. 1994;51(10):838–839. doi:10.1001/archpsyc.1994.03950100086009
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