Figure 1.

Path diagram for a bivariate model. For clarity only one twin is depicted. A1, A2: Genetic variance components; E1, E2: nonshared environmental variance components. For ease of presentation, common environment (C) is omitted from the figure. a₁₁ through a₂₂: genetic path coefficients (or factor loadings) of which a₂₂ represents novel genetic influences on the stress HRV levels, e₁₁ through e₂₂: nonshared environmental path coefficients (or factor loadings) of which e₂₂ represents novel environmental influences on the stress HRV levels. Delta: difference in rest and stress HRV levels. Subtracting the rest HRV level from the level of the stress HRV level is established by setting the path coefficients originating from the two phenotypes (and pointing towards delta) to −1 and 1. Formulas for the different heritability estimates are as follows:

$${\mathrm{h}}^2\text{total}(\text{rest})={{\mathrm{a}}_{11}}^2/({{\mathrm{a}}_{11}}^2+{{\mathrm{e}}_{11}}^2){\mathrm{h}}^2\text{total}(\text{stress})=({{\mathrm{a}}_{12}}^2+{{\mathrm{a}}_{22}}^2)/({{\mathrm{a}}_{12}}^2+{{\mathrm{a}}_{22}}^2+{{\mathrm{e}}_{12}}^2+{{\mathrm{e}}_{22}}^2){\mathrm{h}}^2\text{specific}(\text{stress})={{\mathrm{a}}_{22}}^2/{({\mathrm{a}}_{12}}^2+{{\mathrm{a}}_{22}}^2+{{\mathrm{e}}_{12}}^2+{{\mathrm{e}}_{22}}^2){\mathrm{h}}^2\text{delta}(\text{stress}-\text{rest})={(({\mathrm{a}}_{12}-{\mathrm{a}}_{11})}^2+{{\mathrm{a}}_{22}}^2)/{(({\mathrm{a}}_{12}-{\mathrm{a}}_{11})}^2+{{\mathrm{a}}_{22}}^2+{({\mathrm{e}}_{12}-{\mathrm{e}}_{11})}^2+{{\mathrm{e}}_{22}}^2).$$