Comparisons of the similarity of MZ and DZ twin pairs yield information about the degree of influence that can be attributed to genetic and environmental factors for a particular outcome (Plomin et al., 2001). The basic genetically informative twin model partitions variance in a behavior into additive genetic influences (A), common environmental influences (C), and unique environmental influences (E). Genetic influences correlate 1.0 between monozygotic (MZ) twins, who share all of their genes identical-by-descent, and 0.5 between dizygotic (DZ) twins, who share, on average, 50% of their segregating genes, as do ordinary siblings. Common environmental effects, as defined in biometrical twin modeling, refer to all environmental influences that make siblings more similar to one another. By definition, these influences correlate 1.0 between both MZ and DZ twins. Unique environmental influences are uncorrelated between co-twins and have the effect of decreasing the covariance between siblings.
