on average, 50% of their segregating genetic variation, as do ordinary siblings. The basic genetically informative twin model partitions variance in a behavior into additive genetic influences (A), common environmental influences (C) [or dominant genetic influences (D); C and D cannot be simultaneously modeled with only twin data], and unique environmental influences (E). In twin models, genetic influences are set to correlate 1.0 between MZ twins, and 0.5 between DZ twins. Common environmental effects, as defined in biometrical twin modeling, refers to all environmental influences that make siblings more similar to one another. Unique environmental influences are those factors that have the effect of decreasing the covariance between siblings and include measurement error.
