Abstract
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, phobia, bulimia, and alcoholism), as assessed by personal interview, in 1030 female-female twin pairs from the Virginia Twin Registry with known zygosity. We use a newly developed model-fitting approach which treats perceived zygosity as a form of specified familial environment. In 158 of the 1030 pairs (15.3%), one or both twins disagreed with the project-assigned zygosity. Model fitting provided no evidence for a significant influence of perceived zygosity on twin resemblance for any of the five disorders. Although limited in power, these results support the validity of the EEA in twin studies of psychiatric disorders.
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References
Akaike, H. (1987). Factor analysis and AIC.Psychometrika 52:317–332.
American Psychiatric Association (1980).Diagnostic and Statistical Manual of Mental Disorders, 3rd ed., American Psychiatric Association, Washington, DC.
American Psychiatric Association (1987).Diagnostic and Statistical Manual of Mental Disorders, rev. 3rd ed., American Psychiatric Association, Washington, DC.
Clifford, C. A., Hopper, J. L., Fulker, D., and Murray, R. M. (1984). A genetic and environmental analysis of a twin family study of alcohol use, anxiety, and depression.Genet. Epidemiol. 1:63–79.
Goodman, R., and Stevenson, J. (1989). A twin study of hyperactivity. II. The aetiological role of genes, family relationships and perinatal adversity.J. Child. Psychol. Psychiat. 30:691–709.
Heath, A. C., Jardine, R., and Martin, N. G. (1989). Interactive effects of genotype and social environment on alcohol consumption in female twins.J. Stud. Alcohol. 60:38–48.
Kaprio, J., Koskenvuo, M., and Rose, R. J. (1990). Change in cohabitation and intrapair similarity of monozygotic (MZ) cotwins for alcohol use, extraversion, and neuroticism.Behav. Genet. 20:265–276.
Kendler, K. S. (1983). Overview: A current perspective on twin studies of schizophrenia.Am. J. Psychiat. 140:1413–1425.
Kendler, K. S., Heath, A. C., Martin, N. G., and Eaves, L. J. (1986). Symptoms of anxiety and depression in a volunteer twin population: The etiologic role of genetic and environmental factors.Arch. Gen. Psychiat. 43:213–221.
Kendler, K. S., MacLean, C. J., Neale, M. C., Kessler, R. C., Heath, A. C., and Eaves, L. J. (1991). The genetic epidemiology of bulimia nervosa.Am. J. Psychiat. 148:1627–1637.
Kendler, K. S., Neale, M. C., Kessler, R. C., Heath, A. C., and Eaves, L. J. (1992a). The genetic epidemiology of phobias in women: the inter-relationship of agoraphobia, social phobia, situational phobia and simple phobia.Arch. Gen. Psychiat. 49:273–281.
Kendler, K. S., Neale, M. C., Kessler, R. C., Heath, A. C., and Eaves, L. J. (1992b). Generalized anxiety disorder in women: a population based twin study.Arch. Gen. Psychiat. 49:267–272.
Kendler, K. S., Neale, M. C., Kessler, R. C., Heath, A. C., and Eaves, L. J. (1992c). A population based twin study of major depression in women: The impact of varying definitions of illness.Arch. Gen. Psychiat. 49:257–266.
Kendler, K. S., Heath, A. C., Neale, M. C., Kessler, R. C., and Eaves, L. J. (1992d). A population-based twin study of alcoholism in women.JAMA 268:1877–1882.
Loehlin, J. C., and Nichols, R. C. (1976).Heredity, Environment and Personality: A Study of 850 Sets of Twins, University of Texas Press, Austin.
Lykken, D. T., McGue, M., Bouchard, T. J., and Tellegen, A. (1990). Does contact lead to similarity or similarity to contact?Behav. Genet. 20:547–561.
Lytton, H. (1977). Do parents create, or respond to, differences in twins?Dev. Psychol. 13:456–459.
Martin, N. G., Eaves, L. J., Heath, A. C., Jardine, R., Feingold, L., and Eysenck, H. J. (1986). Transmission of social attitudes.Proc. Natl. Acad. Sci. 83:4364–4368.
Matheny, A. P., Jr. (1979). Appraisal of parental bias in twin studies: Ascribed zygosity and IQ differences in twins.Acta Genet. Med. Gemmellol. 28:155–160.
Matheny, A. P., Jr., Wilson, R. S., and Dolan, A. B. (1976). Relations between twins' similarity of appearance and behavioral similarity: Testing an assumption.Behav. Genet. 6:343–351.
Morris-Yates, A., Andrews, G., Howie, P., and Henderson, S. (1990). Twins: A test of the equal environments assumption.Acta Psychiat. Scand. 81:322–326.
Munsinger, J., and Douglass, A., II (1976). The syntactic abilities of identical twins, fraternal twins, and their siblings.Child Dev. 47:40–50.
Neale, M. C. (1991).Statistical Modelling with Mx, Department of Human Genetics, Medical College of Virginia, Virginia Commonwealth University, Richmond.
Plomin, R., Willerman, L., and Loehlin, J. C. (1976). Resemblance in appearance and the equal environments assumption in twin studies of personality traits.Behav. Genet. 6:43–52.
Robins, L. N., and Helzer, J. E. (1985).Diagnostic Interview Schedule (DIS): Version III-A, Washington University School of Medicine, St. Louis, MO.
Rose, R. J., Kaprio, J., Williams, C. J., Viken, R., and Obremski, K. (1990). Social contact and sibling similarity: facts, issues, and red herrings.Behav. Genet. 20:763–778.
Scarr, S. (1968). Environmental bias in twin studies.Eugen. Q. 15:34–40.
Scarr, S., and Carter-Saltzman, L. (1979). Twin method: Defense of a critical assumption.Behav. Genet. 9:527–542.
Spence, J. E., Corey, L. A., Nance, W. E., Marazita, M. L., Kendler, K. S., and Schieken, R. M. (1988). Molecular analysis of twin zygosity using VNTR DNA probes.Am. J. Hum. Genet. 43(3):A159.
Spitzer, R. L., Williams, J. B., and Gibson, M. (1987).Structured Clinical Interview for DSM-III-R, Biometrics Research Department, New York State Psychiatric Institute, New York.
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Kendler, K.S., Neale, M.C., Kessler, R.C. et al. A test of the equal-environment assumption in twin studies of psychiatric illness. Behav Genet 23, 21–27 (1993). https://doi.org/10.1007/BF01067551
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DOI: https://doi.org/10.1007/BF01067551
Key Words
- Twin studies
- equal-environment assumption
- psychiatric disorders
- depression
- alcoholism
- anxiety disorders