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 environmental influences are uncorrelated across siblings. They are, however, allowed to correlate across time points within individuals (e.g., E at time 1 for sib. 1 is only correlated with E at time 2 for sib. 1). Genetic effects, however, are correlated across siblings and time points to the extent that the siblings share genetic material. This specification is denoted in the model as rA*. For example, the cross-twin cross-time genetic correlation for monozygotic twins would simply be the genetic correlation (rA* = 1 × rA), but for dizygotic twins, this pathway would be constrained to be equal to half of the genetic correlation (rA* = 0.5 × rA). Because the same individual retains the same genotype across time points, the within-sibling cross-time pathway is equal to rA.
