Falconer's formula

The formula is $${\displaystyle {H_b}^2=2(r_{mz}-r_{dz})}$$ where ${H_b}^2$ is the broad sense heritability, ${\displaystyle r_{mz}}$ is the (monozygotic, MZ) identical twin correlation, and $r_{dz}$ is the (dizygotic, DZ) fraternal twin correlation. Falconer's formula assumes the equal contribution of environmental factors in MZ pairs and DZ pairs. Therefore, additional phenotypic correlation between the two pairs is due to genetic factors. Subtracting the correlation of the DZ pairs from MZ pairs yields the variance in phenotypes contributed by genetic factors. The correlation of same sex MZ twins is always higher than the DZ twin correlation with various sexes and thus all gender differences are evaluated as heritable. To avoid this error, only genetic studies comparing MZ twins with the same sex DZ twins are valid. Correlations between $A={H_b}^2$ (additive genetics) and ${\displaystyle C}$ (common environment) must be included in the derivation shown below.

$${\displaystyle r_{mz}=A+C+2\cdot \text{Corr}(A,C)}$$ $${\displaystyle r_{dz}=\frac{1}{2}A+C+2\cdot \text{Corr}(\frac{1}{2}A,C)}$$