A path diagram for a univariate AE model for single occasion data is presented in Figure 1. Variance in this trait is decomposed into latent A and E factors (represented as circles). The correlation between A factors is fixed to specific values depending on genetic relatedness of the siblings. For monozygotic twins (who have nearly identical genotypes), the correlation between the A factors of the siblings is fixed to 1. For dizygotic twins (who share approximately half of segregating genetic material on average), this correlation is fixed to .5. For half-siblings (who share approximately one quarter of segregating genetic material on average), the correlation between A factors is fixed to .25. The E factors are unique for each sibling and are always uncorrelated no matter what sibling type is represented. If the outcome is standardized before analysis, squaring the paths labeled a and e yields the proportion of variance in the phenotype attributable to the latent effects of A and E.
