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

  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},
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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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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Perceived zygosity and perceived similarity by self and others were found to be insignificant biases in the twin study method.
A population-based twin study of alcoholism in women.
The results support the hypothesis that genetic factors play a major role in the etiology of alcoholism in women and women should be well represented in the efforts currently under way to elucidate the molecular basis of the genetic susceptibility to alcoholism.
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Nine commonly used definitions of major depression, which produced life-time prevalence rates ranging from 12% to 33%, were examined and the results of model fitting to twin correlations suggested that the liability to depression results from genetic factors and environmental experiences unique to the individual.
Generalized anxiety disorder in women. A population-based twin study.
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A twin study of hyperactivity--II. The aetiological role of genes, family relationships and perinatal adversity.
  • R. Goodman, J. Stevenson
  • Psychology, Medicine
    Journal of child psychology and psychiatry, and allied disciplines
  • 1989
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