Longitudinal extensions of this methodology decompose both time point specific variation and variation that is stable across time points into genetic and environmental components. Stable genetic or environmental influences include effects that are present at two time points and account for variance in personality. The longitudinal correlated factors model presented in Figure 2 is an example of this type of analysis. In addition to the contribution of A and E at an initial time point (a1 and e1), a second time point is included, and a2 and e2 represent the genetic and environmental influences at the second time point. The latent A and E factors at each time point are correlated (rA and rE) indicating whether it is the same or different genetic and environmental effects present at the two time points. This analysis derives information from cross-sibling cross-time correlations. If one sibling’s initial personality predicts the other sibling’s later personality to a greater degree for pairs that are more genetically related, then this result would be indicative of stable genetic influence across time points. Note that, by definition, the
