phenotype were completely due to genetic variance, we would expect a twin pair correlation of 1.0 for MZ twins and 0.5 for DZ twins. Both MZ and DZ twin pairs share the C influences, so if C were the only source of variance, we would expect a twin pair correlation of 1.0 for both MZ and DZ pairs. E factors are unique for each individual, so if E were the only source of variance, we would expect a twin pair correlation of zero for both MZ and DZ pairs. In reality, phenotypic variance is generally due to a combination of A, C, and E influences and structural equation modelling is used to determine the combination of influences that best matches the observed data.
