Twin studies have been instrumental in building our knowledge of the etiology of common disorders because they provide a mechanism for estimating the relative magnitude of genetic and environmental contributions to disease. Monozygotic (identical) twins share 100% of their genes, and dizygotic (fraternal) twins share on average 50% of their genetic material—the same as in any pair of full siblings. By combining information from monozygotic (MZ) and dizygotic (DZ) twin pairs, an index of heritability can be calculated in the form of a ratio of MZ to DZ twin correlations for a given disorder. Since DZ twins are the same age (unlike other sibling pairs), differences observed between members of DZ twin pairs cannot be attributed to age or cohort differences. A higher correlation among ...

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