1 Introduction

Rather than measuring an individual's phenotype directly, we often have to rely on ratings of the individual made by an observer. An important example is the assessment of children via ratings from parents and teachers. In this chapter we consider in some detail the assessment of children by their parents. Since the ratings obtained in this case are a function of both parent and child, disentangling the child's phenotype from that of the rater becomes an important methodological problem. For the analysis of genetic and environmental contributions to children's behavior, solutions to this are available when multiple raters, e.g., two parents, rate multiple children, e.g., twins. This chapter describes and illustrates simple Mx models for the analysis of parental ratings of children's behavior (Section 11.2. We show how the assumption that mothers and fathers are rating the same behavior in children can be contrasted with the weaker alternative that parents are rating correlated behaviors. Given the stronger assumption, which appears adequate for ratings of some children's behavior problems, the contribution of rater bias and unreliability may be separated from the shared and non-shared environmental components of variation of the true phenotype of the child. The models are illustrated with an application to CBC data (Section 11.2.5