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  (additive genetics) and  (common environment) must be included in the derivation shown below.
The correlation between monozygotic (MZ) twins is higher than that of dizygotic (DZ) twin correlations, regardless of sex. This implies heritability for gender differences. To correct this bias, genetic studies should compare MZ twins with same-sex DZ twins. Both additive genetics and shared environmental factors must be included in the derivation.