We first tested the significance of C latent factors in the model (Table 2). By setting path estimates to 0, beginning with the C6 factor loadings and working our way back, we found that all of the C factors could be dropped from the model (see Table 2, models 2–7). However, because the phenotypic twin correlations clearly indicated that C influences are relevant in the earlier epochs (see Table 1), we opted to retain a single latent C factor, C1, and all of its factor loadings in the model. We next applied the same approach to A factors, and found that factors A3–A6 could be dropped from the model, but A2 could not. To avoid over-fitting, we did not test the significance of E factor loadings. Therefore, the final model is one in which there are two A factors, one C factor and six E factors. Standardized parameter estimates with confidence intervals (CIs) are provided in Fig. 1. Unstandardized path estimates and CIs are provided in Appendix Table A2.
