**DeFries-Fulker
multiple regression method**

**(DeFries & Fulker, 1985, 1988)**

** **

**Click the underlined links for a picture of each step in
the analysis**

** **

The DF model is based on the
differential regression to the mean of MZ and DZ cotwins when proband twins are
selected for extreme scores on a phenotype.

If the proband deficit is due
solely to nonshared environmental influences (for example, an early traumatic
brain injury), then the cotwin shares none of these etiological influences with
the proband. Therefore, the means of both MZ and DZ cotwins should regress to
the population mean.

**Shared
and Nonshared Environment**

If the
proband deficit is due in part to shared environmental influences (for example,
family nutrition), then both the MZ and DZ cotwins are also exposed to the same
shared environmental influences as the probands. Because both MZ and DZ twin
pairs share 100% of the shared environmental variance, the means of both MZ and
DZ cotwins should regress an equal distance back to the population mean.

** **

If the proband deficit is due to genetic influences,
both the MZ and DZ cotwins will regress back to the population mean, but the DZ
cotwins will regress farther.

**The Equation**

The basic DF regression
model is as follows:

C = B_{1 }P + B_{2 }R
+ K

where C is the expected cotwin
score, P is the proband score, R is the coefficient of relationship (1 for MZ pairs,
0.5 for DZ pairs), and K is the regression constant. The B_{1}
coefficient represents the partial regression of the cotwin score on the
proband score, and provides a measure of twin resemblance irrespective of
zygosity. The B_{2} parameter represents the partial regression of the
cotwin score on the coefficient of relationship, and after appropriate
transformation of the data provides a direct estimate of the heritability of
extreme scores on the trait under consideration (h^{2}_{g}). After adjustment of the standard errors of the
regression coefficients to correct for the double entry of concordant pairs,
the significance of the B_{2} parameter provides a statistical test of
the extent to which extreme scores are attributable to genetic influences.