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The power of twin designs arises from the fact that twins may be either identical (monozygotic (MZ), i.e. developing from a single fertilized egg and therefore sharing all of their polymorphic alleles) or fraternal (dizygotic (DZ), i.e. developing from two fertilized eggs and therefore sharing on average 50% of their alleles, the same level of genetic similarity found in non-twin siblings).
An important assumption of the twin model is the equal environment assumption [27] that monozygotic twins have the same shared environmental experiences as dizygotic twins. If, for example, monozygotic twins tend to have more similar experiences than dizygotic twins—and these experiences themselves are not genetically mediated through gene ...
For example, one study found no statistically significant difference in self-reported wellbeing between middle-aged monozygotic twins separated at birth and those reared in the same household, suggesting that happiness in middle-aged adults is not based in environmental factors related to family rearing.
In terms of correlation statistics, this means that theoretically the correlation of tests scores between monozygotic twins would be 1.00 if genetics alone accounted for variation in IQ scores; likewise, siblings and dizygotic twins share on average half alleles and the correlation of their scores would be 0.50 if IQ were affected by genes ...
A limit of this design is the common prenatal environment and the relatively low numbers of twins reared apart. A second and more common design is the twin study in which the similarity of identical and fraternal twins is used to estimate heritability.
Twins are concordant when both have or both lack a given trait. [1] The ideal example of concordance is that of identical twins, because the genome is the same, an equivalence that helps in discovering causation via deconfounding, regarding genetic effects versus epigenetic and environmental effects (nature versus nurture).
Identical Strangers: A Memoir of Twins Separated and Reunited is a 2007 memoir written by identical twins Elyse Schein and Paula Bernstein and published by Random House. [1] The authors, born in New York City in 1968 to Leda Witt, daughter of Nathan Witt , were separated as infants, in part, to participate in a " nature versus nurture " twin ...
Thomas J. Bouchard Jr. (born October 3, 1937) is an American psychologist known for his behavioral genetics studies of twins raised apart. He is professor emeritus of psychology and director of the Minnesota Center for Twin and Adoption Research at the University of Minnesota.