In DF regression, the first step is to select probands in a twin study with extreme scores on the trait being studied, and to regression to the mean among their co-twins depending on whether the probands are part of a MZ or DZ twin pair.
DF regression is thus based on the assumption that, to the extent that genetic factors play a role in causing the extreme variation in the studied trait, the magnitude of regression to the mean should be greater in DZ co-twins of probands than in their MZ counterparts.
[5] This should result in MZ co-twins being more similar to their probands on the trait under study than DZ twins are to theirs.
B1 represents a measure of co-twin relatedness separate from that of zygosity, while B2 can be converted into an estimate of the heritability of extreme scores on the trait.
[5] By comparing MZ and DZ co-twins in this manner, an estimate of "group heritability" is generated.