Twin Studies


Twin studies provide useful tools for genetic and epigenetic research and also for studying the effects of nongenetic variables while controlling for genetic variation. Classic twin studies compare the phenotypic similarity of monozygotic to dizygotic twin pairs sampled from a population in order to estimate the relative influences of genetic and environmental factors on that phenotype in that population. Twin similarities may be of genetic or environmental origin; in twins reared apart, we see genetically influenced similarities emerge in very different environments. Phenotypically discordant monozygotic twins may reveal nonshared, nongenetic sources of influence on the development of that phenotype. Virtual twins, like adoptees, are biologically unrelated, and can teach us how children's rearing environments may either similarly or differentially affect their development, while holding age and cohort constant. Children‐of‐twins designs can help us detect gene‐correlated environments and distinguish between genetic and cultural factors in the transmission of complex phenotypes.

Key Concepts:

  • Twin studies can be used to explore a range of questions, from genetic variation in a trait in a population to the effects of specific epigenetic changes on gene expression to questions in evolutionary psychology.

  • Monozygotic (MZ) twins, also known as identical twins, typically share 100% of their genome and dizygotic (DZ) twins, also known as fraternal twins, share, on average, 50% of their genome. Classic twin studies use this information to estimate the relative proportions of genetic and environmental influence on a given phenotype in a given population.

  • Nonadditive genetic effects can make MZ twins more than twice as similar as DZ twins.

  • Effects of shared environments can make MZ twins less than twice as similar as DZ twins.

  • Studies of twins reared apart combine aspects of both twin study and adoption study designs. Remarkable similarities between MZ twins reared apart suggest a strong influence of genetic factors, and especially, of genes in specific combinations, on behavioural phenotypes.

  • Phenotypically discordant MZ twins may help researchers to isolate environmental and developmental contributors to, and results of, the development of that phenotype.

  • Phenotypically discordant MZ twins also help scientists to identify epigenetic changes that create within‐pair differences in gene expression and explain some of the within‐pair phenotypic differences.

  • Children‐of‐twins studies are useful in estimating the degree to which parental genotype is correlated with the offspring‐rearing environment (passive gene–environment correlation) and also in separating genetic transmission from environmental transmission (the effects of parental rearing environment).

Keywords: children‐of‐twins; discordant twins; epigenetics; heritability; structural equation modelling; twins; twin methods; twins reared apart; virtual twins

Figure 1.

Path diagram of the simple twin model in which additive genetic factors (A), common environmental factors (C) and unique environmental factors (E) are latent variables all contributing to observed or manifest variables of twin phenotype.



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Segal N (2007) Indivisible by Two: Lives of Extraordinary Twins. Cambridge: Harvard University Press.

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Trumbetta, Susan L(Sep 2012) Twin Studies. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0005242.pub2]