We freely estimated means and covariances of P3 amplitude, beta power, the time-frequency components that emerged from the above analysis, and EXT without imposing any latent structure on the data (i.e., without the additive genetic (A), shared environmental (C), and non-shared environmental (E) latent factors). This served as a baseline model. We subsequently fit an ACE model using a Cholesky parameterization. Based on the strength of prior evidence regarding associations between the candidate endophenotypes and genetic risk for EXT, we entered them in the following order: P3 amplitude, time-frequency components, beta power, and EXT (see Figure 1 for an illustration of the Cholesky model). We tested several submodels by fixing parameters at 0 in order to conduct significance tests of the importance of different parameters. Specifically, we constrained to 0 paths from EXT to the first three latent A factors to determine whether genetic influences on the endophenotypes account for significant genetic variance in EXT. We subsequently assessed whether genetic factors beyond A1 (which represent genetic variance in externalizing shared with all the endophenotypes) would be significant, and particularly whether
