Figure 1.

The psychometric model. Maternal and paternal ratings are linear functions of the latent phenotypes of the twins (P twin 1 and P twin 2), and rater specific variance (Am, Cm, Em, Af, Cf, and Ef). When constraining Am and Af (underlined) to zero, the model represents a restricted rater bias model with Cm and Cf representing mother's and father's bias and Em and Ef representing residual error.