The Impact of Variation in Twin Relatedness on Estimates of Genetic and Environmental Influences

By taking advantage of the natural variation in genetic relatedness among identical (monozygotic: MZ) and fraternal (dizygotic: DZ) twins, twin studies are able to estimate genetic and environmental contributions to complex... [ view full abstract ]

By taking advantage of the natural variation in genetic relatedness among identical (monozygotic: MZ) and fraternal (dizygotic: DZ) twins, twin studies are able to estimate genetic and environmental contributions to complex human behaviors. Recently concerns have been raised about the accuracy of twin studies in light of findings of genetic and epigenetic changes in twins. One of the concerns raised is that MZ twins are not 100% genetically (and epigenetically) similar because they show variations in their genomes and epigenomes leading to inaccurate estimates of heritability.

The current study presents findings from a simulation study that examined the degree of bias in estimates of heritability and environmentality when the genetic similarity of MZ twins differs from 1.00 and when the genetic similarity of DZ twins differs from .50. The findings suggest that in the standard biometric model when MZ or DZ twin similarity differs from 1.0 or .50, respectively, the variance that should be attributed to genetic influences is instead attributed to nonshared environmental influences, thus deflating the estimates of genetic influences and inflating the estimates of nonshared environmental influences. Although estimates of genetic and nonshared environmental influences from the standard biometric model were found to deviate from “true” values, the bias was usually smaller than 10 percentage points indicating that the interpretations of findings from previous twin studies are mostly correct. 

In contrast to the fixed genetic factor correlations in the standard biometric model, the alternative model sets the genetic factor correlations of MZ and DZ twins to be free parameters that need to be estimated. The findings suggest that in the alternative model, all the estimates of MZ and DZ twins’ genetic factor correlations and genetic, shared environmental and nonshared environmental influences are within 95% confidence intervals about the “true” parameter values used in generating the data.