Twin Studies

Origins and Assumptions of the Twin Model

The identification of familial clustering of a disorder is only the starting point for investigating its heritability. Because parents provide both genes and environment to offspring, attributions of genetic versus environmental causes of disease cannot be properly made in a traditional parent-offspring study. Studies of twins reared apart allow for a clear distinction to be drawn between genetic and environmental factors in the etiology of disorders. Variability in outcomes between individuals with the same genes in two different environments are attributed to factors that distinguish the two environments. However, the relatively unusual circumstances under which twins are separated at birth need to be considered in interpreting findings (including their possible relationship to parenting behaviors and/or heritable traits). Even more important, the rarity of twins being reared apart creates significant obstacles in the acquisition of samples that are sufficiently large to detect differences between affected and unaffected individuals, especially in the study of relatively uncommon diseases.

Studies of twins reared together do not suffer from the above limitations and, although distinguished from single births by shorter gestational periods, disadvantages associated with twin status do not appear to persist beyond 5 years of age. The prevalence or risk factors for numerous adult health conditions, including psychiatric disorders, do not differ between twins and singletons, making findings from twin studies generalizable to the larger population.

The twin method is based on the premise that the environments of etiologic relevance to the trait being studied do not differ significantly between DZ and MZ twins. The equal environments assumption (EEA) is critical in the interpretation of findings from twin studies because it is the basis on which distinctions between MZ and DZ similarity in a given trait are attributed to genetic rather than environmental sources. It has been argued that MZ twins experience environmental conditions that differ from those of DZ twins in that MZ twins are treated more similarly by parents and other individuals in their social environments and that they typically spend more time together and enjoy a closer emotional bond. Whereas critics of the twin method have argued that these apparent violations of the EEA invalidate findings from twin studies, twin researchers have argued for testing the assumption in a number of ways, including measuring the relationship between environmental similarity and outcomes under study and, in cases in which parents are misinformed about zygosity of twins, comparing the impact of actual versus perceived zygosity on twin resemblance. Little evidence of violations of the EEA has been produced, but twin researchers continue to promote rigorous testing and adjustments for violations when they are found.

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