It is not uncommon to find that a model in which some factors are designated as purely additive genetic (and therefore correlate 1.0 between MZ twins and 0.5 between DZ twins), whereas others are purely environmental, fits the data well. In this situation, estimating latent genetic factor scores can be particularly valuable, although there is no barrier to estimating latent factor scores even when a factor is highly, but not completely, heritable. This latent factor score method was successful in a study of anxiety disorders,65 in which Diagnostic and Statistical Manual of Mental Disorders diagnoses and neuroticism trait scores were analyzed jointly in a multivariate twin model to derive a genetic latent trait factor score. The same approach could be applied to joint analyses of objective measurements of ADHD-related phenotypes such as RT or Actigraph measurements and more subjective but widely accepted ratings of attention and hyperactivity.
