Not really identical: Epigenetic differences in monozygotic twins and implications for twin studies in psychiatry


  • F. Nipa Haque,

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    • F. Nipa Haque is completing her MSc in pharmacology at the University of Toronto with Dr. Albert Wong at the Centre for Addiction and Mental Health (CAMH) in Toronto, Ontario. The focus of her research is gene–environment interaction in the development of neuropsychiatric disease.

  • Irving I. Gottesman,

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    • Irving I. Gottesman holds the Irving and Dorothy Bernstein Professorship in Adult Psychiatry and is a Senior Fellow in the Department of Psychology at the University of Minnesota Medical School. He is also Sherrell J. Aston professor of Psychology Emeritus at the University of Virginia. One of the founders of post-WW II behavioral genetics, he began his first twin study on the genetics of personality in 1957 which became his doctoral dissertation at the University of Minnesota. His strategies with normal and psychiatric twins have ranged from heritability to “epigeneticability” in partnership with Arturas Petronis and Albert Wong, and to unexpressed genotypes in the offspring of schizophrenic twins with Aksel Bertelsen.

  • Albert H.C. Wong

    Corresponding author
    • Centre for Addiction and Mental Health, 250 College Street, Room 711, Toronto, ON M5T 1R8, Canada.
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    • Albert H.C. Wong is a neuroscientist and psychiatrist at CAMH, and an associate professor of Psychiatry at the University of Toronto. His main interests are the regulation of behavior by genetic and epigenetic factors, transcription, and gene–environment interactions.

  • How to cite this article: Haque FN, Gottesman II, Wong AHC. 2009. Not really identical: Epigenetic differences in monozygotic twins and implications for twin studies in psychiatry. Am J Med Genet Part C Semin Med Genet 151C:136–141.


Classical twin studies in the field of psychiatry generally fall into one of two categories: (1) those designed to identify environmental risk factors causing discordance in monozygotic (MZ) twins and (2) those geared towards identifying genetic risk factors. However, neither environment nor differences in DNA sequence can fully account for phenotypic discordance among MZ twins. The field of epigenetics – DNA modifications that can affect gene expression – offers new models to understand discordance in MZ twins. In the past, MZ twins were regarded as genetically-identical controls for differing environmental conditions. In contrast, the evolving current concept is that epigenetic differences between MZ twins may modulate differences in diverse phenotype, from disease to personality. In this article, we review some twin studies, and discuss the dynamic interactions between stochastic, environmental, and epigenetic variables that influence neurobiological phenotypes. © 2009 Wiley-Liss, Inc.