Skip to main content

Twin Studies in Psychiatry and Psychology: Science or Pseudoscience?

Abstract

Twin studies are frequently cited in support of the influence of genetic factors for a wide range of psychiatric conditions and psychological trait differences. The most common method, known as the classical twin method, compares the concordance rates or correlations of reared-together identical (MZ) vs. reared-together same-sex fraternal (DZ) twins. However, drawing genetic inferences from MZ–DZ comparisons is problematic due to methodological problems and questionable assumptions. It is argued that the main theoretical assumption of the twin method—known as the “equal environment assumption”—is not tenable. The twin method is therefore of doubtful value as an indicator of genetic influences. Studies of reared-apart twins are discussed, and it is noted that these studies are also vulnerable to methodological problems and environmental confounds. It is concluded that there is little reason to believe that twin studies provide evidence in favor of genetic influences on psychiatric disorders and human behavioral differences.

This is a preview of subscription content, access via your institution.

REFERENCES

  1. Myerson A: The Inheritance of Mental Diseases. Baltimore, Williams & Wilkins, 1925.

    Google Scholar 

  2. Kallmann F: The Genetics of Schizophrenia: A Study of Heredity and Reproduction in the Families of 1,087 Schizophrenics. New York, J. J. Augustin, 1938.

    Google Scholar 

  3. Faraone S, Tsuang, M: Methods in psychiatric genetics. In: Tsuang M, Tohen M, Zahner G, eds. Textbook in Psychiatric Epidemiology. New York: Wiley-Liss, 1995, pp. 81-134.

    Google Scholar 

  4. Galton F: The history of twins as a criterion of the relative powers of nature and nurture. Journal of the Anthropological Institute of Great Britain and Ireland 5:391-406, 1875.

    Google Scholar 

  5. Siemens H: Die Zwillingspathologie [Twin Pathology]. Berlin, Springer Verlag, 1924.

    Google Scholar 

  6. Jackson D: A critique of the literature on the genetics of schizophrenia. In: Jackson D, ed. The Etiology of Schizophrenia. New York, Basic Books, 1960, pp. 37-87.

    Google Scholar 

  7. Joseph J: Don Jackson's “A critique of the literature on the genetics of schizophrenia”-A reappraisal after 40 years. Genetic, Social, and General Psychology Monographs 127:27-57, 2001.

    Google Scholar 

  8. Joseph J: Not in their genes: A critical view of the genetics of attention-deficit hyperactivity disorder. Developmental Review 20:539-567, 2000.

    Google Scholar 

  9. Bouchard T: Genetic and environmental influences on adult personality: Evaluating the evidence. In: Hettema J, Deary I, eds. Basic Issues in Personality. Dordrecht, The Netherlands: Kluwer Academic Publishers, 1993, pp. 15-44.

    Google Scholar 

  10. Gottesman I, Shields J: Schizophrenia and Genetics: A Twin Study Vantage Point. New York, Academic Press, 1972.

    Google Scholar 

  11. Kendler K, Neale M, Kessler R, et al: Parental treatment and the equal environment assumption in twin studies of psychiatric illness. Psychological Medicine 24:579-590, 1994.

    Google Scholar 

  12. Morris-Yates A, Andrews G, Howie P, Henderson S: Twins: A test of the equal environments assumption. Acta Psychiatrica Scandinavica 81:322-326, 1990.

    Google Scholar 

  13. Rose R: Twin studies and psychosocial epidemiology. In: Tsuang M, Kendler K, Lyons M, eds. Genetic Issues in Psychosocial Epidemiology. New Brunswick, NJ: Rutgers University Press, 1991, pp. 12-32.

    Google Scholar 

  14. Scarr S: Environmental bias in twin studies. Eugenics Quarterly 15:34-40, 1968.

    Google Scholar 

  15. Scarr S, Carter-Saltzman L: Twin method: Defense of a critical assumption. Behavior Genetics 9:527-542, 1979.

    Google Scholar 

  16. Kringlen E: Heredity and Environment in the Functional Psychoses: An Epidemiological-Clinical Study. Oslo, Universitetsforlaget, 1967.

