A test of the equal-environment assumption in twin studies of psychiatric illness

@article{Kendler1993ATO,
  title={A test of the equal-environment assumption in twin studies of psychiatric illness},
  author={Kenneth S. Kendler and Michael C. Neale and Ronald C. Kessler and Andrew C. Heath and Lindon J. Eaves},
  journal={Behavior Genetics},
  year={1993},
  volume={23},
  pages={21-27}
}
The traditional twin method is predicated on the equal-environment assumption (EEA)—that monozygotic (MZ) and dizygotic (DZ) twins are equally correlated in their exposure to environmental events of etiologic importance for the trait under study. In 1968, Scarr proposed a test of the EEA which examines the impact of phenotypic similarity in twins of perceived versus true zygosity. We apply this test for the EEA to five common psychiatric disorders (major depression, generalized anxiety disorder… 
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It is shown that the most crucial of these, namely, the equal environments assumption (EEA), may not hold and differences in twin correlations might be at least partly explained by treatment effects from parents, teachers, peers, and so on.
The "equal environments assumption" in MZ-DZ twin comparisons: an untenable premise of psychiatric genetics?
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Many MZ-DZ pedigree studies have dubious scientific value, given the non-viable premise of the EEA, as well as the misleading operational definition of what has been called "heritability".
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