 The ACE model is effectively a visual representation—in the form of a path analysis—of the underlying mathematics of heritability estimation, known as ‘Falconer’s formula’ (Falconer and McKay 1998). The formula is derived from an observed population statistic: the phenotypic correlation between pairs of MZ and DZ twins. For any phenotype the MZ correlation or ‘\(r_{\MZ}\)’ is typically higher than the DZ correlation or ‘\(r_{\DZ}\)’. Perhaps unsurprisingly, genetically identical twins are more similar than fraternal twins. The implication is that the greater the genetic similarity, the greater the phenotypic similarity. Thus, narrow-sense, quantitative genetic heritability is intuitively defined as the difference between MZ and DZ correlations: the greater the difference, the higher the heritability. On the flip side, if there is minimal difference between MZ and DZ correlations, it would imply that genetics have little to do with variance in the trait in question, and result in a low heritability estimate.