    Google Scholar 

  17. Rosanoff A, Handy L, Plesset I, Brush S: The etiology of so-called schizophrenia psychoses. American Journal of Psychiatry 91:247-286, 1934.

    Google Scholar 

  18. Carey G, DiLalla D: Personality and psychopathology: Genetic perspectives. Journal of Abnormal Psychology 103:32-43, 1994.

    Google Scholar 

  19. Lyons M, Kendler K, Provet A, Tsuang M: The genetics of schizophrenia, In: Tsuang M, Kendler K, Lyons M, eds. Genetic Issues in Psychosocial Epidemiology. New Brunswick, NJ, Rutgers University Press, 1991, pp. 19-52.

    Google Scholar 

  20. Kendler K: Overview: A current perspective on twin studies of schizophrenia. American Journal of Psychiatry 140:1413-1425, 1983.

    Google Scholar 

  21. Lytton H: Do parents create, or respond to, differences in twins? Developmental Psychology 13:456-459, 1977.

    Google Scholar 

  22. Kendler K: The genetics of schizophrenia: A current perspective. In: Meltzer H, ed. Psychopharmacology: The Third Generation of Progress. New York, Raven Press, 1987, pp. 705-713.

    Google Scholar 

  23. Joseph J: The equal environment assumption of the classical twin method: A critical analysis. Journal of Mind and Behavior 19:325-358, 1998.

    Google Scholar 

  24. Pam A, Kemker S, Ross C, Golden R: The “equal environment assumption” in MZ-DZ comparisons: An untenable premise of psychiatric genetics? Acta Geneticae Medicae et Gemellologiae 45:349-360, 1996.

    Google Scholar 

  25. Joseph J: The genetic theory of schizophrenia: A critical overview. Ethical Human Sciences and Services 1:119-145, 1999.

    Google Scholar 

  26. Harris J: The Nurture Assumption: Why Children Turn Out the Way They Do. New York, The Free Press, 1998.

    Google Scholar 

  27. Segal N: Entwined Lives: Twins and What They Tell Us About Human Behavior. New York, Dutton, 1999.

    Google Scholar 

  28. Wright L: Twins: And What They Tell Us About Who We Are. New York, John Wiley & Sons, 1997.

    Google Scholar 

  29. Wright W: Born That Way. New York, Alfred A. Knopf, 1998.

    Google Scholar 

  30. Hearnshaw L: Cyril Burt: Psychologist. Ithaca, NY, Cornell University Press, 1979.

    Google Scholar 

  31. Bouchard T, Lykken D, McGue M, et al: Sources of human psychological differences: The Minnesota study of twins reared apart. Science 250:223-228, 1990.

    Google Scholar 

  32. Kamin L: The Science and Politics of IQ: Potomac, MD, Lawrence Erlbaum Associates, 1974.

    Google Scholar 

  33. Farber S: Identical Twins Reared Apart: A Reanalysis. New York, Basic Books, 1981.

    Google Scholar 

  34. Dusek V: Bewitching science. Science for the People 19:19-22, 1987.

    Google Scholar 

  35. McGue M, Bouchard T: Adjustment of twin data for the effects of age and sex. Behavior Genetics 14:325-343, 1984.

    Google Scholar 

  36. Joseph J: Separated twins and the genetics of personality differences: A critique. American Journal of Psychology 114:1-30, 2001.

    Google Scholar 

  37. Rose R: Separated twins: Data and their limits. Science 215:959-960, 1982.

    Google Scholar 

Download references

Author information

Authors and Affiliations

Authors

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Joseph, J. Twin Studies in Psychiatry and Psychology: Science or Pseudoscience?. Psychiatr Q 73, 71–82 (2002). https://doi.org/10.1023/A:1012896802713

Download citation

  • Issue Date:

  • DOI: https://doi.org/10.1023/A:1012896802713

  • twins
  • twin method
  • genetics
  • psychiatry
  • psychiatric genetics
  • schizophrenia