Elements of structural equation modeling in twin pairs for decomposition of trait determination. Boxes at the bottom represent measured variables in 2 twins within a twinship. Arrows represent causal pathways. Circles represent latent variables: the components of variance A (additive genetic variance), C (shared environmental variants), and E (unique, unshared environmental variance). Twin pair correlations are specified: genetic correlation (a), shared environmental correlation (c), and unique environmental correlation (e).