Figure 2. The path specification of a bivariate Cholesky decomposition.

Note: Circles denote unobserved or latent variables while rectangles denote observed or manifest variables. Single-headed arrows denote asymmetric or causal effects while double-headed arrows denote symmetric or covariances between variables. To simplify the presentation of the models, solid lines for the covariance between the shared environmental variance components indicate that the covariance is fixed at 1 for all types of twins while the dashed lines for the covariance between the additive genetic variance components indicate that the correlation is fixed at 1 for Mz twins and 0.5 for Dz twins.